Advances in magneto-ionic materials and perspectives for their application Cite as: APL Mater. 9, 030903 (2021); https://doi.org/10.1063/5.0042544 Submitted: 31 December 2020 . Accepted: 05 February 2021 . Published Online: 05 March 2021 M. Nichterwitz, S. Honnali, M. Kutuzau, S. Guo, J. Zehner, K. Nielsch, and K. Leistner COLLECTIONS Paper published as part of the special topic on Magnetoelectric Materials, Phenomena, and Devices This paper was selected as an Editor’s Pick ARTICLES YOU MAY BE INTERESTED IN Emerging opportunities for voltage-driven magneto-ionic control in ferroic heterostructures APL Materials 9, 040904 (2021); https://doi.org/10.1063/5.0045820 Machine-learning free-energy functionals using density profiles from simulations APL Materials 9, 031109 (2021); https://doi.org/10.1063/5.0042558 Strain-gradient effects in nanoscale-engineered magnetoelectric materials APL Materials 9, 020903 (2021); https://doi.org/10.1063/5.0037421 APL Mater. 9, 030903 (2021); https://doi.org/10.1063/5.0042544 9, 030903 © 2021 Author(s). APL Materials PERSPECTIVE scitation.org/journal/apm Advances in magneto-ionic materials and perspectives for their application Cite as: APL Mater. 9, 030903 (2021); doi: 10.1063/5.0042544 Submitted: 31 December 2020 • Accepted: 5 February 2021 • Published Online: 5 March 2021 M. Nichterwitz,1,2 S. Honnali,1 M. Kutuzau,1 S. Guo,1 J. Zehner,1,3 K. Nielsch,1,3 and K. Leistner1,a) AFFILIATIONS 1 Leibniz IFW Dresden, Helmholtzstrasse 20, Dresden 01069, Germany 2 Physical Chemistry, Technische Universität Dresden, Dresden 01062, Germany 3 Institute of Material Science, TU Dresden, Dresden 01062, Germany Note: This paper is part of the Special Topic on Magnetoelectric Materials, Phenomena, and Devices. a) Author to whom correspondence should be addressed: k.leistner@ifw-dresden.de ABSTRACT The possibility of tuning magnetic material properties by ionic means is exciting both for basic science and, especially in view of the excellent energy efficiency and room temperature operation, for potential applications. In this perspective, we shortly introduce the functionality of magneto-ionic materials and focus on important recent advances in this field. We present a comparative overview of state-of-the-art magneto- ionic materials considering the achieved magnetoelectric voltage coefficients for magnetization and coercivity and the demonstrated time scales for magneto-ionic switching. Furthermore, the application perspectives of magneto-ionic materials in data storage and computing, magnetic actuation, and sensing are evaluated. Finally, we propose potential research directions to push this field forward and tackle the challenges related to future applications. © 2021 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/10.1063/5.0042544., s I. MAGNETO-IONIC MATERIALS AS A NOVEL Magneto-ionic control of magnetic materials is a novel APPROACH TO ENERGY-EFFICIENT MAGNETIC approach in the field of magnetoelectricity. The magnetic material SYSTEMS in this case is in contact with a solid or liquid electrolyte. By the application of an external voltage, ionic motion and electrochemical Magnetic materials are important in a plethora of indus- reactions are triggered which can reversibly affect the performance trial applications, ranging from large-scale hard magnets in wind of the magnetic material (Fig. 1). As the field is rapidly evolving, turbines and micromagnets in microelectromechanical systems very diverse material/electrolyte systems are studied, and several (MEMS) to magnetic nanostructures for data storage and process- denominations and categorizations, such as redox-based, electro- ing devices. Specific magnetic properties required for the individ- chemical, ion-exchange, or magneto-ionic control of magnetism, ual applications are usually irreversibly set through the design of are in use.4–6 In this perspective, we consider voltage-tunable mag- phase, composition, microstructure, and shape during the fabrica- netic materials systems in which the underlying mechanism is based tion process. During the operation of magnetic devices, the direc- on ionic motion and electrochemical reactions as magneto-ionic tion of magnetization is conventionally controlled by the applica- materials. The main advantage of magneto-ionic materials, in con- tion of external magnetic fields, often realized by electromagnets or, trast to most other magnetoelectric materials, is that they can be in nanoscale devices, by large spin polarized electric currents. For operated at room temperature and at low voltage. Since chemical both cases, Joule heating and associated energy dissipation present changes are involved, the non-volatile setting of a magnetic state by a severe challenge. This problem continuously triggers numerous voltage is possible, requiring only a small current. This is in con- research activities on magnetoelectric (ME) materials1–3 in which trast to the volatile magnetoelectric effects achieved by capacitive the magnetic material is controlled by the electric voltage instead of charging via the electrochemical double layer and associated surface electric current, thereby reducing energy consumption. electronic structure changes in similar gating architectures.7,8 Thus, APL Mater. 9, 030903 (2021); doi: 10.1063/5.0042544 9, 030903-1 © Author(s) 2021 APL Materials PERSPECTIVE scitation.org/journal/apm FIG. 1. Magneto-ionic systems consist of a magnetic material in contact with a solid or liquid electrolyte. The magnetic properties, such as saturation magne- tization and coercivity, are controlled by voltage-triggered ionic motion and electrochemical reactions. This is promising for a drastic energy saving in comparison to conventional electric current-controlled magnetic devices in many possible applications. magneto-ionic materials offer a route toward a reversible manipu- A. Magneto-ionic control of saturation magnetization lation of magnetic properties and are highly promising for ultralow The voltage-control of the saturation magnetization (M S ) by power magnetic devices. magneto-ionic mechanisms has been demonstrated in a large vari- The first reports on electrochemical voltage control of mag- ety of systems. To give an overview on the attainable M S varia- netism in ultrathin metal films dealt with oxygen-based interfacial tions per defined voltage (V) change, the magnetoelectric (ME)- reduction/oxidation and associated changes in magnetic anisotropy voltage coefficients, ΔM S /|ΔV|,4 are plotted in Fig. 3 for selected and coercivity.9–13 Since then, numerous magneto-ionic material magneto-ionic systems and as a function of the time between the two systems consisting of magnetic metals and/or oxides in combina- tion with a solid or liquid electrolyte have been investigated; an overview of these advances until 2018/19 can be found in recent reviews on magnetoelectric materials.4–6,14 Within the past two years, significant progress has been achieved, such as magneto- ionic materials that function on the base of proton15–17 or nitro- gen ion18 transport, the extension toward 3D magneto-ionic sys- tems,19,20 and novel magneto-ionic functionalities such as tunable exchange bias.21,22 In this expanding field, most reports still deal with the exploration of fundamental phenomena. With the key advan- tages of energy efficiency and room temperature operation, how- ever, targeted research toward specific applications is expected in the future. In this perspective, we give an overview on important lat- est advances in magneto-ionic research, discuss the challenges and opportunities of magneto-ionic materials, and propose future research strategies. II. RECENT ADVANCES IN MAGNETO-IONIC RESEARCH In the field of magneto-ionic materials, significant advances have been made within the past two years, especially regarding room FIG. 2. Examples for voltage control of magnetic properties by ion migration and temperature operation, switching speed, reversibility, and the exten- electrochemical reactions for various material systems. (a) Out-of-plane hysteresis sion of magneto-ionic tunability to diverse magnetic phenomena loops corresponding to the virgin state and the first switching cycle for the proton- based magneto-ionic control in the Pt/Co(0.9 nm)/GdOx layer system. Reprinted and functional materials. To illustrate the broad range of magneto- with permission from Tan et al., Nat. Mater. 18, 35 (2019). Copyright 2019 Springer ionic materials, Fig. 2 gives recent examples for voltage-tunable mag- Nature. (b) Out-of-plane hysteresis loops of an L10 CoPt film (2.8 nm) polarized in netization curves in ultrathin single metal films15 [Fig. 2(a)], metal LiClO4 in DMC/EC at different voltages (adapted from Reichel et al.13 ). (c) Out-of- alloy films13 [Fig. 2(b)], oxide superlattices17 [Fig. 2(c)], and three- plane hysteresis loops of a [(La0.2 Sr0.8 MnO3 )1 (SrIrO3 )1 ]20 superlattice under ionic dimensional oxyhydroxide nanoplatelet structures19 [Fig. 2(d)]. liquid gating. Figure reproduced with permission from Yi et al., Nat. Commun. 11, In the following, we will focus on important advances from the 902 (2020). Copyright 2020 Author(s), licensed under a Creative Commons Attri- past two years in light of the magnetic property, which is controlled bution 4.0 License. (d) Voltage-controlled ON-switching of magnetization by the electrochemical reduction of 3D FeOOH nanoplatelets in a 1M LiOH electrolyte by magneto-ionic means and with respect to application-relevant (adapted from Nichterwitz et al.19 ). aspects, such as the switching speed and reversibility. APL Mater. 9, 030903 (2021); doi: 10.1063/5.0042544 9, 030903-2 © Author(s) 2021 APL Materials PERSPECTIVE scitation.org/journal/apm FIG. 3. Comparison of the ME-voltage coefficients for (a) an absolute and (b) relative variation in M S for selected magneto-ionic systems as a function of the time elapsed between the setting of the two different M S states considered in ΔM S . The ME-voltage coefficients are given for the largest reversible M S change. All data refer to room temperature measurements, except if otherwise stated. In (a), for reasons of comparability, only studies from which ΔM S in emu cm−3 could be directly extracted are depicted. In (b), the maximum value of M S is set as the reference value to ensure comparability: ΔM S,rel = [M S1 (V 1 ) − M S2 (V 2 )]/M S1 (V 1 ) with M S1 (V 1 ) > M S2 (V 2 ). All data refer to room temperature measurements, if not stated differently [np—nanoporous]. voltage-induced M S states. The data for very recent studies (from to modulate M S to a large extent. For instance, a 70% decrease in M S 2019 to 2020) are shown in red to highlight the current activities in is reversibly achieved by Li intercalation in ZnFe2 O4 for an applied the field. It should be noted that the time does not always represent voltage of just 1.25 V.23 For most systems in Fig. 3(a), the time scales the switching speed inherent to the material system, but sometimes for the magneto-ionic measurements range from minutes to hours, also the time required for the magnetic measurement (which might which is very long with regard to applications. However, it must be be slower than the actual possible switching speed). Thus, the pre- acknowledged that most studies focused on the fundamental under- sentation in Fig. 3 gives the currently demonstrated time scales for standing of the mechanisms, targeted large tunable volumes, and uti- magneto-ionic M S -control. lized time-intensive measurements such as in situ superconducting Absolute values for the M S variation per volt [Fig. 3(a)] are quantum interference device (SQUID) magnetometers.26,27 mainly reported for magneto-ionic systems based on ferrites and Recently, Ning et al.30 reported non-volatile magneto-ionic other transition metal oxides in thick film (≥100 nm) or in bulk control of M S in epitaxial SrCo0.5 Fe0.5 O3−δ (SCFO) thin films form.23–30 Several systems, with both the liquid or solid electrolyte, via ionic liquid gating. Reversible and continuous control of are directly derived from battery research here and measured with in the magnetization up to 100 emu cm−3 was demonstrated at room situ magnetometry.23,25,26 In some cases, the magnetically active elec- temperature, which was ascribed to voltage-induced changes in trodes are prepared by mixing transition metal oxide nanoparticles the oxygen stoichiometry. At negative voltage (−2 V), oxygen is with binders and conductive nanoparticles.23,24,26,27 This procedure extracted from the perovskite SCFO, which leads to the decrease optimizes the electrochemical operation, but at the same time, the in the magnetic moment (80% decrease in M S after 2 min). After magnetic moment per volume is diluted. Figure 3(a) shows that, prolonged gating, the nonmagnetic brownmillerite phase forms and so far, the largest ME-voltage coefficients for M S are in the range thus ferromagnetic ordering is suppressed (M S ∼ 0 after 3 min). of several tens of emu cm−3 /volt. This seems small in comparison The ferromagnetic state can be recovered by applying a posi- to conventional magnetic materials exhibiting high M S (e.g., CoFe tive voltage (+2 V) at which oxygen is reinserted. In this way, alloy with M S up to 1950 emu cm−3 ); however, in the studied sys- reversible ON/OFF control of M S is obtained by alternating the tems with small overall M S , the magneto-ionic effects are sufficient polarity of the gating voltage. The switching speed of few minutes is APL Mater. 9, 030903 (2021); doi: 10.1063/5.0042544 9, 030903-3 © Author(s) 2021 APL Materials PERSPECTIVE scitation.org/journal/apm remarkable, especially in view of the relatively large film thickness of gated via a GdOx layer.34 Here, voltage pulses of ±10 V were applied up to 150 nm. and continuous changes in magnetization were recorded down to De Rojas et al.18 proposed the use of nitrogen ions instead of a time scale of milliseconds. Di et al.12 also reported variations of oxygen ions to improve the magneto-ionic switching speed. They M S (up to 60% M S reduction) of an ultrathin Co layer. The effects demonstrate a reversible ΔM S of 1 emu cm−3 upon the electrolytic were attributed to voltage-triggered reversible Co surface oxidation gating of CoN (85 nm) thin films in propylene carbonate (PC) with in the liquid alkaline electrolyte. The effects occur within a very small solvated Na+ species. To reach a ΔM S of 1 emu cm−3 , in this system, voltage range (ΔV of 0.4 V) and are demonstrated in a time scale of voltages of ±4 V and switching times around 4 min are required. about 40 s. A higher voltage of ±8 V and a longer time of 6 min are required For nanoporous Pd(Co) polarized in 1M KOH solution, to achieve the same effect in Co3 O4 films of the same thickness. This Gößler et al.16 demonstrated a fully reversible voltage-induced ON difference between the magneto-ionic behavior of CoN and Co3 O4 is and OFF switching of magnetization for a voltage change of about discussed with regard to the lower activation energy for ion diffusion 1 V. This strong magneto-ionic effect is discussed with regard to and the lower electronegativity of nitrogen compared to oxygen. In a electrochemical hydrogen sorption, which affects the magnetic cou- further study, de Rojas et al.28 revealed the importance of the gating pling of the Co clusters via the Ruderman–Kittel–Kasuya–Yoshida- geometry for the magneto-ionic efficiency for Co3 O4 . The insertion type interaction in the Pd matrix. In consequence, magneto-ionic of a conductive buffer layer underneath the semiconducting and switching between the ferromagnetic and the superparamagnetic magneto-ionically active Co3 O4 film (130 nm) to realize a bottom state is achieved. gate electrode yields faster switching and an increase in the ME- There have been remarkable advances also with regard to voltage coefficients from around 0.01 emu cm−3 V−1 to 0.11 emu the voltage-control of magnetization in perovskite transition metal cm−3 V−1 .28 For the magneto-ionic control of Co3 O4 28 and CoN,18 oxides by ionic mechanisms.17,36,37 In superlattices comprised of in comparison to Li intercalation in spinel ferrite systems,23 the ME- alternating one unit cell of SrIrO3 and La0.2 Sr0.8 MnO3 , protons and voltage coefficients are significantly smaller, but faster switching is oxygen ions can be transferred using ionic liquid gating, which trig- demonstrated. gers reversible phase transitions.17 As a result, voltage-controlled The relatively scarce data for absolute ME-voltage coefficients ON/OFF switching of M S [Fig. 2(c)] is demonstrated. Vasala et al.36 (with ΔM S in emu cm−3 ) in Fig. 3(a) are connected to measurement studied the electrochemical (de)intercalation of fluoride ions into limitations for magneto-ionic materials, as they are often utilized in a two-layer La1.3 Sr1.7 Mn2 O7 system to tune the magnetization by the ultrathin film or nanoporous form. One challenge is the difficulty exploiting the high sensitivity of the magnetic states to the Mn oxi- to accurately determine the volume of these materials. A second dation state and the distance between the perovskite building blocks. challenge is that the very small magnetic moments, as in ultrathin They achieve a 67% change in M S for low applied voltages (<1 V), films, are usually not measurable with conventional magnetome- which is reflected in a high ME-voltage coefficient [Fig. 3(b)]. So far, try, especially if in situ characterization requires further addition of the large magneto-ionic effects are restricted to a low temperature the electrolyte and cell components. Therefore, for ultrathin films, (e.g., 10 K) here. surface-sensitive magnetic techniques based on the anomalous Hall effect (AHE) or the magneto-optical Kerr effect (MOKE) are uti- lized to probe relative changes in magnetization during magneto- B. Magneto-ionic control of magnetic anisotropy and coercivity ionic switching.15,31,32 To give a more comprehensive overview, ME- voltage coefficients for the relative variation in M S over time are The control over magnetic anisotropy and coercivity (H C ) is plotted in Fig. 3(b). Here, the great variety and extended activities a key requirement for the design of magnetic materials in most regarding magneto-ionic material systems become obvious. Effec- types of applications. Figure 4 gives an overview on ME-voltage tive magneto-ionic control of M S can also be achieved in systems coefficients for H C achieved by magneto-ionic mechanisms, again containing metallic ferromagnets, such as Fe,33 Co,34 FePt,10 CoPt,13 as a function of the time passed between the settings of the two and Pd(Co).16 states. Most studies deal with ultrathin Co films exhibiting per- In the FeOx /Fe system, the magneto-ionic switching relies pendicular magnetic anisotropy (PMA).9,11,12,15,38 In such films, the on the reversible electrochemical transformation (surface oxida- anisotropy and spin reorientation can be controlled by magneto- tion/reduction) between weakly magnetic iron oxide and ferromag- ionic mechanisms, resulting in an ON/OFF-switchable coercivity netic iron metal in alkaline aqueous solution.35 In this oxygen-ion and remanence [Fig. 2(a)]. Recently, large magneto-ionic effects on based system, Duschek et al. achieved relative variations of M S up anisotropy and H C have also been reported for Fe films with in-plane to 64% in continuous FeOx /Fe films (2 nm)32 and almost ON/OFF uniaxial anisotropy39 and for typical hard magnetic materials such switching of M S in FeOx /Fe nanoisland geometry33 for a voltage as L10 CoPt13 and SmCo5 .40 This is a promising sign that magneto- change of just 1 V. The increased magneto-ionic effect in FeOx /Fe ionic control can be transferred to a wide range of usable magnetic nanoislands, as compared to the thin film geometry, is attributed to materials in the future. the larger surface/volume ratio, which promotes the interfacial oxi- For the magneto-ionic control of anisotropy switching in ultra- dation/reduction reactions. Recently, a continuous voltage-triggered thin Co layers voltage-gated via GdOx , advances in switching speed ON switching of magnetism based on the reduction of paramagnetic and reversibility are notable. Tan et al.15 demonstrated the bene- β-FeOOH nanoplatelets to ferromagnetic Fe [Fig. 2(d)] has been fits of protons over oxygen ions as the functional ions and achieved achieved in the same electrolyte.19 fast (100 ms) and highly reversible (2000 cycles) anisotropy switch- The insertion and removal of oxygen is also proposed as the ing at room temperature. In previously utilized GdOx /Co structures, origin for the voltage-induced change in M S in Co films (0.8 nm) which relied on oxygen ion migration, significant magneto-ionic APL Mater. 9, 030903 (2021); doi: 10.1063/5.0042544 9, 030903-4 © Author(s) 2021 APL Materials PERSPECTIVE scitation.org/journal/apm compared different proton conducting gate oxides and their effect on the switching speed for the magneto-ionic control of Co lay- ers. The remanence ratio approaches zero within a time scale of >80 s for gadolinium doped ceria (GDC) and barium cerium yttrium zirconate (BZCY), while it takes less than 1 s for yttria-stabilized zirconia (YSZ). The Pt/Co/YSZ/Au structure is further optimized toward a Pd/Co/Pd/YSZ/Pt architecture in which Pt is used as top electrode to catalyze the electrochemical reaction and Pd serves as a hydrogen loading layer as well as a protection layer between the YSZ and the Co. Here, the fastest switching speed of <2 ms is achieved for a thickness of 10 nm for YSZ and with a gate voltage of +6 V. Huang et al.42 recently showed that voltage-tunable PMA on the base of oxygen ion migration can also be achieved in Pt/Co/CoO/TiO2 (TaOx ) heterojunctions. In combination with the spin current generated from the Pt layer, this makes voltage control of the spin-orbit torque (SOT) induced perpendicular magnetization switching of Co possible. Besides the substantial work on magneto-ionic control of ultra- thin Co films with PMA, an extension to films with in-plane anisotropy has recently been reported. Zehner et al.39 achieved a fully reversible low voltage-induced collapse of coercivity and rema- FIG. 4. Comparison of the ME-voltage coefficients for variation in HC for selected magneto-ionic systems as a function of the time elapsed between the settings nence in FeOx /Fe films (10 nm) with uniaxial in-plane anisotropy of the two different HC states considered. The ME-voltage coefficients are given during electrolytic gating at low voltage in 1M LiOH aqueous for the largest reversible HC change. All data refer to room temperature mea- solution [Figs. 5(a) and 5(b)]. In the initial FeOx /Fe films, Néel surements, if not stated different (aq.—aqueous solution, DMC/EC—dimethyl car- wall interactions stabilize a blocked state with high coercivity. bonate and ethylene carbonate, PC—propylene carbonate, YSZ—yttria-stabilized With dedicated and angle-resolved in situ Kerr microscopy, work- zirconia, and np—nanoporous). ing in combination with liquid electrolyte gating, inverse changes in coercivity and anisotropy and a coarsening of the magnetic microstructure [Fig. 5(c)] were probed. The quantitative analysis effects required high temperatures or were restricted to the edges of the anisotropy and domain size changes allowed us to reveal of the patterned electrodes.9 For the proton-based mechanism, the the redox-induced change in Néel domain wall interactions and authors suggested that protons, generated by atmospheric water in the microstructural domain wall-pinning sites as the origin for splitting at the Au top electrode, are transported through GdOx to the voltage-triggered magnetic deblocking. This reversible modula- the Co interface.15 For this mechanism, a sufficiently humid atmo- tion of local defects to tune H C goes beyond state-of-the-art thin sphere is required. The magnetic anisotropy switching is ascribed film magneto-ionics in which extrinsic properties such as coer- to the high sensitivity of the interface anisotropy of Co to adsorbed civity are usually changed by controlling the intrinsic magnetic H. In the same work, anisotropy switching is also observed for a anisotropy. With this approach, voltage-assisted 180○ magnetization Au/GdOx /Pd/Co/Pd structure, which is in this case explained by the switching with a high energy efficiency is achieved within seconds. hydrogenation of the Pd layer adjacent to the Co layer. Lee et al.41 Zehner et al.21 further extended the magneto-ionic functionality to FIG. 5. Voltage-control of coercivity and magnetic domains in FeOx /Fe films with uniaxial in-plane anisotropy. (a) In-plane hard axis magnetization curve in pristine state and at −0.02 V (oxidation) and at −1.1 V (reduction) showing the voltage-induced collapse of coercivity and remanence. (b) Angular dependence of coercivity for the pristine (=oxidized) and the reduced state. (c) Magnetic domains in reduced and oxidized state as observed by in situ Kerr microscopy (adapted from Zehner et al.39 ). APL Mater. 9, 030903 (2021); doi: 10.1063/5.0042544 9, 030903-5 © Author(s) 2021 APL Materials PERSPECTIVE scitation.org/journal/apm in-plane exchange bias thin films, which are important for stable and magnetic data storage, a temporary reduction of coercivity during artificial magnetization distributions.43 The authors could demon- the writing process (magnetization switching) is desirable. So far, strate voltage-controlled non-volatile programming of exchange this is accomplished by heat-assisted magnetic recording,57 which, bias fields in FeOx /Fe/IrMn heterostructures with fully shifted hys- however, involves the generation of heat with a laser. Here, pro- teresis loops by a redox-based transition of the ferromagnetic layer. vided that the switching speed is improved, magneto-ionic mech- Recent advances are reported also for the magneto-ionic anisms to control coercivity may be a route to more energy-efficient control of coercivity and magnetization in bulk-like materials. voltage-assisted magnetic recording. Navarro-Senent et al.20 studied mesoporous Co–Pt/CoO microdisks The device structure and nanoscale ion transportation mecha- (>300 nm) in PC with traces of NaOH. The voltage-driven oxygen nism in some magneto-ionic devices are comparable with those of migration promoted a partial reduction of CoO to Co and led to resistive switching devices, which are used for the non-volatile resis- a strong decrease in coercivity (−85%) at −10 V, which could be tive random access memory (RRAM).58 It has been experimentally partially recovered. In a subsequent study, Navarro-Senent et al.44 demonstrated that device resistance can be switched among multi- investigated the behavior of nanoporous Co–Pt films covered by states to achieve an ultra-high density/capacity storage.59 In analogy, 10 nm AlOx or HfOx in the same electrolyte. Differences in the non-volatile ion transportation triggered by voltage could result in magneto-ionic behavior of uncoated Co–Pt films are ascribed to the progressive changes in the magnetization value in magneto-ionic oxygen acceptor behavior of HfOx and the oxygen donor behavior materials. Thus, we propose that the stable multi-level intermediate of AlOx . states observed in many magneto-ionic materials could be extended For micrometer-sized SmCo5 , Ye et al.40 showed a huge mod- into a multi-state magnetic storage concept. ulation of coercivity (ΔH C of 1 T for ΔV < 1 V) by electrochemical Current-driven magnetic domain wall motion has raised hopes hydrogenation in 1M KOH aqueous electrolyte. Even though very for new memory or logic devices, but so far, high threshold cur- long timkes are required for the largest H C change, voltage-assisted rent densities and defect pinning effects pose challenges to the fur- magnetization reversal could be demonstrated within around 3 min. ther development of this concept.60 Magneto-ionic approaches have Further improvements in switching speed are proposed by the already shown that domain-wall propagation fields can be tuned reduction of the particle size, the introduction of additional diffusion through oxygen ion-based magneto-ionic control of ultrathin Co paths, or optimized device geometries. layers61 and Fe films.39 Offering the advantage of lower threshold current densities and topologically protection,62 skyrmions are an alternative candidate to domain wall memory logic devices. One III. APPLICATION PERSPECTIVES way to stabilize skyrmions is via the Dzyaloshinskii–Moriya inter- FOR MAGNETO-IONIC MATERIALS action (DMI). By adjusting the strength of the DMI, it is possi- In the following, application perspectives for magneto-ionic ble to tune the domain wall chirality and the skyrmion’s winding materials in the fields of memory devices, neuromorphic comput- number, making it more suitable for practical applications. Interest- ing, spin-torque nano-antennas, sensors, and magnetic actuation are ingly, with regard to potential magneto-ionic control, DMI can be discussed. induced by oxygen chemisorption on a ferromagnetic surface.63 A Digital memory is a crucial part in our current digital era. recent work demonstrated that the DMI strength can be (so far irre- The advent of internet of things, machine learning, and artificial- versibly) tuned by oxygen-ion migration at the ferromagnet/heavy intelligence (AI) based applications, such as image recognition50 metal (Co/Pt) interface.31 Furthermore, gas phase experiments have or protein folding,51 impose strict requirements on the processing shown that skyrmions can be induced in ultrathin Fe upon hydro- speed and energy efficiency of data-centric tasks. Already today, the gen exposure.64 This indicates that modulation of skyrmions by fast worldwide energy consumption of the data centers of all cloud ser- proton-based magneto-ionic control may be feasible.15 Fine tuning vice providers is about 200 TW/year.52 Magnetic hard disk drives and reversibility of skyrmion parameters with high endurance, for (HDD) remain a conventional way to store data in computers and example, through hydrogenation, could be favorable for the next large data centers, but the electric currents required for data writing generation spin-orbitronic devices. are associated with the energy loss due to Joule heating. Also, charge In general, the limiting factors for future applicability in based memories, such as flash and dynamic/static random-access data technology might be, in the first place, the reversibility and memory (RAM), have high energy consumption and further device switching speeds. In order to compete with the state-of-the-art miniaturization becomes problematic mainly because of increasing technology, a stability over >1015 cycles and switching speeds leakage currents.53 The use of magnetic elements for RAM seems between 10 ms (HDD) and 10 ns [spin transfer torque (STT) RAM] appealing due to non-volatility and fabrication compatibility with are desirable.65 CMOS.54 Additionally, complex magnetic textures can be used for The practical applications of AI require processing of large vol- information storage.55,56 In such magnetic memory concepts, elec- umes of data, which, in conventional computing architecture (sep- tric currents are still used to control magnetization dynamics. The arate processing and storage units), is energy expensive and limited combination of magnetic memory concepts with energy-efficient by von Neumann bottleneck and the memory wall.66 Neuro-inspired magneto-ionic control could be a route to reduce the operational computing chips that emulate the working principles of the biologi- energies for next generation data storage media. cal brain are expected to perform better and more energy efficient.67 For HDD, control of coercivity or magnetization of the stor- Memristive elements are shown to emulate synaptic properties.68 age media by a voltage, instead of a magnetic field, is appealing Since latest magneto-ionic approaches resemble the redox memris- to reduce the energy consumption. In high anisotropy magnetic tors in its operational principles, they may serve as an alternative thin films, which are required for data stability in high density building block for neuromorphic hardware. An artificial synapse and APL Mater. 9, 030903 (2021); doi: 10.1063/5.0042544 9, 030903-6 © Author(s) 2021 APL Materials PERSPECTIVE scitation.org/journal/apm different synaptic functions based on the magneto-ionic setting of an energy-efficient alternative to standard micro-electromagnets, different magnetization states were already demonstrated in the Co especially as the demand for high switching speed is less crucial thin film.34 than for memory devices. As an example, in some magnetophoretic Magnonics is gaining a lot of popularity for wave-based com- devices, particle transport in a liquid above a magnetic film is real- puting69 and microwave electronic70 applications. It has been pre- ized by magnetic actuation in a time scale of seconds via the modula- dicted that terahertz (THz) spin waves can be excited by domain tion of artificial magnetic domain patterns.85 The required external wall motion induced by STT and SOT.71 Large bandwidths of fre- magnetic field is so far realized by electromagnets. Direct magneto- quency modulation can be achieved, which is important for signal ionic control of the magnetic domains21 could potentially replace processing72 and in telecommunication technologies. It was theoret- magnetic field control in such devices in a succinct way. ically demonstrated in antiferromagnets that the wavelength of the To achieve sufficient stray fields for actuation, a high energy emitted spin waves depends on material lattice constants.73 One of product of the magnet is required. Thus, materials with high satu- the limitations to excite the spin waves is the high threshold cur- ration magnetization, as a precondition for a high remanence, and rent density in materials with strong exchange interactions. Here, sufficient H C will be favorable here. Furthermore, magneto-ionic the magneto-ionic mechanism such as ion intercalation or hydro- materials exhibit the unique feature of different magnetic states (H C , genation may be an approach to tune the lattice constants and the remanence) being able to be set by an external voltage, and this exchange interactions to obtain the desired wavelength. opens the door to a voltage-programmable actuation functionality. Spin-torque nano-oscillators are magnetic tunnel junctions that emit microwave signals when a direct current induces sus- tained magnetization precession. A frequency tuning of 50 MHz IV. POTENTIAL RESEARCH DIRECTIONS has already been demonstrated in an electrically gated spin Hall AND CONCLUDING REMARKS nano-oscillator by engineering the electronic bands at the ferromag- While the technology readiness of this emerging topic is netic/oxide interface.74 However, an all ionic control of PMA would still at the fundamental research levels 1 and 2 (1—basic princi- potentially lead to higher modulation of damping,75 thereby directly ples observed and 2—technology concept formulated), the recent influencing the auto-oscillation threshold current. research works clearly show that magneto-ionic control can be The opportunities to tune magnetic properties by a low voltage extended to a broad range of material systems and magnetic func- are also of interest for magnetic sensors. Modern magnetic field sen- tionalities. In the following, we propose potential research directions sors rely on the giant magnetoresistance (GMR) or tunneling mag- to push this field forward, especially in view of potential applications. netoresistance (TMR) effect in functional layer stacks and are key So far, magneto-ionic control is demonstrated for academic to the information technology and numerous scientific and indus- material systems and the material choice has primarily been con- trial measurement applications. Voltage control of GMR and TMR ducted on the base of the targeted magneto-ionic mechanism. Now, stacks via the electric field control of the magnetic anisotropy is with the basic knowledge on magneto-ionic principles, the research proposed as a promising route to tuning and enhancing the sensi- could extend toward magnetic materials, which exhibit optimized tivity and the linear range of magnetic sensors.76,77 With magneto- initial magnetic properties with regard to application. Here, research ionic approaches, ultralow voltage and non-volatile programming can rely on long-standing engineering approaches to design mag- of magnetic sensors may be within reach, but this remains largely netic materials, including the optimization of the microstructure, unexplored. One recent study on oxide/metal layers demonstrates a texture, intergranular phases, or composition in alloys. For example, tunable magnetoresistance (MR) and even a voltage-controlled sign magneto-ionic control of bulk ferrites and rare earth hard magnets is change in MR by magneto-ionic mechanisms.78 so far restricted to isotropic materials.29,40 An extension to textured A different type of potential sensor application relates to the materials could push forward the application potential for actua- interplay of electrochemistry and magnetism, which is inherent to tion, as the accessible energy product could be increased. Further- magneto-ionic materials. The magnetic state not only depends on more, the search for magneto-ionic mechanisms to control antifer- the applied voltage but also on the type of electrolyte species, pH, romagnetic layers, which are currently hot candidates for spintronic and hydration state, and thus, it can be regarded as a magnetic fin- devices, seems promising. In this regard, Mustafa et al.22 recently gerprint to sense solution, solid-state, or atmospheric chemistry. demonstrated voltage control of exchange bias by the intercalation of Such a direct magneto-chemical sensor would have the advantage Li in antiferromagnetic Co3 O4 , placed adjacent to the ferromagnetic that functionalized magnetic particles, which are often utilized in Co layer. magneto-chemical sensing platforms,79 are not required. A first The energy efficiency of magneto-ionic systems is the most example in this direction is in situ characterization of (de)lithiation intriguing aspect for their application potential. Research works processes in Lix CoO2 cathodes for Li ion batteries by utilizing in situ on magneto-ionic control already clearly demonstrate that a large SQUID magnetometry.80 variety of magnetic properties can be tuned by a low voltage. The Magnetic actuation is important in a plethora of modern tech- electrochemical reactions involve electric currents, but these are nologies, including microfluidic chips,81,82 (micro)robotics,83 and orders of magnitude smaller than those required for electric cur- MEMS.84 Up to now, the magnetic actuation functionality requires rent controlled magnetic devices. In addition, as the mechanism an external magnetic field as the control parameter, which is usu- relies on chemical changes, the effects are often non-volatile and ally provided by an electromagnet. This becomes problematic par- thus do not require a continuous power supply. For the FeOx /Fe ticularly when downscaling to the microscale, as here, high current system, magneto-ionic switching energies in the range of few fem- densities are required to achieve sufficient magnetic field strength.84 tojoules (for a 50 nm device) are projected,39 which is com- Switchable micromagnets by magneto-ionic control could present parable to the lowest reported switching energies for magnetic APL Mater. 9, 030903 (2021); doi: 10.1063/5.0042544 9, 030903-7 © Author(s) 2021 APL Materials PERSPECTIVE scitation.org/journal/apm tunnel junction devices.86 Considering that the switching speed models. At the same time, the presence of a functional liquid is is not yet optimized, this indicates that magneto-ionic mecha- a critical point for implementation in most technologies. There- nisms can yield an improvement in the energy efficiency of sev- fore, research efforts should also target a transfer from solid/liquid eral orders of magnitude. Nevertheless, quantitative information systems to all-solid-state magneto-ionic systems wherever possible. on the energy consumption of magneto-ionic systems is still too Recent examples are the use of ionic gels,90 hydrated oxides,15 and scarce and should be provided more often in the future to high- devices that are derived from solid state battery architectures.25,36 light the competitiveness with alternative approaches in magnetic On a more futuristic level, solid/liquid magneto-ionic principles technologies. may become useful in brain-inspired computing concepts91 or for The limitation of the switching speed, due to the required ionic magnetic actuation in microfluidic devices.85 motion, is considered as one of the main challenges for the appli- So far, research on magneto-ionic materials focused, to a large cation of magneto-ionic materials. Consequently, future research extent, on the basic science. Thus, in order to move from academic should address this issue. For ultrathin Co films with PMA, which research to the application of magneto-ionic materials, the develop- is the most-studied magneto-ionic material, the optimization of the ment of various laboratory demonstration prototypes is an essential device architecture, film thickness, and gate oxide has successfully future task. increased the switching speed to the range of (sub)milliseconds41 In summary, this perspective gives an overview about impor- and, for small effects, also to the sub-nanosecond regime.49 With tant research advances in the field of magneto-ionic control of this, these systems approach the required switching speeds for magnetism. These include the growing diversity of magneto-ionic memory applications. Only for few systems besides Co, e.g., for materials systems and functionalities and the improvements toward the FeOx /Fe system, a switching speed in the range of seconds is high switching speeds. We discuss various application perspec- reported,39 which may suffice for actuation and sensor applications. tives for magneto-ionic materials in memory, computing, actuation, Time scales of hours to days, which occur for many magneto-ionic and sensor technologies and suggest potential research directions. systems (see Figs. 3 and 4), seem unacceptable from an application For instance, the quantification of switching energy and switching standpoint. In many cases, actually, the maximum possible switch- speeds and the development of magneto-ionic prototype systems ing speed may not be known due to measurement restrictions. Thus, seem important to highlight the energy saving potential and increase a deeper understanding and an improvement of the key aspects for the competitiveness for future applications. the switching speed is an important research strategy. In a first step, time-resolved magnetic measurements would offer access to the AUTHORS’ CONTRIBUTIONS “true” switching speed. Second, interface- and defect-engineering could be pursued as a promising route to optimize the ion transport M.N. and S.H. contributed equally to this work. pathways and thus the kinetics, similar to the successfully applied concepts in the field of memristive systems.87 ACKNOWLEDGMENTS The reversibility of magneto-ionic effects is a further impor- tant issue that defines the possible application areas. At first glance, The authors acknowledge funding by the DFG (Project No. magneto-ionic systems without a structural phase transformation LE 2558 2-1). This project has received funding from the European (such as Li intercalation in spinel structure24 ) or with purely inter- Union’s Horizon 2020 research and innovation programme under face effects (such as surface effects at ultrathin Co films12,15 ) seem the Marie Sklodowska-Curie Grant Agreement No. 861145. favorable here. In contrast, many magneto-ionic systems involve a phase transformation, which exponentiates tuning possibilities DATA AVAILABILITY but may decrease reversibility due to strain-related fatigue or irre- versible microstructure changes. In such systems, good reversibility The data that support the findings of this study are available is often found at the surface/interface regions or for nanoparticu- from the corresponding author upon reasonable request. late morphologies.28,33,88,89 Thus, the minimization of the affected volume, e.g., via interface functionalization in nanostructured mate- REFERENCES rials, seems to be a promising route to high reversibility. Further- 1 J.-M. Hu and C.-W. Nan, APL Mater. 7, 080905 (2019). more, the unprecedented possibilities of magneto-ionic fine tun- 2 E. Y. Vedmedenko, R. K. Kawakami, D. D. Sheka, P. Gambardella, A. Kirilyuk, ing of material properties by voltage at room temperature, even A. Hirohata, C. Binek, O. Chubykalo-Fesenko, S. Sanvito, B. J. 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