Electrolyte-gated magnetoelectric actuation: Phenomenology, materials, mechanisms, and prospective applications Cite as: APL Mater. 7, 030701 (2019); https://doi.org/10.1063/1.5080284 Submitted: 07 November 2018 . Accepted: 01 January 2019 . Published Online: 06 March 2019 Cristina Navarro-Senent , Alberto Quintana , Enric Menéndez , Eva Pellicer , and Jordi Sort COLLECTIONS This paper was selected as Featured This paper was selected as Scilight ARTICLES YOU MAY BE INTERESTED IN A comprehensive review of electrolyte-gated magnetoelectric actuation Scilight 2019, 110002 (2019); https://doi.org/10.1063/1.5095779 Opportunities and challenges for magnetoelectric devices APL Materials 7, 080905 (2019); https://doi.org/10.1063/1.5112089 The development of 2D materials for electrochemical energy applications: A mechanistic approach APL Materials 7, 030902 (2019); https://doi.org/10.1063/1.5085187 APL Mater. 7, 030701 (2019); https://doi.org/10.1063/1.5080284 7, 030701 © 2019 Author(s). APL Materials RESEARCH UPDATE scitation.org/journal/apm Electrolyte-gated magnetoelectric actuation: Phenomenology, materials, mechanisms, and prospective applications Cite as: APL Mater. 7, 030701 (2019); doi: 10.1063/1.5080284 Submitted: 7 November 2018 • Accepted: 1 January 2019 • Published Online: 6 March 2019 Cristina Navarro-Senent,1 Alberto Quintana,1 Enric Menéndez,1 Eva Pellicer,1 and Jordi Sort1,2,a) AFFILIATIONS 1 Departament de Física, Universitat Autònoma de Barcelona, E-08193 Cerdanyola del Vallès, Spain 2 Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluís Companys 23, E-08010 Barcelona, Spain a) Author to whom correspondence should be addressed: Jordi.Sort@uab.cat ABSTRACT Manipulation of the magnetic behavior of materials with voltage (i.e., magnetoelectric actuation) has become a topic of intense research during the last years. Apart from its obvious interest from a basic science standpoint, control and eventual switching of the magnetization without applying any external magnetic field (or spin polarized current) has the potential to drastically reduce the power consumption of magnetic devices due to the lack (or minimization) of Joule heating dissipation effects. Herein, an overview of the state-of-the-art of electrolyte-gated magnetoelectric actuation (where an electric field is applied using an electrolyte, either liquid or solid) is provided. The different types of mechanisms responsible for voltage-driven mag- netic actuation (surface charging, ionic migration, also termed “magneto-ionics,” reduction/oxidation reactions, and ferroelec- tric/ferromagnetic coupling) are summarized. The various effects (changes in coercivity, anisotropy easy axis, exchange bias field, saturation magnetization, Curie temperature, etc.) observed in the different types of materials investigated so far (mainly metallic thin films and semiconductors, porous alloys, and nanocomposite structures) are described. The potential applications of electrolyte-gated magnetoelectric actuation in devices as well as the current challenges in the field are also reviewed with the aim of providing the basic ingredients for further prospects and technological advancements in this area. © 2019 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5080284 I. INTRODUCTION wasted in the form of heat dissipation by the Joule effect. For this reason, there is a growing interest in finding alternative Magnetic devices, such as micro-/nano-electromechanical methodologies to process magnetic information and operate systems (MEMS/NEMS), computer hard disks, magnetoresis- magnetic/spintronic devices using voltage (or electric field), tive random-access memories (MRAMs), or spintronic sys- instead of electric currents. tems, are conventionally operated using magnetic fields. In The grounds of the magnetoelectric effect (ME) were pos- conventional recording media, such magnetic fields are gen- tulated already in 1894 by Curie3 although the first experimen- erated with electromagnets in which relatively high electric tal observations date from 1960.4 The magnetoelectric effect currents are involved.1 In MRAMs, the spin switching is also refers to the influence of an external magnetic field on the achieved by passing a current through a neighboring metal- electric polarization of certain materials or, conversely, the lic strip which generates the magnetic field. Recently, spin- effect of the external electric field on the magnetic proper- polarized electric currents are being used to orient the mag- ties of materials. A traditional approach to tailor magnetism netic bits in magnetic memories via spin-transfer torque.2 In by means of voltage is to take advantage of strain-mediated all these cases, due to the presence of flowing currents, a large magnetoelectric coupling in piezoelectric-magnetostrictive fraction of the energy employed to control magnetization is hybrid materials. In such systems, the applied electric field APL Mater. 7, 030701 (2019); doi: 10.1063/1.5080284 7, 030701-1 © Author(s) 2019 APL Materials RESEARCH UPDATE scitation.org/journal/apm induces strain in the piezoelectric counterpart, which is trans- metallic films and nanoporous metallic alloys in aqueous elec- ferred to the magnetic material, causing changes in the trolytes.20–22 Subsequent studies showed the possibility to orientation of its net magnetic moment via the inverse magne- modify the magnetic properties of metal/metal oxide het- tostriction or Villari effect.5 This effect has been successfully erostructures23,24 or even oxide materials (e.g., Fe2 O3 , Co3 O4 ) exploited in transducers, magnetic actuators, and a variety of with voltage when immersing them either in aqueous25 or other hybrid devices.6–9 However, in spintronic devices, this even non-aqueous26 electrolytes, in all cases assisted by redox approach has some disadvantages. First, in thin films, there reactions or O2− ion motion. In the solid state, O2− can dif- is a reduced piezoelectric response due to the clamping with fuse back and forth from the material of interest toward an the substrate. In addition, repetitive strain action may ulti- oxygen source/sink (e.g., a high O2− mobility thin film, such mately cause mechanical fatigue and device failure. There as HfO2 or Gd2 O3 , deposited next to the FM layer), depend- have been some attempts to induce magnetoelectric effects ing on the voltage polarity.27–32 Interestingly, not only oxygen in single-phase materials exhibiting simultaneously magnetic migration but also cationic movement has been reported.33 and electric coupled ferroic orders, also known as “multifer- Alternatively, the properties of FM or ferrimagnetic oxides roics.”10 Unfortunately, despite intensive recent efforts, most have also been tuned by voltage-driven lithiation (Li+ incor- single phase multiferroic materials consist of complex oxides poration using suitable electrolytes).34–37 These effects have that exhibit their properties basically at low temperatures been sometimes referred to as “electrochemical reactions” or (with a few exceptions11 ). Therefore, there is a technological “magneto-ionic effects” although these reactions are typically demand for alternative approaches to manipulate magnetism not restricted to the surface but can affect a significant frac- with electric field at room temperature. tion of the overall film thickness, depending on the exper- About two decades ago, several studies showed the pos- imental conditions and the ability of the system to trigger sibility to modify the magnetic properties of diluted magnetic ion migration. In any case, magneto-ionics has gained huge semiconductors directly with electric field, without strain- interest recently since the induced effects are non-volatile. mediated effects, due to the changes induced in the carrier This is opposite to simple charge accumulation, where the density (donors or holes) which mediate the exchange inter- magnetic changes instantaneously disappear when voltage is actions responsible for the onset of ferromagnetism in these removed unless a ferroelectric (FE) layer (with high rema- materials.12,13 As a result, variations in coercivity (HC ), Curie nent electric polarization) is used as a dielectric. Despite being temperature (TC ), magnetization (M), or magnetic easy axis non-volatile, the effects caused by ion migration can be par- were reported when applying electric fields of the order of tially or totally reversed when the sign of the electric field 0.5–5 V/Å. Such effects in semiconductors occur at pene- is inverted. Electrochemical processes also overcome one of tration depths within the Debye radius, which is around 10– the main drawbacks of pure charge accumulation, i.e., the 12 nm. Inspired by these studies, similar observations were limited depth in which the phenomenon occurs due to the later made on ultra-thin magnetic metallic films.14–16 How- electric field screening length. Hence, electrochemical reac- ever, in metals, the electric field is more effectively screened tions are observable even in relatively thick films. One of the than in semiconductors and electric charges are accumu- drawbacks of electrochemical processes is, nevertheless, that lated just at the surface, near the interface with the dielectric the effects are not induced instantaneously (until today, the material employed to generate the electric field (within the dynamics is relatively slow, taking place in minutes or hours, Thomas-Fermi screening length which is around 0.5 nm).17 as will be described in Sec. III), hampering the utilization These charges can alter the wave function and the occupancy of magneto-ionics in high-speed memory applications and of the d orbitals of the alloy, eventually inducing changes spintronics. in the magnetic properties since the electric field screening Usually, all the aforementioned mechanisms (see Fig. 1 for depends on the orientation of the spin. It should be noted an overview of these mechanisms) do not occur separately that although the electric field is only effective at the very in voltage-actuated magnetic materials. In most cases, vari- surface of the alloy, the induced magnetic changes can prop- ous effects act synergistically to tailor the magnetic behavior agate up to 20 nm toward the interior of the alloy, due to under the action of an electric field and it is difficult to dis- the spin-spin exchange correlation length. Given the high sur- criminate between them. The influence of electric charging face area-to-volume ratio of nanoporous materials, magneto- on the various magnetic properties of thin films, multilay- electric effects ascribed to electric charge accumulation have ers, and nanostructures has been reviewed to some extent been recently observed in relatively thick (few hundred nm from a theoretical point of view by Brovko et al.38 In turn, thickness) nanoporous metallic films (e.g., Cu-Ni,18 Fe-Cu19 ) Song et al.39 recently reviewed the general mechanisms with ultra-narrow pore walls (∼5 nm), prepared by advanced responsible for voltage control of magnetism. Fusil et al.40 electrodeposition procedures. also summarized the different applications of magnetoelectric Besides strain and electric surface charging, voltage can materials (with special emphasis in multiferroics) in spintron- also induce changes in the oxidation state of ferromagnetic ics. Here we focus on the most relevant studies dealing with (FM) metallic alloys (i.e., reduction/oxidation reactions or electrolyte-gating as a means to generate ultra-high electric oxygen ion migration), particularly when these alloys are fields (resulting in huge magnetoelectric effects) and the new immersed in aqueous electrolytes or grown in direct contact types of materials, electrolytes, and experimental configura- with an ionically conducting oxide buffer layer (i.e., solid elec- tions that have led to unprecedented results on magnetoelec- trolyte).12–24 This approach was first carried out by immersing tric actuation of materials during the last fewyears. APL Mater. 7, 030701 (2019); doi: 10.1063/1.5080284 7, 030701-2 © Author(s) 2019 APL Materials RESEARCH UPDATE scitation.org/journal/apm FIG. 1. Main mechanisms responsible for electrolyte-gated magnetoelectric actua- tion. The most representative types of materials for each case are indicated, together with simple drawings to illus- trate the physical/physicochemical prin- ciples governing each mechanism. II. EXPERIMENTAL CONFIGURATIONS between 0.01 V/Å and 5 V/Å. To obtain these fields, relatively AND MAGNETOELECTRIC MEASUREMENT large voltages need to be applied (sometimes of the order of TECHNIQUES: SOLID DIELECTRIC LAYERS tens or hundreds of V), unless the electrically insulating layers VS. ELECTRIC DOUBLE LAYERS are made extremely thin. The charge accumulated in a con- To apply an electric field on magnetic materials, two denser is Q = ∆V C, where C is the capacity and ∆V is the main geometries are commonly utilized. The first one is the applied voltage. In turn, C = εr ε0 A/d, where εr is the relative field effect transistor (FET) structure, consisting of a source permittivity, ε0 is the vacuum permittivity, A is the area of the electrode, a drain electrode, and a gate electrode [Fig. 2(a)] plates, and d is the thickness of the dielectric layer. Hence, the in which magnetic properties are typically (but not always) accumulated charge for a given voltage-actuated material can assessed by resistive Hall effect measurements. Here the gate be given as voltage is the one that alters the magnetic properties of the Q = ∆Vεr ε0 A/d. (1) FM layer, and the current flowing from the source to the drain is used to track the changes in M. The gate electrode and Generation of hundreds of V is, of course, not very conve- the FM layer are isolated by an insulating (dielectric) layer. nient for practical applications. Hence, ultra-thin dielectrics Another convenient geometry is the condenser-like structure are desirable. However, despite the recent progress in depo- [Fig. 2(b)]. In this case, the FM material is grown on top or sition techniques, such as sputtering, molecular beam epi- below an insulating layer and voltage is applied between the taxy, or atomic layer deposition, the production of solid-state FM material and the counter-electrode grown on the other devices with high-quality ultra-thin dielectric layers (e.g., side of the dielectric layer. 1–2 nm thick Al2 O3 , SiO2 , HfO2 , or MgO) is still very chal- The strength of the electric fields needed to modify the lenging since the presence of structural defects easily result magnetic properties of materials typically falls in the range in pinholes through which the accumulated electric charges tend to leak. Several approaches have been engineered to tackle this problem. The first one is the utilization of insu- lating polymers such as polyimide (PI) or polyvinyl difluo- ride (PVDF).41,42 However, making these polymers very thin (by, e.g., spin coating) is not straightforward and, anyhow, the voltage they can afford without dielectric breakdown is also rather limited. A second approach is to employ liquid electrolytes. This includes (i) polar aprotic organic solvents (such as ethylene carbonate or propylene carbonate) and (ii) ionic liquids [e.g., diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide (DEME-TFSI), 1-ethyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM- TFSI or EMI-TFSI)] or other electrolytes such as LiPF6 dis- FIG. 2. Schematic drawings of the (a) field-effect transistor and (b) condenser-like solved in ethylene carbonate or KClO4 -polyethylene oxide as geometry employed to apply electric fields on magnetic materials. a means to generate the electric field. Aqueous electrolytes APL Mater. 7, 030701 (2019); doi: 10.1063/1.5080284 7, 030701-3 © Author(s) 2019 APL Materials RESEARCH UPDATE scitation.org/journal/apm TABLE I. Representative materials, typical electrolytes, and most prominent magnetoelectric effects induced by “pure” surface charging (electrostatic surface charge accumulation). Note that the following acronyms are used in the table: PC: propylene carbonate; LiClO4 : lithium perchlorate; EC: ethylene carbonate; DEME-TFSI: N,N- diethyl-N-(2-methoxyethyl)-N-methylammonium bis(trifluoromethylsulfonyl)-imide; CsClO4 : cesium perchlorate; PEO: polyethylene oxide; EMI-TFSI: 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; P(VDF-HFP): poly(vinylidene fluoride)-poly(vinylidene fluoride-co-hexafluoropropylene); HC : coercivity; PM: paramagnetic; FM: ferromagnetic; TC : Curie temperature; MR : remanent magnetization; DW velocity: domain wall velocity; PMA: perpendicular magnetic anisotropy; FM-HMI transition: ferromagnetic-hard magnetic insulating transition; MAE: magnetic anisotropy energy; M: magnetization; HR : resonance field (ferromagnetic resonance experiments). Electrolyte Material Magnetic parameter Variation Reversibility References Fe-Pt and Fe-Pd −4.5% (FePt) PC HC Yes (∆V, ∼5 min) 14 ultra-thin films +1% (FePd) PC Co-Pd film M ≈ 4.5% ... 50 LiClO4 in EC Nanoporous Au-Fe alloy M 0.2% Yes 53 ≈ 0.5 % (Pd74 Ni26 ) Partially (∆V, ∼1 h) LiClO4 in EC Nanoporous Pd-Ni alloy M 55 ≈ 0.3% (Pd35 Ni65 ) Yes (∆V, ∼1 h) DEME-TFSI (Ti,Co)O2 film M PM-FM switching ... 49 CsClO4 in PEO TC ∆TC ≈ +17% EMI-TFSI Co ultra-thin film MR Yes (∆V, 0.5–1 h) 59 −14% HC Domain wall EMI-TFSI Fe-Pt/Al2 O3 film Domain walls Yes (∆V, ∼min) 60 propagation Domain walls ∆DW velocity = 4.2 factor EMI-TFSI CoFeB/MgO film Yes (∆V, ∼s) 62 HC ∆HC = 1.3 factor PM–FM switching TC DEME-TFSI LSMO ultra-thin film ∆TC = 14–26 K Yes (∆V, ∼min) 65 M ∆M = 98.5%–128% PC Nanoporous Cu-Ni film HC −32% ... 18 Pseudo-ordered PC HC −25% Partially (0 V, 5 min) 19 porous FeCu film LiClO4 in EC Nanoporous Pd-Co alloy M 3% Partially (∆V, ∼2 h) 56 CoFeB/MgO/HfO2 PMA EMI-TFSI ∆PMA = 0.108 mJ/m2 Yes (0 V, slow time) 61 ultra-thin film DW velocity LiClO4 in PC Nanoporous LSMO M 1.8%–2.5% Yes (∆V, ∼h) 63 TC DEME-TFSI LSMO film ∆TC ≈ 21% ... 64 FM-HMI transition EMI-TFSI in PVDF (Pt/Co)2 /Pt film MAE 3.16 × 105 J/m3 Yes (0 V, -) 67 EMI-TFSI in Pt/Fe/Pt film Anisotropy field ∆HR = 114 Oe Yes (∆V, ∼min) 68 P(VDF-HFP) TABLE II. Representative materials, typical electrolytes, and most prominent magnetoelectric effects induced by surface reduction/oxidation reactions. The following acronyms are used in the table: KOH: potassium hydroxide; NaOH: sodium hydroxide; LiClO4 : lithium perchlorate; DMC: dimethyl carbonate; EC: ethylene carbonate; MAE: magnetic anisotropy energy; m: magnetic moment; MS : saturation magnetization; RAH,S : anomalous Hall resistivity at saturation; M: magnetization. Electrolyte Material Magnetic parameter Variation Reversibility References 1M KOH Nanoporous Pd-Ni alloy M 24.6% Yes (∆V, ∼2 h) 21 0.01M KOH Co ultra-thin film MAE 0.36 erg/cm2 ... 20 1M KOH γ-Fe2 O3 /Pt-nanocomposite M 4.2% Yes (∆V, ∼min) 25 Fe-O/Fe films M 64% MR /MS 1M KOH Fe3 O4 /Fe/(001) L10 Partially (∆V, -) 24 PMA MAE = 55% FePt ultra-thin film MAE 0.001M–6M KOH Fe film MS 20% ... 22 1M NaOH Nanoporous Cu-Ni MS +33% Yes (∆V, 10 min) 72 1M KOH FeOx /Fe nanoislands M ∆MS ≈ ∆RAH,S = +86% Yes (∆V, -) 23 Porous γ-Fe2 O3 /Pt- 1M KOH M ∆m = 10.4% Yes (∆V, ∼5 min) 71 nanocomposite HC ∆HC = +200% LiClO4 in DMC/EC CoPt ultra-thin films No 73 MS ∆MS = +4% APL Mater. 7, 030701 (2019); doi: 10.1063/1.5080284 7, 030701-4 © Author(s) 2019 APL Materials RESEARCH UPDATE scitation.org/journal/apm (with dissolved KOH or NaOH) are also utilized when the aim double layer” (EDL), which is extremely thin. Electric fields is only to induce voltage-driven redox reactions on the mag- of the order of hundreds of MV/cm can be reached apply- netic material immersed in the solution. The different types of ing moderate voltages.18 The EDL is simply the solid-liquid electrolytes together with the main experimental observations capacitor formed between a solid electrode and dissolved on various types of voltage-controlled magnetic materials are ions in close proximity with a solvent acting as a dielec- summarized in Tables I–III. tric.43 The Gouy–Chapman–Stern is the most popular theory In liquid electrolytes, low voltages can generate reason- to model the EDL capacitance and to study the ionic dynamics ably high electric fields due to the formation of the “electric at electrode/electrolyte interfaces.44 A flat EDL is created by TABLE III. Representative materials, typical electrolytes, and most prominent magnetoelectric effects induced by voltage-driven ion migration (magneto-ionics), including ion incorporation from the electrolytes. The following acronyms are used in the table: EMI-TFSA: 1-ethyl-3-methylimidazolium and bis(trifluoromethylsulfonyl)amide; P(VDF-HFP): poly(vinylidene fluoride-co-hexa-fluoropropylene); DEME-TFSI: N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis(trifluoromethylsulfonyl)-imide; TMPA+ /TFSI− : N,N,N- trimethyl-N-propylammonium/bis(trifluoromethylsulfonyl)imide; LiPF6 : lithium hexafluorophosphate; LiTFSI: lithium bis(trifluoromethanesulfonyl)imide; DMC: dimethyl carbonate; EC: ethylene carbonate; DEC: diethyl carbonate; PC: propylene carbonate; TC : Curie temperature; MS : saturated magnetization; HC : coercivity; HE : exchange bias field; RAH,S : anomalous Hall resistivity at saturation; MA: magnetic anisotropy; TB : superparamagnetic blocking temperature; Tt : transition temperature; HR : ferromagnetic reso- nance field; Keff : effective anisotropy constant; MAE: magnetic anisotropy energy; MR /MS : remanence to saturation magnetization ratio; DW velocity: domain wall velocity; M: magnetization. Electrolyte Material Magnetic parameter Variation Reversibility References EMI-TFSA in SrRuO3 film TC ∆TC = 30 K Yes (∆V, -) 89 P(VDF-HFP) MS ∆MS ≈ +11%–21% DEME-TFSI Co/Ni/HfO2 films PMA −53% Yes (∆V, ∼30 min) 31 TC ∆TC ≈ 10% MS ∆MS ≈ ∆RAH,S = +70% Co/Ni/HfO2 films Yes (∆V, ∼10 min) DEME-TFSI HE ∆HE , ≈ 54% 32 [Co/Ni]3 /HfO2 films Partially (∆V, ∼30 min) HC ∆HC ≈ 74% MaN2401 Co ultra-thin film HC ∆HC ∼ 68% Yes (∆V, ∼30 min) 81 Tt-warm ≈ −10% DEME-TFSI FeRh/HfO2 film Tt Yes (∆V, -) 82 Tt-cool ≈ −31% DEME-TFSI Co ultra-thin film MA ∆HR = 219 Oe Yes (∆V, 5 min) 85 TMPA+ /TFSI- Cu/Ni ultra-thin films TB ∆TB = 28 ◦ C Yes (∆V, -) 86 FM insulator-FM SrCoO3− δ , SrCoO2.5 , metal-AFM DEME-TFSI Phase transition Yes (∆V, ∼min) 90 and HSrCoO2.5 films insulator phase transition Pr0.65 (Ca0.75 Sr0.25 )0.35 MnO3 DEME-TFSI Phase transition FM fraction (∼±0.15%) Yes (∆V, -) 96 films LiPF6 in Fe-Pt ultra-thin films MS ∆MS ≈ ∆RAH,S = 8% Yes (∆V, -) 98 DMC/EC LiPF6 in MS ∆Keff = −25% FePt/iron oxide composites Yes (∆V, -) 99 DMC/EC Keff ∆MS ≈ ∆RAH,S = +4% LiPF6 in γ-Fe2 O3 nanoparticles M ∆m ≈ 20 Am2 /kg Yes (∆V, ∼h) 100 EC/DEC LiTFSI in Nanoporous Co0.5 Ni0.5 Fe2 O4 M ∆M ≈ 2.5%–5 % Yes (∆V, ∼min) 101 EMIM-TFSI and CoFe2 O4 films LiTFSI in Nanoporous α-LiFe5 O8 films M ∆M ≈ 4% Yes (∆V, ∼h) 102 EMIM-TFSI MS ∆Ms ≈ 157.14 emu/g LiPF6 in PC Nanoscale α-Fe2 O3 -based film Yes (∆V, -) 34 HC ∆HC ≈ 134.4 Oe PC Co3 O4 film Magnetic transition FM–PM transition Yes (∆V, 1 h) 72 MAE GdOx Co/GdOx ultra-thin films MR /MS 0.75 erg/cm2 Yes (∆V, 10 s) 28 HC −80% (interface) Partially GdOx AlOx /GdOx /Co films M 29 −38% (bulk) (∆V, 40 min) AlOx Co/AlOx ultra-thin film Domain wall velocity From 0.4 to 4 µm/s ... 75 MS ∆MS = 20% Partially PC Nanoporous Fe-Cu films 87 HC ∆HC = 100% (∆V, 40 min) EMI-TFSI in LSCO ultra-thin film TC ∆TC = 7% ... 92 P(VDF-HFP) APL Mater. 7, 030701 (2019); doi: 10.1063/1.5080284 7, 030701-5 © Author(s) 2019 APL Materials RESEARCH UPDATE scitation.org/journal/apm a charged plane in contact with an electrolyte solution com- posed of solvent molecules, counter-ions (ions with the charge of the opposite sign than the plane), and co-ions (ions with the charge of the same sign as the plane) [see Fig. 3(a)].45 A buildup of counter-ions near the surface creates an excess electric potential in the surface neighborhood. This effect is partic- ularly pronounced when a polar medium is used, wherein the surface charges of the solid attract counter-ions distributed in the polar solvent.46 This potential drops over some dis- tance, from the value at the surface to some lower Galvani potential in the bulk of the solution. The effective thickness of the EDL increases with increasing counter-ion size and with decreasing bulk concentration of ions, whereas either a decrease or increase in the effective thickness may be caused by an increase in the surface charge. The distance from a solid flat surface to the inner Helmholtz plane is around the order of one bare ionic radius, roughly 0.1 nm, while that to the outer Helmholtz plane is roughly one hydrated ionic radius, about 0.5 nm.47 The diffuse layer can extend a few nm from the sur- face of the electrode. When non-flat surfaces are involved, as would be the case of nanoporous materials [Fig. 3(b)], the structure of the EDL becomes more complex, being influ- FIG. 4. Schematic illustration of the experimental setups typically used for magne- toelectric measurements in a liquid configuration using (a) a magneto-optic Kerr enced by entropy effects and resulting in non-homogeneous effect setup (MOKE) and (b) a vibrating sample magnetometer (VSM). potential distributions.48 To perform magnetoelectric measurements using liq- uid electrolytes, conventional magnetometry techniques such quantum interference device (SQUID), or vibrating sample as the magneto-optic Kerr effect (MOKE), superconducting magnetometry (VSM) are typically assembled together with electrolytic cells that allow for in situ application of voltage while hysteresis loops are being recorded (see Fig. 4). Although not straightforward, in situ structural characterization of the investigated materials under an applied electric field is some- times performed in order to elucidate the mechanism respon- sible for the observed changes in magnetic properties. In most cases, though, the materials are structurally characterized ex- situ (i.e., after voltage removal). This is useful only when volt- age causes permanent changes in the structure of the films (e.g., magneto-ionics or redox reactions). III. MATERIALS AND MAIN EXPERIMENTAL OBSERVATIONS The vast majority of materials used in electrolyte-gated magnetoelectric experiments are prepared in the form of thin films (sometimes lithographed) using sputtering, pulse laser deposition (for oxides), or electrodeposition (for met- als or semiconductors) (see Fig. 1). Ultra-thin metallic films (0.5–4 nm) or nanoporous alloys (both with very high surface area-to-volume ratio) are required to observe pure surface charging effects since the accumulated carrier density is pro- portional to the overall surface area [Eq. (1)]. Oxygen ions’ migration and redox reactions are generally promoted when the alloys are immersed in protic solvents that can provide oxygen to the magnetic system. Magneto-ionic effects can be also observed in systems where the magnetic layer is put in direct contact with oxide layers (e.g., HfO2 , Gd2 O3 ) that can absorb or release oxygen to the magnetic material under volt- FIG. 3. Schematic representation of the formation of the electric double layer (ELD) age application (i.e., acting as solid electrolytes). Also inter- onto the surface of (a) a flat electrode and (b) a nanoporous electrode. esting are the magnetoelectric effects originating from the APL Mater. 7, 030701 (2019); doi: 10.1063/1.5080284 7, 030701-6 © Author(s) 2019 APL Materials RESEARCH UPDATE scitation.org/journal/apm coupling between FE and FM layers, sometimes exacerbated and, due to the spin-spin exchange coupling, the effect is then by using ionic liquids. In the following, the main types of propagated. electrolyte-gated materials and magnetoelectric observations The influence of applied voltage on “magnetic proxim- due to charge accumulation, redox, magneto-ionic phenom- ity effects” was investigated by Obinata et al.51 in a Pd film ena, and FE/FM coupling, in all cases assisted or mediated by deposited onto a FM Co/Pt bilayer gated using an ionic liq- liquid or solid electrolytes, are summarized. uid electrolyte. The surface magnetic moment induced by the proximity effect on the polarized Pd layer could be tailored A. Pure charge accumulation magnetoelectric effects by the external voltage. The electric field supposedly induced Pioneering studies on electric field manipulation of the changes in the density of states at energy levels close to the magnetic properties due to charge accumulation were car- Fermi level, hence modifying the net moment of Pd. Quite ried out on diluted magnetic semiconductors (DMSs). Specif- recently, ionic liquid gating has been also shown to induce ically, Ohno and co-workers12 successfully modified TC in a changes in the Ruderman–Kittel–Kasuya–Yosida (RKKY) inter- (In, Mn)As ultra-thin film (4 nm), taking advantage of the hole- action in FeCoB/Ru/FeCoB and [Pt/Co]/Ru/[Pt/Co] multi- dependent ferromagnetism of this system. Upon application layers as a result of a voltage-induced disturbance of itinerant of an electric-field of ±1.56 MV/cm, which corresponds to a electrons inside the synthetic antiferromagnetic heterostruc- change in the carrier density of the order of 10−12 cm−1 , the tures and the concomitant change in the Fermi energy level.52 authors successfully tuned TC by ∓2 K. Changes in HC and the This resulted in pronounced changes in the overall shapes anisotropy easy axis direction were observed in a similar DMS of the hysteresis loops for applied voltages of the order of a subject to voltage, where an electric field was applied through few V. a solid dielectric layer using a FET configuration.13 Despite Controlled reversible changes in M (0.1%–0.2%) were the promising results shown in these preliminary studies, the also reported as a function of voltage in porous Au-Fe low temperature character of the magnetic properties of DMS alloys immersed in lithium perchlorate dissolved in ethylene made it necessary to move toward more robust, room tem- acetate.53 The observed effect was partially ascribed to the perature FM materials, such as high-TC oxide diluted mag- Coulomb pressure effect (i.e., voltage-induced strain54 ) due netic semiconductors49 or metallic magnetic materials, where to the surface charging. It should be noted that the pore wall the use of an electrolyte can further assist and improve the thickness in this material is close to 50–100 nm; hence, varia- magnetoelectric effect on such systems. tions in the MAE ascribed to pure charging are anticipated to A huge breakthrough was achieved by Weisheit et al.14 be very small. By using the same electrolyte, Ghosh et al.55,56 who observed the first surface charging magnetoelectric were also able to alter the magnetic properties of porous Pd- effect in metallic ultra-thin films. The authors were able to Ni and Pd-Co alloys, in a reversible manner, but again to a tune the HC of FePt and FePd films by a factor of 4.5% and rather limited extent (a few % variation in M). The effect was 1%, respectively. The trends for FePt and FePd were opposite again attributed to elastic strain due to disturbance of atomic for the same applied voltage, which was a strong confirma- bonding with the excess charge accumulated on the metal sur- tion for the theoretical predictions about this magnetoelectric face (i.e., magnetoelastic effect). Specifically, Ghosh55 showed effect being directly linked to the material’s band structure. that the changes in M with voltage depended on the Ni con- To apply a sufficiently high electric field without leakage cur- tent in the Pd1−x Nix alloy. M decreased when a negative volt- rents, EDL gating using propylene carbonate with dissolved age was applied on Pd74 Ni26 , whereas the opposite behavior Na+ and OH– ions was employed. The reported changes were was observed on Ni-rich specimens (Pd35 Ni65 ). Moreover, the thickness-dependent, evidencing the surface character of the initial value of M in Pd74 Ni26 could no longer be recovered phenomenon. The observed changes in HC were attributed to after cycling for several hours.55 Non-recovery of the initial voltage-driven variations in the magnetic anisotropy energy mass magnetization, σM , was also observed in Pd-Co,56 where (MAE). This work can be considered as a seminal one concern- again σM did not reach the original value after several cycles. ing voltage-actuation of metallic materials, and several studies Although the authors pointed to “pure” surface charging as focused on “surface charging” effects in magnetic materials the origin of the observed magnetic changes, some structural followed afterwards (see Table I). changes are likely to occur on the samples as well, particularly While Weisheit et al. reported changes in HC which almost after long-term cycling. vanished in a 4 nm thick film, Zhernenkov et al.50 reported The paramagnetic susceptibility of a sintered porous net- changes in M in a thicker (18.5 nm) CoPd film. Since Kerr work of interconnected palladium nanocrystallites, electro- spectroscopy cannot provide depth resolution and quantita- statically charged using an electrolyte, was also reported to tive assessment of M, the authors employed polarized neu- vary in a reversible manner, due to voltage-induced strain, by tron reflectivity to evaluate the M profile as a function of Drings et al.57 Subkow and Fähnle58 provided further theo- depth. Gating was also performed using propylene carbonate. retical interpretation to the observed effects in nanoporous An enhancement of M, scaling linearly with the applied volt- materials. They demonstrated through ab initio simulations age, was observed within approximately 7 nm from the surface that charge accumulation can lead to changes in the surface of the film. This thickness is much larger than in the case of magnetic anisotropies of the nanoparticles which form the FePt or FePd.14 The affected region is, in fact, of the order nanoporous network, assuming that only those surfaces that of the magnetic exchange length for CoPd, thus suggesting are not at grain boundaries in direct contact with neighboring that, upon charging, a perturbation in the Fermi level occurs nanoparticles become charged. APL Mater. 7, 030701 (2019); doi: 10.1063/1.5080284 7, 030701-7 © Author(s) 2019 APL Materials RESEARCH UPDATE scitation.org/journal/apm Significantly larger surface charging magnetoelectric effects have been recently observed in Cu-Ni,18 Co-Pt, and Fe- Pt nanoporous alloys prepared by micelle-assisted electrode- position, for which the pore wall width could be reduced to 5–7 nm [see Fig. 5(a) for an example of the typical morphology of these films]. The HC values in these systems could be dras- tically reduced [by about 25%–32%, as shown in Figs. 5(b) and 5(c)] by properly choosing the sign and strength of the applied voltage (which was generated using an anhydrous propylene carbonate electrolyte containing a low amount of Na+ and OH– ions). This reduction in HC allows for magnetization reversal at significantly lower applied magnetic fields and, therefore, reduces Joule heating effects during writing of the magnetic information. Contrary to the seminal work by Weisheit et al., here the overall thickness of the films could easily exceed 0.5 µm. Similar results were obtained in hierarchically porous Fe-Cu films prepared by electrodeposition on colloidal tem- plated substrates.19 These results, ascribed to changes in the MAE (as revealed by ab initio calculations18 ), are thus very appealing for energy-efficient magnetic actuation. Beyond the unquestionable technological relevance of these results, inter- esting observations emerge when the behavior of the different investigated nanoporous alloys is compared [Fig. 5(c)]. First, the variations in HC not always occur for the same voltage polarity. Namely, while HC in Co-Pt and Fe-Cu decreases for negative applied voltages, HC in Cu-Ni decreases for positive voltages. On the other hand, only an increase in HC is observed for Fe-Pt under application of negative voltages. These dissim- ilar trends for the various investigated alloys might be under- stood from ab initio calculations. For a given crystallographic phase and orientation, the MAE as a function of applied electric field might vary differently with the electric field. Namely, a positive slope in MAE vs. electric field was obtained in Co-Pt,38 whereas a negative slope was found in Cu-Ni.18 Additionally, the observed effects do not linearly vary with voltage (and in some cases they are only observed for one voltage polarity). This might be due to the dissimilar thick- nesses of the ELD which forms when the FM material is neg- atively or positively polarized as the ion species involved in each case are different. Thus, an asymmetric magnetoelectric effect could actually be anticipated due to the dependence of the EDL structure on the sign of the applied voltage. Further experiments are required to shed light on these issues. Simultaneously to all these studies on metallic FM alloys, Yamada and coauthors49 demonstrated for the first time that electric field could be used to manipulate the magnetic prop- erties of a diluted semiconductor whose TC is above room temperature. The selected material was Co-doped TiO2 , and voltage was applied through the formation of an EDL. The authors demonstrated the same effects using two differ- ent electrolytes: the ionic liquid DEME-TFSI and polyethy- lene oxide with dissolved CsClO4 . Through Anomalous Hall FIG. 5. (a) Typical morphology of a nanoporous Co-Pt film prepared by micelle- Effect (AHE) measurements, they showed that by apply- assisted electrodeposition, as evidenced by high-resolution scanning electron ing few volts (Vapplied < 4 V), a clear FM hysteresis loop microscopy (HR-SEM) (on-top view), and (b) hysteresis loops corresponding to emerged from the initial paramagnetic signal of the sample. nanoporous Co-Pt films, measured under voltage application while the sample This FM signal, which could not be due to sample FM impu- remains immersed in propylene carbonate with solvated Na+ and OH– ions in rities, scaled with the applied voltage. Moreover, the authors solution. (c) Relative changes in coercivity for different nanoporous metallic alloys showed a non-monotonic dependence of TC with the carrier prepared by electrodeposition, i.e., Fe-Pt, Co-Pt, Cu-Ni,10 and Fe-Cu.11 In all cases, the effects are mainly ascribed to surface charge accumulation. density. APL Mater. 7, 030701 (2019); doi: 10.1063/1.5080284 7, 030701-8 © Author(s) 2019 APL Materials RESEARCH UPDATE scitation.org/journal/apm By means of EDL gating using the ionic liquid EMI-TFSI, liquid and PVDF dissolved in dimethylformamide and acetone a drastic tuning of TC in a cobalt ultra-thin film was demon- baked at 45 ◦ C) as electrolytes for solid-state systems (i.e., for strated by Shimamura et al.59 By simply applying ±2 V, the flexible, wearable electronic devices).67–69 Zhao et al. reported authors demonstrated that TC could be modified up to 100 K, changes in the perpendicular anisotropy of [Pt/Co] multilay- hence drastically altering the magnetic properties of the film. ers, including a reorientation toward the in-plane direction, Not only changes in M, HC , or TC have been obtained through with voltage values as low as 4 V.59 Variations in the RKKY electrolyte gating. Herrera and colleagues showed, for exam- interactions have been also observed in synthetic antiferro- ple, that the domain wall (DW) dynamics of a FePt thin film magnets using an analogous methodology.69 In turn, Wang can be tailored with an external electric field.60 Namely, the et al.68 demonstrated reversible and non-volatile changes in number of magnetic domains can be increased (and their size magnetic anisotropy in Pt/Fe/Pt trilayers directly deposited decreased) by applying a negative voltage due to a strength- onto a polyimide flexible substrate after applying 1.8 V. ening of the magnetic anisotropy. Conversely, upon positive voltage, a de-pinning of the domain walls was observed due B. Surface redox reactions using oxidizing to the reduction in the magnetic anisotropy. In turn, Liu electrolytes et al.61,62 showed that the DW velocity in CoFeB/MgO/HfO2 Electrochemical reactions have also been proposed as a and CoFeB/MgO can be also tailored using an electric field means for tuning the magnetic properties of materials. The applied through an ionic liquid gate. Specifically, the velocity concept relies on exploiting (whenever possible, reversibly) increases for positive voltages, while negative voltages tend oxidation-reduction reactions of a starting metallic (or metal to reduce it. The observed modulation scales by a factor 4.2 oxide) material in the presence of an aqueous alkaline elec- using relatively small applied voltages (±0.8 V). A reorientation trolyte. A charge-transfer reaction between the electrolyte of the magnetic easy axis with voltage, from perpendicular- and the material leads to changes in the oxidation state of the to-plane to in-plane with voltage, was also reported by these metal (or oxidized metal). As a result, variations in the resulting authors. This is an interesting effect for voltage-driven MRAM magnetic properties, typically M, are achieved. Compared to applications. pure surface charge accumulation phenomena, wherein only Besides the aforementioned studies dealing with mag- the topmost atomic layers are involved, redox reactions may netic metallic alloys and semiconductors, there are also a affect a higher volume of the material. Although oxidation- few reports on magnetoelectrically actuated lanthanum stron- reduction reactions are set to occur at the surface level, the tium manganite (La1−x Srx MnO3 or LMSO),63–65 a ceramic thickness affected is of the order of several nanometers, and perovskite-type oxide material with a complex phase dia- therefore the changes observed tend to be larger. gram that exhibits doping-dependent metal-to-insulator and Concerning the measurement setup, a three-electrode paramagnetic-to-FM transitions (e.g., for certain composi- cell filled with an alkaline electrolyte, commonly KOH in a tions, these oxides show a high electric conductivity at high concentration ranging from 0.01M to 1M, is utilized in most temperatures). Voltage-induced carrier doping in LMSO was of the studies (see Table II). In order to accurately record first reported to modify the Mn4+ /(Mn3+ + Mn4+ ) ratio, result- changes in the magnetic properties of the material while ing in changes in M close to TC 63 and the eventual gener- oxidation-reduction reactions take place, the cyclic voltam- ation of hard magnetic and insulating regions embedded in metry (CV) technique is combined in situ with either SQUID a soft magnetic matrix (i.e., a phase transition) that could magnetometry, AHE, or MOKE. This allows for a direct corre- enable four-state memory devices.64 More recently, Moli- lation between redox processes and variations in the magnetic nari et al.65,66 have shown the possibility to tune the mag- properties of the material under study. netic properties of manganite-based supercapacitors using Among the FM metals and alloys subjected to voltage- the ionic liquid DEME-TFSI as an electrolyte. In a film con- control of M through electrochemical reactions, iron-based sisting of 3 nm thick La0.74 Sr0.26 MnO3 with TC = 246 K, electric electrodes are the most exploited by far. For example, the charging at T = 235 K induces changes in the manganese oxi- electrochemical reduction of magnetite to metallic iron in dation state, altering the spin-spin interactions and leading strong alkaline solutions has been intensively studied by CV70 to a reversible suppression of ferromagnetism.65 The authors because the process of reduction is advantageous for several have reported that by tuning the potential window, M can applications like catalysis, batteries, recycling of metal scraps, be tuned in-phase and anti-phase, thus opening avenues for and regeneration of corroded steel surfaces. The pathway this material to be used in transducers. A synergetic effect from iron oxide to metallic iron (and vice versa) and thus the between electrostatic surface charging (electric-double-layer potentials at which the different intermediate species form are capacitance) and electrochemical (pseudocapacitance) doping relatively well known, and this has encouraged the commu- [i.e., arising from chemisorptions of DEME and TFSI ions to the nity working in magnetoelectric phenomena to take Fe-based La1−x Srx MnO3 (LSMO) surface] has been shown to lead to large materials as candidate electrodes for electrochemical-control changes in M for this system (by 33%), even at room temper- of their magnetic properties. ature, with relatively small external voltages, of the order of a Leistner and co-workers studied a 10 nm thick sputtered few V.66 Remarkably, the properties of LMSO can be also tuned iron film in KOH at various concentrations (0.001M–6M) by by Li+ ion intercalation, as it will be discussed in Sec. III C. combined CV with AHE measurements.22 The starting Fe film Finally, interesting recent studies have explored the pos- already contained a thin oxide/hydroxide passivation layer sibility of using ionic gels (comprising, e.g., EMIM-TFSI ionic which was reduced by polarization at −1.3 V vs. Standard APL Mater. 7, 030701 (2019); doi: 10.1063/1.5080284 7, 030701-9 © Author(s) 2019 APL Materials RESEARCH UPDATE scitation.org/journal/apm calomel electrode (SCE) for 10 min before the experiments iron oxide layer was naturally formed on the as-sputtered Fe were initiated. The authors concluded that 1M KOH was the thin film. The presence of the permanent FePt underlayer, optimal concentration in order to enable reduction toward with perpendicular magnetic anisotropy, allowed for changes metallic Fe upon negative polarization and avoid severe dis- in the average magnetization orientation and switching from solution (and hence, irreversibility) with positive polarization rigid magnet to exchange spring magnet regime (the latter in the potential window between −1.4 V and −0.2 V vs. SCE. thanks to the Fe/FePt interface). The electroreduction of the A variation of the saturation magnetization (∆MS ) up to 20% Fe3 O4 layer to metallic Fe caused an increase in the over- was achieved when the potential was switched from −1.26 V all thickness of the metallic Fe and its decrease when the to −0.18 V vs. SCE, which was attributed to the re-oxidation of polarization was reversed. As a result, M, the remanence-to- the ca. 2 nm thick Fe layer. saturation magnetization ratio (MR /MS ), and the perpendicu- Similarly, the occurrence of naturally passivated Fe sur- lar magnetic anisotropy could be reversibly tuned. An almost faces was exploited in a following study from Leistner’s ON–OFF voltage-induced magnetic transition was observed in group.23,24 Here, the authors studied more sophisticated the nanoislands for relatively thin structures23 (see Fig. 6). architectures consisting of [Fe3 O4 /Fe/(001) L10 FePt] trilay- In a different study, Topolovec et al.25 followed the ers and FeOx /Fe nanoislands in 1M KOH, wherein the topmost changes in the magnetic moment of γ-Fe2 O3 (maghemite) FIG. 6. Magnetization curves as measured by AHE for FeOx /Fe nanoislands with layer thicknesses of (a) 11 nm, (b) 7 nm, and (c) 4 nm in the as-deposited state and in 1 mol L−1 KOH at Eocp (open circuit potential, with no exter- nal applied voltage), Eox (oxidizing potential of −0.2 VSCE , potential with respect to a saturated calomel reference elec- trode), and Ered (reduction potential of −1.26 VSCE ). The insets depict the relative E-induced changes in MS . Panel (d) indicates that by applying suitable voltage values to the system, FeOx can be essentially reduced to Fe, thus result- ing in drastic changes in MS (maximized for the smaller layer thickness, i.e., inset of panel c). Reprinted with permis- sion from Duschek et al., J. Mater. Chem. C 6, 8411 (2018). Copyright 2018 Royal Society of Chemistry. APL Mater. 7, 030701 (2019); doi: 10.1063/1.5080284 7, 030701-10 © Author(s) 2019 APL Materials RESEARCH UPDATE scitation.org/journal/apm nanoparticles (4 nm in size) previously mixed with commer- processes. Namely, the oxidation of the Cu-Ni film mainly cial Pt nanoparticles and pressed into a pellet. A 4% change occurred on the Cu counterpart of the solid solution, resulting in the magnetic moment was observed in a 1M KOH medium in a Ni-enriched alloy, thus with a higher MS . After apply- during combined CV-SQUID magnetometry studies under an ing small negative voltages (e.g., −2 V), Cu was reincorporated applied field of 5 kOe at 300 K. Such a change was not due back into the Cu-Ni solid solution (i.e., not forming any sec- to any chemical interaction at the γ-Fe2 O3 /Pt interface level ondary phase), resulting in a reduction in MS back to its initial but was solely ascribed to the γ-Fe2 O3 nanoparticles. In par- value, hence allowing for a full reversibility of the process. ticular, the authors observed an increase in M upon negative Finally, it is worth mentioning that in some cases, redox charging and a decrease in it with positive charging, within the reactions also occur even when no aqueous electrolytes are potential window between −650 mV and +300 mV vs. the Au used. For example, Reichel et al.73 reported drastic changes wire quasi-reference electrode. The changes were reversible in the magnetic properties of ultra-thin CoPt films when and ascribed to the formation of Fe3 O4 upon negative bias- immersed and electrolyte-gated using LiClO4 in dimethyl ing, which would make M to increase. Aside from the chemi- carbonate-ethylene carbonate. cally induced variation of M, electronic contributions, namely, charging-induced of the magnetic anisotropy and magnetoe- C. Magneto-ionics and ion intercalation lastic coupling, could not be ruled out. A further increase in Magneto-ionics is a very generic term that has been tra- M (up to 10.4%) had been observed in the same material, also ditionally utilized to describe the propagation of electromag- by Würschum’s team, when bringing the system to the per- netic waves by an ionized medium in the presence of an colation limit,71 although a detailed correlation between the external magnetic field (e.g., the Earth magnetic field).74 In ongoing electrochemical processes and magnetic behavior is recent years, this term has been applied to refer to voltage- provided in Ref. 25. driven ionic motion (i.e., ion diffusion) in magnetic materi- Besides Fe-based materials, the influence of electro- als. Most frequently, magneto-ionic effects have been studied chemically driven changes in the magnetic response of Co in oxide-containing systems. In this case, the material itself and Pd-Ni alloys has also been investigated. For exam- includes the source of structural oxygen. Namely, it consists ple, Allongue’s team observed a variation of the magnetic of either an oxide layer or a heterostructure that comprises anisotropy (KS ) in a Co ultra-thin film produced by electrode- oxide counterparts which act as donors or acceptors of oxy- position on Au (111)/Si(111) substrates. The resulting epitaxial gen. This is different from Sec. III B, where the described Co(0001)/Au(111) specimens were treated in 0.01M KOH to redox reactions involved oxygen coming from the surround- reversibly oxidize the Co surface, and the changes in KS were ings (e.g., oxidative liquid media) and affected mainly the sur- followed by MOKE and further confirmed by density func- face of the magnetic material (i.e., oxygen ions did not neces- tional theory calculations. An increase in KS value by 0.36 erg sarily diffuse deep toward the interior of the studied system). cm−2 was attributed to the formation of a Co-OH layer from Furthermore, in addition to oxygen, there are ion migration the initially reduced 3.1 ML Co-H material (i.e., H was proven processes in which other species (e.g., Li+ ions) are used to to be adsorbed onto the Co surface). Electrochemically speak- tune the magnetic properties. This phenomenon is termed ing, this structural change accounted for an electrochemical “ion intercalation” in this section. charge passed through the system of 0.4 mC cm−2 .20 Bauer et al.28 were the first to use the term “magneto- With the same philosophy, Ghosh21 ascribed the ionic control” to describe voltage-induced oxygen trans- reversible changes in M for a nanoporous Pd67 Ni33 alloy to port in magnetic films. Namely, they demonstrated that, in hydrogenation phenomena. In this case, the electrode was Co/GdOx bilayers, oxygen migration (and, thus, interfacial fabricated by consolidating Pd67 Ni33 nanoparticles of ca. 6 nm oxygen chemistry) could be controlled with electric voltage, in diameter into a disk shape and monitorized when immersed allowing for a modulation of magnetic properties (in this in 1M KOH by using a physical property measurement sys- case, perpendicular magnetic anisotropy) to an extent never tem (PPMS) or VSM magnetometry and CV. Observed changes achieved before by any other magnetoelectric means.28 In this in the mass magnetization up to 24.6% with negative biasing case, voltage actuation was carried out in a solid state config- were attributed to hydrogen absorption (i.e., incorporation of uration and GdOx was the ionic conductor (solid electrolyte) atomic hydrogen in the Pd-Ni lattice). Meanwhile, almost no used to provide the oxygen ions. Similarly, Bi et al.27 showed changes were observed upon positive biasing due to the for- that the easy axis of a thin Co film could be tuned with volt- mation of an antiferromagnetic NiO passivation layer at the age when Co was grown in contact with GdOx . These studies surface of the porous Pd67 Ni33 disk. stem from previous results from Schellekens et al.,75 Bauer Interesting magnetoelectric effects have been reported et al.,30,76 Chiba et al.,77 and Bernand-Mantel et al.78 who had recently by Quintana et al.72 in single-phase (face-centered demonstrated that domain wall motion in magnetic films and cubic) Cu20 Ni80 (at. %) solid solution alloy films (with nanowires could be tuned with electric field using AlOx or sub-10 nm pore size) immersed in a 1M NaOH aqueous solu- GdOx gating. Magnetic domains could be effectively trapped tion. Contrary to previous expectations, this system shows (pinned) in certain regions of the nanowires taking advantage an increase (rather than a decrease) in the MS (up to of this effect.76 Gilbert et al.29 performed depth-profile struc- 33% enhancement with respect to the as-prepared sample) tural and magnetic analyses, using polarized neutron reflec- after oxidation (i.e., applying positive voltages). The effect tometry, to understand how the depth-dependent oxygen was ascribed to the selectivity of the electrochemical redox diffusion affects the magnetic properties of a relatively thick APL Mater. 7, 030701 (2019); doi: 10.1063/1.5080284 7, 030701-11 © Author(s) 2019 APL Materials RESEARCH UPDATE scitation.org/journal/apm (15 nm) Co layer deposited onto AlOx /GdOx . In this case, the and oxygen-deficient, insulating, non-ferromagnetic regions authors demonstrated that magnetization was tailored along would co-exist forming percolated networks. the first 10 nm and that the process was reversible up to a Next we focus on voltage-induced oxygen migration phe- 92%. Zhou et al.32 used HfO2 with an ionic liquid deposited on nomena in magnetic materials which are gated using non- top to tune the magnetic properties of a Co/Ni bilayer. Very aqueous electrolytes, including ionic liquids (see Table III), recently, Tan et al.79 have demonstrated that different ambi- without the assistance of GdOx , HfO2 , or analogous adja- ent conditions (e.g., presence of oxygen or hydrogen in the cent layers. Similar to the work by Bauer et al.,28 the atmosphere during magneto-ionic experiments in Co/GdOx room temperature perpendicular magnetic anisotropy of bilayers) can lead to very dissimilar magneto-ionic effects, SiO2 /Co films could be controllably modified by Zhao et al.85 opening the door for H+ pumping controlled spin orbitronic by means of electric-field-induced O transport, where an ini- devices. Voltage-driven oxygen transport across the interface tial ultra-thin cobalt oxide layer between Co and the ionic between HfO2 and Co/Ni resulted in partial oxidation of the liquid provided the oxygen ions. The superparamagnetic metals, leading to the formation of antiferromagnetic oxides blocking temperature in Cu/Ni ultrathin films (presumably (NiOx and CoOx ) and non-volatile changes in the magnetic exhibiting Ni/Cu intermixing or containing Ni clusters properties. An exchange bias effect (loop shift) was observed onto the Cu layer) could be tuned by 28 ◦ C with volt- at low temperatures, which depended on the thickness of the age, using N,N,N-trimethyl-N-propylammonium (TMPA+ ) and voltage-induced antiferromagnetic oxide layers. Electric-field bis(trifluoromethylsulfonyl)imide (TFSI− ) as ionic liquid.86 reversible tuning of TC and MS at low temperature (mea- Large magneto-ionic effects (leading to changes as high sured by Hall resistance) was also observed by Yan et al.31 as 20% for MS and beyond 100% for HC ) were reported in in Co/Ni/HfO2 structures with a frozen ionic liquid (which nanoporous Fe-Cu films prepared by sputtering and electro- still maintains the electric double layer even while being in chemical dealloying, when immersed in propylene carbonate the solid state). In this work, variations in the d orbitals of Co, (i.e., in an anhydrous electrolyte).87 In this case, the large driven by voltage, were also claimed to influence the result- observed magnetic changes were caused by controlled and ing magnetic properties, besides the ionic migration effects. reversible electric-field-driven nanoscale phase transforma- Control of positive exchange bias using voltage was demon- tions between face-centered cubic (fcc) and body-centered strated by Gilbert et al.80 in Gdx Fe1−x /NiCoO bilayer films, cubic (bcc) structures. These phase transitions were in turn taking advantage of the strong oxygen affinity of gadolin- due to selective redox reactions induced by the applied volt- ium. Reversible electric field control of exchange bias is very age. The oxygen in the sample presumably came in small appealing for energy-efficient spintronic devices. Magneto- amounts from the OH– ions in solution in the propylene car- ionic control of the coercivity of Co ultra-thin films deposited bonate although an oxide passivation layer was probably also onto SiO2 and gated using an inorganic polymer was demon- formed at the surface of the samples following the synthesis strated by Tan et al.81 A maximum variation up to 68% was since they were stored in air. achieved by this means, and the effect was ascribed to oxy- Very recently, Quintana et al.26 reported voltage-driven gen ions’ migration from the SiO2 substrate to the Co films. O and Co redistribution in 100 nm-thick Co3 O4 films through The utilization of an inorganic polymer (MaN2401) as an insu- electrolyte-gating using propylene carbonate, allowing for the lating layer for electric field control is of particular interest controlled generation and suppression of ferromagnetism. A to overcome the formation of pinholes in conventional oxides negative voltage reduced Co3 O4 to Co (ferromagnetism: ON), (which, if grown by atomic layer deposition, require of thermal whereas the process could be reversed by applying a positive heating and relatively long deposition times) and also to avoid bias, aimed at oxidizing Co back to Co3 O4 (paramagnetism: sophisticated encapsulation techniques that are required if OFF). These gate-induced O and Co migrations were driven ionic liquids are to be implemented into real spintronic by mixed vacancy clusters as evidenced by positron annihi- devices. lation spectroscopy. Ionic transport was promoted at grain The antiferromagnetic to FM transition in ultrathin FeRh boundaries and further assisted by the formation of diffusion films deposited on MgO (100) substrates and capped with HfO2 channels that incorporated large amounts of O. The process has also been successfully modified by electrical manipula- was self-sustained in the sense that no external source/sink tion using ionic liquid-gating through the reversible transport of oxygen is required (Fig. 7). of oxygen ions, supplied by the HfO2 layer, across the inter- Also remarkable are the recent results by Navarro-Senent face with FeRh.82 Also interesting is the control of the mag- et al.88 on nanoporous CoPt + CoOx microdisks, electrolyte- netic properties of LMSO films (40 nm thick), coated with Gd gated using propylene carbonate, in which magnetoelectric capping layers, through interfacial oxygen migration.83 Even effects between the metallic and the oxide counterparts have without applying voltage, oxygen is leached from the film and a been found to reduce the HC by 88% and the Kerr signal Gd2 O3 layer is formed, leading to changes in M and anisotropy amplitude by 60%. This material combined (i) enhanced elec- that affect the overall thickness of the film. Similarly, the ionic trostatic charge accumulation at the surface of the ultra-thin distributions in (La,Sr)CoO3 films (36 nm thick) could also nanopore walls comprised in the micro-disks together and (ii) be controlled in the solid-state by depositing a Gd capping magneto-ionic phenomena, where voltage-driven O2− migra- layer and subsequent application of voltage.84 The observed tion promoted a partial and controlled reduction from CoO trends in M and magnetoresistance suggested a significant to Co. The effects were found to be reversible but with a voltage-driven phase separation, in which metallic FM regions relatively slow dynamics. APL Mater. 7, 030701 (2019); doi: 10.1063/1.5080284 7, 030701-12 © Author(s) 2019 APL Materials RESEARCH UPDATE scitation.org/journal/apm Remarkably, in this context of ionic liquid-gating, Lu et al. showed that, on top of voltage-driven oxygen migration, hydrogen transport could also be activated by electric fields and in an independent manner from the O diffusion. This dual-ion migration allowed for reversible phase transforma- tions among three different material phases: the perovskite SrCoO3−δ (FM), the brownmillerite SrCoO2.5 (antiferromag- netic), and HSrCoO2.5 (weakly FM), hence for an electric- field control of three different magnetic ground states.90 Interesting results were also reported in electrolyte-gated SrTiO3 91 and La0.5 Sr0.5 CoO3−δ .92 In the first case, the authors could discriminate between pure charging, reversible, mag- netoelectric effects (for low applied voltage), and persistent electrochemical processes for voltages larger than 3.75 V.91 Walter et al.92 showed that mixed electrostatic and redox mechanisms occur in La0.5 Sr0.5 CoO3−δ depending on the voltage polarity. Magnetoelectric effects were also reported in cobaltite/manganite (SrCoO3−x /La0.45 Sr0.55 MnO3−y ) het- erostructures, where a reversible manipulation of oxygen vacancies using DEME-TFSI ionic liquid was shown to result in magnetic phase transitions and drastic changes in HC , exchange bias field, and TC .93 Walter et al.94 also showed that ionic gels can be effectively used to tailor the mag- netic properties of La0.5 Sr0.5 CoO3−δ using voltage values of the order of a few V, which are sufficient to induce changes in the oxygen vacancy density, as evidenced using synchrotron hard x-ray diffraction and polarized neutron reflectometry. Dhoot et al.95 reported a transition from a ferromagnetic metal to an insulating ground state with positive voltage in La0.8 Ca0.2 MnO3 films gated using EMIM:TFSI. This resulted in changes in the colossal magnetoresistance values, which were found to be reversible upon changing the polarity of the applied voltage. Similar to all the aforementioned examples, a pronounced electric-field effect was demonstrated in phase- separated Pr0.65 (Ca0.75 Sr0.25 )0.35 MnO3 thin films, particularly at low temperatures, using DEME-TFSI as electrolyte (elec- tric double layer transistor). The effect of the electric field in this material couples with that of the applied magnetic field, leading to pronounced changes in electroresistance and mag- FIG. 7. (a) Schematic illustration of the wire connection to apply voltage on a netoresistance at around 80 K. The effects were ascribed to Co3 O4 film, grown by atomic layer deposition, in which Cu wires were attached an increase in the ferromagnetic fraction in this oxide man- to the sample using indium. The welding was performed at the Co3 O4 sur- ganite film.96 Analogously, changes in the magnetic proper- face. Insulating hot glue spacers were used to avoid short-circuits during VSM ties of Pr1−x (Ca1−y Sry )x MnO3 were also reported by Hatano measurements.18 (b) Hysteresis loop recording for this system under applica- tion of voltage using a propylene carbonate electrolyte. Although the sample et al., again using the electric double layer transistor with an is initially not magnetic [black curve in (b)], a clear ferromagnetic hysteresis ionic liquid. The electric field was found to drastically change loop emerges when the film is subjected to the action of a negative electric the temperature-dependence of the electric resistivity and, field (OFF → ON magnetic transition). Reprinted with permission from Quintana concomitantly, had an effect on the metal (FM)-to-insulator et al., ACS Nano 12, 10291 (2018). Copyright 2018 American Chemical transition.97 Society. Based on previous knowledge from Li-ion batteries and supercapacitors, non-oxidative lithium based electrolytes have also been employed for magnetoelectric purposes. For In ionically gated SrRuO3 thin films, the onset of ferro- instance, Leistner et al.98 used a non-aqueous LiPF6 -based magnetism could be reversibly modulated by voltage-induced electrolyte and observed that the magnetic moment of FePt oxygen migration in SrRuO3 (i.e., by the creation and annihi- films could be tuned by reversible O ion transport com- lation of oxygen vacancies in the SrRuO3 lattice). The tran- ing from the native iron oxide surface layer. Based on this sition temperature could be decreased up to around 30 K work, Leistner et al.99 further investigated voltage-driven O for small positive voltages and, then, recovered when apply- transport in electrolyte-gated FePt/iron oxide composites ing the same voltage value but with opposite polarity.89 where a reversible change in MS and magnetic anisotropy was APL Mater. 7, 030701 (2019); doi: 10.1063/1.5080284 7, 030701-13 © Author(s) 2019 APL Materials RESEARCH UPDATE scitation.org/journal/apm achieved. In these studies, though, Li+ ions were not interca- heterostructures comprise variations of the charge accumu- lated in the structure of the investigated materials. lation (and, in turn, the FE polarization) that lead to changes Ions other than oxygen can also be controllably inserted in strain or chemical bonding at the FE/FM interface. An or extracted by means of electric fields to tailor magnetic effective way to polarize FE polymer films is to use elec- properties of materials. In fact, increasing the number of trolytes (including aqueous ones) since the formation of the transferrable ion species is of great benefit to enrich mate- EDL aids to polarize the FE counterpart in a controlled rial functionalities. Due to its crucial role in batteries, Li manner.105 has been commonly chosen as the mobile specie to migrate Herklotz and co-workers demonstrated the reversible in some magnetoelectric studies. Typically, the mobile Li+ control of interfacial magnetism in a LSMO and PZT multifer- ions are provided by non-aqueous organic liquids contain- roic heterostructure through ionic-liquid assisted FE switch- ing LiPF6 . For example, Dasgupta et al.100 showed that the ing.106 They pointed out that the use of ionic-liquid for magnetic properties of maghemite (γ-Fe2 O3 ) can be con- switching the FE polarization provides several advantages, trollably tuned in a reversible manner by voltage-induced Li which include (i) its feasibility to thinner FE films since the insertion or extraction. More recently, Zhang et al.34 demon- leakage currents are smaller through the ionic liquid and (ii) strated electric-field-driven lithiation (magnetic moment the ionic liquid allows a reversible in situ switching of the enhancement)/delithiation (magnetic moment suppression) FE polarization in large areas of the FE while avoiding the of nanoscale α-Fe2 O3 -based batteries, enabling strong fer- risk of pinholes compared to the common metallic top elec- romagnetism as an extra degree of freedom in the field of trodes. The authors concluded that the origin of the enhance- energy storage. Similarly, voltage-driven Li+ ion intercalation ment/reduction in the interfacial magnetization in this sys- has been recently shown to be effective to tune the M val- tem relied on the hole accumulation/depletion produced by ues of nanoporous Co0.5 Ni0.5 Fe2 O4 and CoFe2 O4 by roughly the FE polarization. 5% and 2.5%, respectively, in a rather reversible manner, using Similarly, Nishino et al.107 reported the use of an ionic liq- 20 wt. % LiTFSI dissolved in EMIM-TFSI as an electrolyte in uid for the switching of the PZT polarization aiming at a FE a potential window 1.5–3.3 V.101 The authors obtained larger control of the transport properties in SrRuO3 and Nb-doped variations in M by extending the potential window to 1.1–3.4 V, SrRuO3 . The ionic liquid was used in order to promote the for- but then the effects were highly irreversible. mation of an EDL at the interface between the FE (PZT) and Besides the aforementioned oxides, Li+ intercalation the ionic liquid, which was very effective for inducing polar- was also studied in α-LiFe5 O8 mesoporous thin films using ization reversal in the FE. Interestingly, although the study was LiTFSI in EMIM-TFSI as electrolyte. This spinel ferrite could focused on the modulation of the carrier density in SrRuO3 , a reversibly store Li+ in a potential ranging from 3.2 to 1.4 V. change in TC was observed, from 115 K to 90 K. The electrochemical reactions proceeded in a topotactic way, As mentioned in the introduction, in multiferroic mate- which allowed control and tuning of M at room temperature rials, the ME effect can be induced in both directions (i.e., (up to 4%) in a reversible manner.102 Finally, oxygen deficient either voltage can tune the magnetic properties of the FM or LiFe5 O8−x was used as a target ferrimagnetic material in which a change in the electrical response of the FE can be induced magnetic properties were tailored by voltage, in the solid by means of a magnetic field applied to the FM). Based on state, at the nanoscale, taking advantage of the relatively low the latter idea, Chau et al.108 studied the magnetoelectric activation energy of this oxide for Li+ ion migration.103 Li+ de- effect (changes in charge density induced by applied mag- intercalation/intercalation resulted in changes in the resistive netic fields) in Terfenol-D (FM)/PZT (FE)/electrolyte-polymer switching properties of this oxide, as well as in the mag- matrix composite materials. They demonstrated that the pres- netic domain pattern (imaged by magnetic force microscopy) ence of the electrolytic-polymer matrix increases the mag- and M. netoelectric coefficient compared to heterostructures with conventional insulating matrices. Even though the aim of the D. Electrolyte-gated ferroelectric/ferromagnetic work was not the control of magnetism, the results sug- heterostructures gest that such a system could be a potential candidate for This section is devoted to electrolyte-gated heterostruc- electrolyte-gated magnetoelectric actuation. tured multiferroic materials, which mainly refer to FE/FM bilayers actuated using liquid electrolytes (e.g., ionic liquids). Conventionally, most of the investigated multiferroic het- IV. APPLICATIONS, CHALLENGES, AND FUTURE erostructures used for electric-field control of magnetism, in PROSPECTS fact, do not incorporate electrolytes on their systems. For this In recent years, exciting new results in the field reason, electrolyte-assisted control of magnetism in multifer- of electrolyte-gated magnetoelectric actuation have been roic structures is still considered to be rather underexplored. obtained in metallic, semiconducting, and dielectric oxide In general terms (not necessarily actuated using elec- thin films and nanoporous structures. At present, there is a trolytes), the most widely studied FE/FM systems encompass huge potential to use these materials in a variety of innova- La1−x Srx MnO3 (LSMO) or La1−x Cax MnO3 (LCMO) combined tive applications, such as energy-efficient spintronic devices with FE BaTiO3 (BTO) or Pb(Zr0.2 Ti0.8 )O3 (PZT), commonly (e.g., tunnel junctions in voltage-driven MRAMs) or multi- known as FM-FE perovskite oxide heterostructures.40,104 The state magnetic memories.40 These applications go beyond the underlying magnetoelectric mechanisms in these multiferroic conventional utilization of magnetoelectricity109 in magnetic APL Mater. 7, 030701 (2019); doi: 10.1063/1.5080284 7, 030701-14 © Author(s) 2019 APL Materials RESEARCH UPDATE scitation.org/journal/apm actuators, transducers, or, more recently, energy har- be achieved with a better control of the microstructure of the vesters.110 Furthermore, new avenues have been opened layers or by purposely introducing structural defects (i.e., ion recently on the utilization of magnetoelectric heterostruc- irradiation). tures in biomedicine, for example for wireless electric stim- In general, the mechanisms responsible for magneto- ulation of cells (e.g., osteoblasts or neurons)111–113 or to electric actuation are still rather poorly understood and are enhance the effectiveness of drug delivery.114,115 In these lat- often more complex than as initially anticipated. This hinders ter cases, magnetoelectric actuation is performed in liquid deeper control of the voltage-driven changes in the magnetic media although in most applications (e.g., physical ones), all- properties. As an example, “pure” surface charging effects turn solid configurations are preferred. Nonetheless, the use of liq- out to not to be always instantaneous and, what is even worse, uid electrolytes offers important advantages: (i) an enhanced they are not always fully reversible and reproducible after ion mobility (compared to solid electrolytes), which allows long-term cycling (see Table I), hence suggesting structural for ion intercalation (penetration of ions from the electrolyte changes at the surface of the magnetic materials of interest. toward the material of interest), (ii) generation of ultra- Further in-depth structural investigations, preferably using high electric fields (due to the very narrow thickness of the in situ surface characterization techniques, are, therefore, EDL) at the surface of the magnetic material, and (iii) pos- urgently needed. sibility to easily penetrate inside 3D nanoporous magnetic Progress beyond the state-of-the-art in the use of mag- alloys and oxides, hence allowing for a more effective elec- netoelectric materials for spintronics and magnetic memory tric surface charging and eventual redox or magneto-ionics devices would certainly benefit from the implementation of effects. all-solid configurations, either by using solid ionic conductors There are some challenges that need to be overcome in as dielectrics (such as Gd2 O3 or HfO2 ) or organic ferroelectrics order to fully exploit the technological applications of state- (e.g., PVDF). In the case of nanoporous magnetic alloys, the of-the-art magnetoelectric materials. First, it is important to latter might be more convenient since polymer precursors induce all these effects at room temperature. Therefore, fur- might penetrate more easily inside the small pores when ther magnetoelectric studies on new types of diluted magnetic compared to oxide dielectrics although recent progress has semiconductors with TC above room temperature need to be been also made on coating the inner surfaces of nanoporous performed in order to take advantage of this class of materials. magnetic alloys with dielectric oxides using atomic layer Also important is to be able to induce magneto-ionic effects deposition.116 in solid-state devices without thermally assisted ionic diffu- sion. That is, ion migration should occur at room temperature, without heating. ACKNOWLEDGMENTS There is also a need to decrease the required volt- This work was funded by the European Research Coun- age values to a level below the breakdown threshold of the cil under the SPIN-PORICS 2014-Consolidator Grant (Agree- dielectric layer. The insulating oxide layers are typically made ment No. 648454), the Generalitat de Catalunya (2017-SGR-292 relatively thick and defect-free to avoid leakage currents. project), the Spanish Government (No. MAT2017-86357-C3-1- However, the increase in the barrier thickness causes a reduc- R and associated FEDER project), and the European Union’s tion in the effective electric field. To overcome this problem, Horizon 2020 research and innovation programme under the electrolyte-gating using ionic liquids turns out to be a very Marie Skłodowska-Curie Grant Agreement No. 665919. E.P. promising strategy to achieve very large electric fields for rel- acknowledges the Ramon y Cajal (No. RYC-2012-10839) fellow- atively low applied voltages (due to the ultra-narrow thickness ship from MINECO. of the EDL). The role of structural defects at the interface with the electrolyte needs to be further assessed in order REFERENCES to optimize magnetoelectric actuation. Also important is to 1 minimize mechanical fatigue effects under repetitive electric D. Giles, Introduction to Magnetism and Magnetic Materials (CRC Press, Taylor & Francis Group, Boca Raton, Florida, USA, 2016). cycling, which can be substantial in FM/FE bilayers or voltage- 2 B. Dieny, R. C. Sousa, J. Hérault, C. Papusoi, G. Prenat, U. Ebels, D. actuated nanoporous oxides immersed in liquid electrolytes Houssameddine, B. Rodmacq, S. Auffret, and L. D. Buda-Prejbeanu, Int. J. (i.e., electrostriction). Nanotechnol. 7, 591 (2010). Depending on the end application, the type of magneto- 3 P. Curie, J. Phys. 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