Voltage control of magnetism with magneto- ionic approaches: Beyond voltage-driven oxygen ion migration Cite as: Appl. Phys. Lett. 120, 070501 (2022); https://doi.org/10.1063/5.0079762 Submitted: 24 November 2021 • Accepted: 01 February 2022 • Published Online: 16 February 2022 J. de Rojas, A. Quintana, G. Rius, et al. COLLECTIONS This paper was selected as Featured ARTICLES YOU MAY BE INTERESTED IN Random alloy thick AlGaAsSb avalanche photodiodes on InP substrates Applied Physics Letters 120, 071101 (2022); https://doi.org/10.1063/5.0067408 Advances in magneto-ionic materials and perspectives for their application APL Materials 9, 030903 (2021); https://doi.org/10.1063/5.0042544 Spectral modulation of blocked-impurity-band hybrid structure terahertz detector Applied Physics Letters 120, 071102 (2022); https://doi.org/10.1063/5.0082048 Appl. Phys. Lett. 120, 070501 (2022); https://doi.org/10.1063/5.0079762 120, 070501 © 2022 Author(s). Applied Physics Letters PERSPECTIVE scitation.org/journal/apl Voltage control of magnetism with magneto-ionic approaches: Beyond voltage-driven oxygen ion migration Cite as: Appl. Phys. Lett. 120, 070501 (2022); doi: 10.1063/5.0079762 Submitted: 24 November 2021 . Accepted: 1 February 2022 . Published Online: 16 February 2022 J. de Rojas,1,a) A. Quintana,2 G. Rius,3 C. Stefani,4 N. Domingo,4 € mer,5 J. L. Costa-Kra  ndez,1,a) E. Mene and J. Sort1,6,a) AFFILIATIONS 1 Departament de Fısica, Universitat Auto  noma de Barcelona, Cerdanyola del Vallès E-08193, Spain 2 Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, Cerdanyola del Vallès E-08193, Spain 3 Institut de Microelectro nica de Barcelona (IMB-CNM-CSIC), Campus UAB, Cerdanyola del Vallès E-08193, Spain 4 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Cerdanyola del Vallès E-08193, Spain 5 IMN-Instituto de Micro y Nanotecnologıa (CNM-CSIC), Isaac Newton 8, PTM, Tres Cantos, Madrid E-28760, Spain 6  Catalana de Recerca i Estudis Avançats (ICREA), Pg. Lluıs Companys 23, Barcelona E-08010, Spain Institucio a) Authors to whom correspondence should be addressed: juliuscaesar.derojas@uab.cat; enric.menendez@uab.cat; and jordi.sort@uab.cat ABSTRACT Magneto-ionics is an emerging field in materials science where voltage is used as an energy-efficient means to tune magnetic properties, such as magnetization, coercive field, or exchange bias, by voltage-driven ion transport. We first discuss the emergence of magneto-ionics in the last decade, its core aspects, and key avenues of research. We also highlight recent progress in materials and approaches made during the past few years. We then focus on the “structural-ion” approach as developed in our research group in which the mobile ions are already pre- sent in the target material and discuss its potential advantages and challenges. Particular emphasis is given to the energetic and structural benefits of using nitrogen as the mobile ion, as well as on the unique manner in which ionic motion occurs in CoN and FeN systems. Extensions into patterned systems and textures to generate imprinted magnetic structures are also presented. Finally, we comment on the prospects and future directions of magneto-ionics and its potential for practical realizations in emerging fields, such as neuromorphic com- puting, magnetic random-access memory, or micro- and nano-electromechanical systems. C 2022 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http:// V creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0079762 The proliferation of modern electronic devices continues at a which mimics the structure of the biological brain by de-centralizing furious pace, integrated into everything from advanced scientific duties away from single-task units and redistributing both memory equipment to everyday household items. They form the backbone of and computation functions among a large number of neurons con- technologies, such as the Internet of Things, Artificial Intelligence, and nected by synapses. Realizing such structures requires moving beyond Big Data—technologies pushing current computer hardware and volatile hardware units and relying instead on hardware able to offer architectures to their limits, both in terms of functionality and energy analog tuning of properties depending on the stimulus provided, efficiency. Functionally, the ubiquitous von Neumann architecture allowing for “learning” and “forgetting” functionalities, among others. found in nearly all computers, while excellent at computing many In terms of energy efficiency, concerns regarding production and sequential tasks very quickly, struggles to perform complex tasks, such development portend the death of conventional hardware in the com- as pattern recognition on large but imprecise or incomplete data sets. ing 10–15 years. Neural networks have applied neuromorphic princi- The biological brain, however, performs such task orders of magnitude ples to conventional computer hardware with tremendous effect,2–4 more efficiently, inspiring neuromorphic computing,1 a paradigm, but fundamental issues, such as manufacturing limitations, the Appl. Phys. Lett. 120, 070501 (2022); doi: 10.1063/5.0079762 120, 070501-1 C Author(s) 2022 V Applied Physics Letters PERSPECTIVE scitation.org/journal/apl so-called dark or dim silicon, and power and cooling restraints have tune surface charge density in magnetic semiconductors,51–54 ultra- already left current devices struggling to maintain efficiency at dimen- thin metallic films,55–59 and complex magnetic oxides sensitive to car- sions 10 nm,5,6 sparking a search for life after Moore’s Law.7 rier doping at the interface of complex-oxide heterostructures.60–64 Spintronics8,9 has enjoyed success using spin-polarized current to Exchange bias-mediated ME effects, where the interfacial exchange induce low-power magnetization reversal via spin-transfer torque10,11 interaction between an antiferromagnetic ferroelectric and a ferromag- or spin–orbit torque,12,13 but these still rely heavily on electric current netic layer results in a biasing of the magnetization curve away from to manipulate magnetic elements, leading to Joule heating. This con- zero, can be controlled by applying voltage to the ferroelectric layer in cern also extends to, among other technologies, micro- and nanoelec- several systems.65–70 Orbital reconstruction ME effects71 occur when tromechanical systems (MEMS and NEMS) and hard disk drives, an applied field induces orbital reconstructions and hybridizations of where current is used to manipulate magnetic elements. These chal- atoms at the boundary between layers.72–77 To these, we add a rapidly lenges, coupled with expected worldwide electricity demands and envi- growing sub-field of VCM, the electrochemical effect, which has only ronmental impacts of existing technology,14–16 have led to an intense emerged within the last decade. Dielectrics, such as ionic liquids, GdOx, search for low-power, energy-efficient materials with flexible function- HfO2, AlOx, Y2O3-doped ZrO2, and SrCoO2.5,78–86 have opened up the alities. One proposed solution to this challenge is the use of an applied possibility to significantly alter the oxidation states of an adjacent mate- external electric field, instead of current, to modulate the magnetic rial. The electrochemical effect is generally split into two mechanisms: properties of a material. The expanding number of materials and electrostatic (charging) effects and magneto-ionics. The presence of mechanisms that produce this effect has grown into a field of research both effects was shown in ultra-thin La0.5Sr0.5MnO3-d,87 with negative broadly classified as voltage control of magnetism (VCM).17–22 bias, inducing reversible hole accumulation (electrostatic), and positive The intrinsic coupling between an applied magnetic (electric) bias, inducing irreversible oxygen vacancy formation (magneto-ionics). field and a medium’s electric polarization (magnetization) was first Strictly electrostatic effects were demonstrated in the magnetic oxide proposed by P. Curie when discussing the Wiedemann effect,23 a cou- semiconductor (Co,Ti)O2,88 while Walter et al.89 very recently demon- pling later coined the magnetoelectric effect (ME) by Debye.24 Landau strated the fascinating result that diamagnetic (zero-spin) FeS2 could and Lifshitz postulated a linear ME effect in select magnetic crystal be biased into a ferromagnetic state by electrostatically induced band symmetry classes,25 which Dzyaloshinskii extended to predict its exis- filling alone. The evolution of VCM and some important milestones tence in Cr2O3,26 confirmed by experiment soon after.27–30 This sets are presented in the top timeline of Fig. 1(a), with selected works the stage for the pursuit of ME effects in single-phase multiferroics,31,32 referenced. a class of material, which exhibits two or more types of ferroic order.33 Magneto-ionics83,84 first appeared in ultra-thin Co films adjacent However, the fundamental requirements for a material to possess both to GdOx, accomplished by inducing redox reactions at the interface ferroelectric and ferromagnetic order make multiferroics rare,34 push- under an applied bias, changing its magnetic state. A representative ing research beyond single-phase materials. Composite materials35–40 schematic of a typical magneto-ionic structure is shown in Fig. 1(b): a generate the ME effect by stitching together two ferroic phases, cou- dielectric (either liquid or solid-state) is placed in contact with a target pled by some mediating mechanism. Piezo-strain mediated ME effects material (which maybe an oxide or a metal), with voltage applied utilize lattice changes in a ferroelectric material under bias to “strain” a across the film and dielectric, inducing ionic motion. More generally, magnetic material, inducing a change in its magnetic properties.41–50 magneto-ionics uses an applied electric field to insert ions into and Carrier density-mediated ME effects rely on an applied electric field to remove them from a target material, undergoing changes in FIG. 1. (a) Timeline noting representative milestones in voltage control of magnetism and recent developments in magneto-ionics. (b) Schematic of a generic magneto-ionic device common to many systems, with the gate voltage Vg setting or resetting a magnetic state via a redox reaction. Dielectric may be a liquid or solid-state electrolyte, and the activated region may correspond to the interface or beyond the interface into the bulk. Large red circles indicate the ions, which flow toward or away from the target material under bias, such as O2, Hþ, Liþ, N3, or F. (c) Sketch of some magnetic parameters which may be turned in a magneto-ionic system, including saturation magnetization, coercive field, exchange bias, magnetic anisotropy, and Curie Temperature.82,90–97 Appl. Phys. Lett. 120, 070501 (2022); doi: 10.1063/5.0079762 120, 070501-2 C Author(s) 2022 V Applied Physics Letters PERSPECTIVE scitation.org/journal/apl stoichiometry, oxidation state, crystalline structure, as well as large and cyclable changes in magnetic properties [Fig. 1(c)].82,90–97 Magneto- ionic materials also offer several potential advantages for practical applications: They potentially require lower energy to actuate (as low 10 aJ/bit,98,99 much lower than traditional current-using methods, 10 fJ/bit100), no voltage to maintain a set state (nonvolatile) and offer the possibility of functional plasticity (i.e., the ability to undergo non- volatile changes in its structure and properties in response to an exter- nal stimulus), making magneto-ionic materials intriguing candidates to be deployed in neuromorphic or stochastic computing, magnetic random access memory, domain-wall logic, and lab-on-a-chip devi- ces.101–103 Indeed, magneto-ionics has already been identified for potential use in several applications. Magnetic actuation is key in microelectromechanical systems,104–107 where magneto-ionics may prove utile in fabricating multifunctional robotic materials.108 The use of ionic transport to modify magnetic properties has already been observed in Pt/Co(Fe)/MgO, Ta/CoFeB/MgO, and Pt/Co/Ni/HfO2 stacks,79,109,110 representative structures for magnetic random-access memories58,111 and envisioned to allow for low-power tunability of the switching elements via the insulating tunnel barrier. Spin textures, such as skyrmions,112,113 chiral domain walls,114,115 and spin spi- rals,116,117 are stabilized under the Dzyaloshinskii–Moriya interaction (DMI), which have been shown to be tunable using magneto- FIG. 2. Examples of recent progress in magneto-ionics. (a) Modulation of out-of- ionics,118–121 with possible applications in spin-orbitronics.122 plane lattice constant (Dc/c) during voltage cycling [SS refers to Sr(Mn0.5Ir0.5)O3 Continued fundamental research has led to further exploration of dif- and SL refers to [(La0.2Sr0.8MnO3)1(SrIrO3)1]20], showing reversible structural ferent structures, materials, and mobile ions, with successful demon- changes in the digital oxide (SL). Adapted with permission from Yi et al., Nat. stration of magneto-ionic effects using hydrogen,118,123–125 Commun. 11, 902 (2020). Copyright 2020 Springer Nature. (b) Hysteresis loops of lithium,126–130 nitrogen,131–133 and flourine.134 The last two years have a Gd/Ni0.5Co0.5O sample in the as-grown state, after field cooling, subsequent volt- age conditioning, and followed by a second field cooling, showing enhanced seen a flurry of exciting activity, a few of which are highlighted in exchange bias. Adapted with permission from Murray et al., ACS Appl. Mater. Fig. 2. In 2019, Tan et al.123,124 showed that insertion of Hþ into the Interfaces 13(32) 38916–38922 (2021). Copyright 2021 American Chemical interface of Co/GdOx stacks increased the speed of nondestructive tog- Society. (c) Observation of hydrogen-induced domain-wall chirality switching in gling of magnetic anisotropy, demonstrating that the powerful contri- compound spin polarized low energy electron microscopy (SPLEEM), showing as- bution of Hþ plays in altering Co oxidation state under voltage, while grown (left) and hydrogen exposed (right) states. Black and gray areas indicate per- pendicularly magnetized up and down domains, and colors indicate the in-plane ori- recent work by Huang et al. has shown voltage-control of the sublat- entation of magnetization in the domain-wall region. Adapted with permission from tice in ferrimagnetic GdCo and toggling magnetic order.135 Yi et al.91 Chen et al., Phys. Rev. X 11, 021015 (2021). Copyright 2021 American Physical reported reversible and nonvolatile phase transitions at room tempera- Society. (d) Room-temperature in-plane VSM measurements of nanostructured ture under voltage using both Hþ and O2 ions in digital complex metallic alloys conformally coated with insulating oxide nanolayers (CoPt/HfOx) oxides, showing that nondestructive structural changes under bias are under 50 and þ100V, applied for 90 min. Adapted with permission from Navarro- possible [Fig. 2(a)]. Murray et al.136 demonstrated that exchange bias Senent et al., ACS Appl. Mater. Interfaces 12, 14484–14494 (2020). Copyright 2020 American Chemical Society. can be controlled in Gd/Ni1-xCoxO thin films [Fig. 2(b)], where inter- facial Ni1-xCoxO is reduced to magnetic NiCo to establish an exchange bias that can be modulated by up to 35% under voltage conditioning. to superparamagnetic transitions and often lack nonvolatile states. In Zehner et al. and Mustafa et al. also showed the ability to tune addition, oxygen ion migration has often required heat assistance to exchange bias under voltage by inducing redox reactions due to O2 aide ion mobility,82,84,123,124,143 reducing potential energy efficiency. and Liþ, respectively.125,137,138 Chen et al.118 reported inducing Effects beyond the interface have also been explored, although this Dzyaloshinskii-Moriya interaction by chemisorption and desorption brings the added complication of relatively large oxygen ions moving of Hþ, seen in reversible and nonvolatile chirality transitions of mag- through the film, resulting in irreversible structural or compositional netic domain walls [Fig. 2(c)]. Navarro-Senet et al.139,140 and changes as the ions move along grain boundaries and fill vacancies, Robbennolt et al.141,142 have produced dramatic electric field induced limiting reversible toggling to redox reactions at the inter- changes in the magnetic properties of nanoporous, mesoporous, and face.82,119,144,145 As a simple illustration, consider that in the transition nanostructured materials with high surface-to-volume ratios, extend- from metallic Co to CoO, the change in relative cobalt positions while ing magneto-ionic phenomena beyond thin film structures [Fig. 2(d)]. changing phase would approach 20%. To improve cyclability and The changes occurring primarily by redox reactions at the inter- durability, these kinds of stresses and distortions should be minimized, face of heterostructures have motivated studies of magneto-ionic potentially by using the migrating ion in the as-deposited state. Oxide/ effects in thin metal films. However, systems that utilize target ferro- metal heterostructures in thin films and islands have been explored magnetic metal layers, such as Co, are limited to ultra-thin films using Fe/FeOx96,146 and Co/CoOx structures, allowing the reversible (ⱗ5 nm), where patterning on the nanoscale leaves them susceptible removal and insertion of oxygen to toggle magnetic states. In Appl. Phys. Lett. 120, 070501 (2022); doi: 10.1063/5.0079762 120, 070501-3 C Author(s) 2022 V Applied Physics Letters PERSPECTIVE scitation.org/journal/apl particular, on-off switching of magnetism has been observed in iron- completely removed by subsequently applying a bias of þ20 V for based hybrid transition metal oxide/metal,90,140,147 transition metal 60 min (off), i.e., demonstrating nonvolatile on-off control of ferromag- oxide148–151 films, and porous structures. Along this vein, we discuss netism at room temperature. Additionally, the magneto-ionic response our recent work on the structural-ion approach, where the target is found to scale with the increasing voltage; a series of biases (10, material possesses the migrating anions in the as-deposited state, in 25, and 50 V) applied for 60 min results in increasing saturation single layer oxide and nitride films, and so providing a crystal structure magnetization, MS, a result of increased voltage-driven ion migration with more local vacancies and larger ionic pathways, facilitating ion and further reduction of Co3O4 to a metallic Co phase. The series was movement in and out of the target material with reduced structural performed on the same film after applying a positive bias to return the distortion. sample to its original magnetic state (off), showing that cyclability is We begin with magneto-ionics using the structural oxide possible on the scale of 102 s, achievable without thermal assistance. approach in spinel Co3O4 films.150,152 100 nm-thick Co3O4 films, pre- On states are observed to have excellent stability, remaining steady pared by plasma-enhanced atomic layer deposition onto SiO2, were and nonvolatile for months after on-off toggling. (Before and after voltage biased using propylene carbonate (PC) with solvated Naþ M–H loops are provided in the supplementary material.) Interestingly, ions. PC is a polar anhydrous liquid electrolyte, which generates a very the redox process is found to penetrate deep into the film and, under large electric field with low voltage due to an electric double layer high applied voltage (200 V), introduces strong compositional and (EDL), allowing for magneto-ionic effects. Representative hysteresis structural inhomogeneities, as observed by high-angle annular dark- loops of magnetization, M, as a function of applied magnetic field, H, field scanning transmission electron microscopy (HDAAF-STEM) measured by vibrating sample magnetometry (VSM) before and after and electron energy loss spectroscopy (EELS).150 In the top figures of the application of voltage, are shown in Fig. 3(a). The as-grown Co3O4 Fig. 3(b), it is clear that the Co and O distributions are isotropic across film initially shows no ferromagnetic signal, but after biasing under the film, consistent with Co3O4. In the bottom figures of Fig. 3(b), 10 V for 60 min, it shows an apparent hysteresis loop, clear evidence biasing at 200 V clearly reveals a mixture of Co and O concentra- of a ferromagnetic response (on). This ferromagnetic signal is tions. The ion flow is funneled through irregular O-rich diffusion FIG. 3. (a) Normalized magnetic moment, m, vs applied magnetic field, H, of the as-grown Co3O4, the sample treated at 10 V for 60 min, and subsequently treated at þ20 V for 60 min. (b) HAADF, Co and O EELS mappings of a sample negatively biased at –200 V for 30 min, respectively. Adapted with permission from Quintana et al., ACS Nano 12(10), 10291–10300 (2018). Copyright 2021. American Chemical Society. (c) COMSOL simulations of the initial voltage distribution at the moment the transistor-like (top) and the capacitor-like (bottom) configurations are biased. (d) Time evolution of the saturation magnetization (MS vs t) and (e) squareness (MR/MS) and slope of hysteresis loop at the coercive field, HC, normalized to MS for the capacitor- and transistor-like configurations. Adapted with permission from de Rojas et al., Adv. Funct. Mater. 30, 2003704 (2010). Copyright 2020 John Wiley and Sons. Appl. Phys. Lett. 120, 070501 (2022); doi: 10.1063/5.0079762 120, 070501-4 C Author(s) 2022 V Applied Physics Letters PERSPECTIVE scitation.org/journal/apl channels, which are highly nanostructured or partially amorphous, polycrystalline CoN sputtered onto a Cu/Ti buffer layer that was leaving Co-rich areas in its wake. Fascinatingly, the Co-rich and O- biased much in the same way as the above Co3O4, using PC with sol- rich regions are formed by a combination of redistribution of cobalt vated Naþ ions to electrolyte-gate the films at -50 V while measuring and oxygen within the film, as well as oxygen moving toward the liq- M vs H loops. In Fig. 4(a), MS is shown as a function of time, where uid electrolyte. Structural-ion systems also possess nonvolatile proper- the magneto-ionic rate of CoN is clearly much higher than Co3O4 ties, which can be applied to neuromorphic architectures, where films of the same thickness sputtered on identical substrates. MS tuning of oxidation states as a function of depth in the film leads to increases with time, asymptotically, reaching 588 emu cm3 for Co3O4 continuous, analog changes in MS, coercive field, Hc, or the switching and 637 emu cm3 for CoN. The linear magnetization rate of CoN is field distribution (SFD), allowing for hysteretic “learning” or calculated to be 722 memu cm3 s1, a fivefold enhancement over “forgetting” processes depending on the strength and duration of the sputtered Co3O4. However, it should be emphasized that this value applied field. To further extend the utility of cobalt oxide, the effects of may be considerably underestimated; the measurement device (VSM) the gating configuration on magneto-ionic speed were investigated by is limited to measurements on the order of 107 emu and 101 s, so comparing transistor-like with capacitor-like configurations on two any assessment of set and reset events should be taken as a rough identically sputtered Co3O4 films. To understand the effects of device approximation. These promising results suggest that perhaps other configuration, numerical simulations, modeling the initial voltage dis- transition metal nitrides,153 such as the Fe–N system,154 are also tributions for each configuration upon electrolyte-gating, were per- potential magneto-ionic candidates with built-in industrial qualities, formed. The results show a stark difference in the electric potential of including high hardness, melting point, and tunability of properties each configuration [see Fig. 3(c)]. The transistor-like configuration, with nitrogen concentration. Indeed, a study on polycrystalline iron due to the limited conductivity of Co3O4 and the insulating nature of nitrides132 reveals the viability of FeN; MS as a function of time is pre- the SiO2 buffer layer, results in an inhomogeneous electric field along sented in Fig. 4(a) alongside CoN. Interestingly, FeN exhibits a distinct the film’s length and width, weakening further away from the electrical multistep process, with MS initially increasing at a rate slower than TiN contact. The capacitor configuration, on the other hand, forms a CoN but faster than Co3O4, attributed to the appearance of a Fe3N nearly uniform electric field distribution along the length/width of the phase, and then slowing down and reaching a maximum MS of film (due to the electrically conducing TiN layer), resulting in a fully 898 emu cm3, i.e., higher than CoN. Perhaps the most interesting fea- activated Co3O4 layer. The two Co3O4 systems were biased at 50 V ture of nitrogen magneto-ionics, beyond the increased magneto-ionic as magnetic hysteresis loops were continuously recorded. In Fig. 3(d), rates, is the exact way in which the ions are transported from the film. the evolution of the saturation magnetizations, MS, for both systems is An examination of the films after biasing at -50 V using TEM and plotted as a function of time. While both systems show an increase in EELS gave a fascinating insight as to how the different anions move MS, the capacitor configuration shows a near sixfold improvement in through the films [Fig. 4(b)]. After biasing at 50 V, the Co3O4 film total magnetization (119 vs 700 emu cm3), indicating a much stron- redistributes oxygen into irregular O-rich channels, similar to that ger magneto-ionic effect. Indeed, the on state in the capacitor configu- seen in Fig. 3(b). However, in CoN, a well-defined, planar interface ration is activated faster than the transistor-like structure, with over a parallel to the surface is observed, dividing the film in two sublayers 35-fold improvement in magnetization rate (9 vs 325 memu cm3 s1), with two distinct microstructures. No nitrogen is detected in either of showing that the biasing configuration plays a key role in the the two sublayers, escaping beyond the film and into the electrolyte. magneto-ionic performance of structural ion films. Aside from a Sublayer 1 (near the electrolyte) is found to be amorphous-like and much-improved magneto-ionic rate, the on state possesses a nar- less dense, while sublayer 2 (near the electrode) is highly nanocrystal- rower HC distribution [Fig. 3(e)], and the minimal onset threshold line. A similar, although somewhat rougher, sublayer structure is voltage is reduced (4 vs 10 V), a direct consequence of the chosen observed in FeN, reflected in the homogeneous distribution of atomic configuration. The observed TEM and EELS images taken of lamel- Fe and N near the working electrode and a reduced amount of N lae cut from regions distant from the contact are in agreement,152 atoms near the electrolyte side, consistent with the formation of Fe3N. showing a weakened ionic effect in the transistor-like configuration, To understand if this planar migration front is consistent through the although further examination would further illuminate the mecha- voltage range and perhaps correlated with defect structure, variable nisms at play, as analysis until now has not attempted to include energy positron annihilation lifetime spectroscopy experiments for dynamics. For example, TEM/EELS or VEPALS data taken for a CoN were performed and results are shown in Fig. 4(c).132,133 For sim- series of exposure times, both near and far from the electrical con- plicity, only the intensity (I2) of lifetime s2, corresponding to the den- tact, could provide further insight on how ions are distributed under sity of defects in the form of surface states and grain boundaries, is bias using these configurations. While possessing mild activation shown. A monotonic increase in relative intensity I2 is found in the as- rates, applications in neuromorphic and stochastic computing or prepared CoN, whereas relative jumps are found for the treated CoN magnetic MEMS are possible.95 In addition, transition metal oxides, films (highlighted in yellow). The depth position of these relative max- such as Co3O4, are relatively easy to grow in via reactive sputtering, ima increases with the applied voltage (20 to 50 V), confirming the making them well-suited for industrial applications. occurrence of a planar migration front, in agreement with TEM obser- To further extend the potential of the structural ion approach, an vations. The ionic transport upon electrolyte-gating in CoN is, thus, examination of the functionality of ions beyond oxygen in magneto- consistent with a uniform nitrogen migration through vacancies and ionic target materials would likely prove fruitful. Indeed, we have grain boundaries, leaving behind a larger density of grain boundaries. shown that nitrogen132,133 has significant advantages, both energeti- Although not shown here, a migration front is also observed in cally and functionally, over their oxygen-based counterparts. FeN,132 even under the minimal bias needed to activate ionic motion Nitrogen133 magneto-ionics was first observed in paramagnetic, (onset voltage). It must be noted here that this planar front is a unique Appl. Phys. Lett. 120, 070501 (2022); doi: 10.1063/5.0079762 120, 070501-5 C Author(s) 2022 V Applied Physics Letters PERSPECTIVE scitation.org/journal/apl FIG. 4. (a) Evolution of the saturation magnetization as a function of time (MS vs t) for CoN, FeN, and Co3O4 films. (b) HAADF-STEM images and corresponding elemental EELS mappings of the areas marked in orange, of the CoN, FeN, and Co3O4 films subjected to a 50 V for 75 min. Colors corresponding to each element are shown just below the figure. (c) Positron lifetime relative intensity I2 (corresponding to grain boundaries) as a function of positron implantation energy Ep for as-prepared, 20, and 50 V biased CoN films. Relative peaks in intensity of I2 correspond to the position of the moving planar migration front between Co sublayers, highlighted in yellow. (d) Evolution of MS as voltage is increased in steps of 2 V to determine the onset and recovery voltages. Insets show EELS mappings after on and off switching. (e) Calculated total energy per atom, normalized to the minimum energy value, as a function of the position of the inserted ion relative to the surface. The black circles, red squares, and blue triangles correspond to the Co–N, Fe–N, and Co–O energetic paths, respectively. Adapted with permission from de Rojas et al., Nat. Commun. 11, 5871 (2020). Copyright 2020 Springer Nature. Adapted with permission from de Rojas et al., ACS Appl. Mater. Interfaces 13(26) 30826–30834 (2021). Copyright 2021 American Chemical Society. trait, one which can be imagined to allow the imprinting of magnetic onset and reset process, shown in Fig. 4(d). Not only is the onset volt- shapes parallel to the applied field, and potentially allowing for the tog- age for CoN (4 V) found to be lower than that for Co3O4, but it also gling of related magnetic and stoichiometrically controlled crystalline recovers the paramagnetic state under biasing at þ4 V (for Co3O4 a states. This planar front is currently attributed131 to the highly nano- larger positive voltage is required). The insets of Fig. 4(d) show EELS structured nature of the films, isotropic grains, and a defect gradient images of the CoN film during the onset and recovery processes, along the depth of the film, with more defects near the top and fewer revealing that under 4 V biasing, CoN develops a slightly nanopo- near the working electrode, resulting in faster ionic motion occurring rous structure and a decrease in the nitrogen content near the electro- near the surface under voltage, while relatively fewer defects result in lyte, while, after þ4 V biasing, the film is nearly fully recovered with lower ionic rates deeper in the film. This, along with the smaller, iso- very small nitrogen-deficient regions. This polarization symmetry is a tropic grains compared with Co3O4 and lower electrical resistivity distinct advantage of N3 over O2, which requires asymmetric bias- effectively makes the ionic front penetration depth a voltage depen- ing to recover the original state (i.e., a positive bias of þ20 V must be dent quantity. While CoN does generate magnetic phase faster than applied to recover a sample biased at 6 V). Similar results are FeN and with a more clearly defined planar front, the generated ferro- observed in FeN, where complete recovery is achieved under equiva- magnetic phases possess different hysteresis loop shapes. In general, lent positive biasing. To understand this difference between oxygen iron nitride films show a larger increase in coercive field, HC, up to and nitrogen, formation energies of Co–O, Co–N, and Fe–N, shown 102 Oe, compared with 101 Oe in cobalt nitride, indicative of rela- in Fig. 4(e), were calculated by determining minimum energy paths of tively smaller activated magnetic regions, comprising single- or few- N inserted into a transition metal slab, plotted as a function of the domain structures in mostly denitrided regions. This suggests that position of the oxygen or nitrogen relative to the surface (reflective of FeN, while slower, may be more useful for magnetic memory applica- an on-off transition). The critical electric field needed to induce oxygen tions. The energy required to set a magnetic state was evaluated by an or nitrogen motion can be estimated by EC ¼ DV/Dz, where DV is the Appl. Phys. Lett. 120, 070501 (2022); doi: 10.1063/5.0079762 120, 070501-6 C Author(s) 2022 V Applied Physics Letters PERSPECTIVE scitation.org/journal/apl electric potential per atom to overcome the energy barrier and Dz is described here rely on voltage-driven oxygen or nitrogen ion migra- the distance between minima. This critical field is found to be tion, much like ion channels activated with electric stimuli in neurons’ 8.1 V nm1 for Co3O4, 6.6 V nm1 for FeN, and 5.3 V nm1 membranes in the brain. CoN, in agreement with the observed onset voltages. Although the Extending magneto-ionic targets with structural-ions toward crystallographic structures of CoN, FeN, and Co3O4 were not exactly device viability require tackling several hurdles, including the effect of reproduced in the modeling, it is still a reasonable approximation of dimensional reduction and patterning. As a preview, initial steps an off-on process in a polycrystalline film. These results complement toward “imprinting” a magnetic region into a structural-ion target recent calculations, which show that CoN has a lower energy barrier under biasing are shown for a 100 nm thick, polycrystalline Co3O4 compared to Co3O4 as well as previous simulations where CoN was film sputtered onto a Cu/Ti/SiO2 substrate. Cu stripes were then pat- found to have a lower cohesive energy than FeN.155,156 Nitrogen terned atop the Co3O4 film, leaving only the exposed (uncovered) magneto-ionics, thus far, presents a desirable blend of qualities as a Co3O4 regions in contact with PC during biasing. The Cu stripes were magneto-ionic anion, such as requiring lower activation voltages, fabricated by photolithography using a photoresist of stripes with higher cyclability (under symmetric bias), much faster rates of motion dimensions of 2.4 mm  5 lm and a pitch of 12 lm, atop which a and, perhaps most interestingly, a planar ionic motion front that gen- 100 nm thick Cu layer was then sputtered. Once the photoresist was erates a voltage-programmable ion depth profile, potentially allowing lifted off, alternating Cu and exposed Co3O4 stripes were left behind, for the nonvolatile imprinting and destruction of magnetic sublayers. constituting 45% of the area, as shown in Fig. 5(a). We have assigned These features, such as the voltage-tunable analog changes in MS directionality to the stripes on the film, with 0 corresponding to the [Fig. 4d)], may open the door to an eventual utilization of magneto- direction parallel to the lines, and 90 corresponding to the direction ionics in neuromorphic applications. Remarkably, the magnetic signals perpendicular to the lines. Once again using PC, the samples were FIG. 5. (a) Image of the patterned Co3O4 film with a Cu mask. Dark stripes correspond to exposed Co3O4, light stripes correspond to the Cu mask. Inset is a schematic of the film and electrical circuit used for measurements shown in (a)–(d). Directions parallel and perpendicular to the exposed Co3O4 regions are noted 0 and 90 , respectively. (b) Normalized magnetic moment, M, vs applied magnetic field, H, of Co3O4 after treatment at 50 V for 75 min, measured along both the 0 and 90 directions, with asymmetry reflecting the emergence of hard and easy axes. (c) Atomic force microscopy (AFM, top) and magnetic force microscopy (MFM, bottom) measured at (non-saturated) rema- nence after an applied external field of 100 Oe was applied along 90 . Green lines are provided as guides to the eye marking the outline of the Cu mask. (d) MFM measure- ments at remanence after an applied external field of 450 Oe (top) and 450 Oe (bottom) were directed along 90 , showing greater magnetic contrast. (e) AFM measurements of 2 lm radius FeN dots grown onto a Cu/Ti buffer layer. (f) Normalized magnetic moment, M, vs applied magnetic field, H, of FeN dots after sequential treat- ment at 50 and 100 V, for 25 min each, measured in plane. Appl. Phys. Lett. 120, 070501 (2022); doi: 10.1063/5.0079762 120, 070501-7 C Author(s) 2022 V Applied Physics Letters PERSPECTIVE scitation.org/journal/apl biased at 50 V for 75 min, with hysteresis loops measured by VSM motion, alternate approaches have been considered to reduce switch- post-biasing. Biasing not only results in magneto-ionic effects in the ing speeds. The structural ion films studied so far have been relatively exposed Co3O4 stripes but, to a lesser extent, also below the Cu lines, thick when compared to other approaches (50–100 nm). Very recent results in a well-defined patterned magnetic composite. The patterns work on oxide thin films97 and an upcoming work on nitride thin even exhibit a magnetic shape anisotropy depending on the measure- films158 with reduced thicknesses (25 nm) have demonstrated ment direction, as seen in Fig. 5(b), a feature not seen in treated sam- switching speeds <10 s for oxygen ion motion and <1 s for nitrogen ples without patterned Cu stripes. This confirms a magneto-ionic ion motion. To achieve speeds of 1 ms or faster, however, additional generation of magnetic anisotropy. Here, the hard-axis is found to be work must be done, such as further microstructure tuning by, for along the stripe short axis (90 ), consistent with a hard axis due to instance, defect engineering through ion irradiation159 and/or minia- shape anisotropy perpendicular to the stripe long axis, while the easy turization by micro-/nanofabrication to promote surface diffusion axis direction, 0 , is consistent with shape anisotropy observed parallel over bulk diffusion since the latter results in faster ion mobility.160 to the stripe long axis, suggesting that the observed hard and easy axes Moreover, in order to expand technological applications, other forms are the result of selectively activating Co3O4 regions due to the Cu of the dielectric layer used to bias the films should be considered. Up mask. To further explore the magnetic imprinting achieved in the to now, the structural-ion films discussed have been biased using a liq- film, atomic force microscopy (AFM) and magnetic force microscopy uid electrolyte, a common and convenient biasing tool. The massive (MFM) measurements were performed, as shown in Fig. 5(c). The electric fields achieved via EDL formation are excellent for fundamen- topography by AFM (top figure) shows the masked film, with the tal research, as is the relative ease of use (compared with sputtering a broader, lighter color corresponding to the Cu mask, and the darker solid-state ionic conductor) and ability to apply biasing in exotic regions corresponding to the exposed, underlying Co3O4. The mag- motifs outside thin films (foams, nanoporous materials, pillars, etc.). netic profiling by MFM (bottom figure) shows the MFM signal as However, in order to move toward device applicability, solid state het- measured at (non-saturated) remanence after applying an external erostructures using a dielectric sublayer to gate the films will be neces- magnetic field of 100 Oe to the film. A stripe pattern can be identified sary. The fastest room temperature switching in oxygen-based systems along the 0 direction, with clear magnetic domain structure whose has reached speeds up to 0.2 ms,80 utilizing the very high oxygen sizes match the width of the exposed vs Cu-masked regions, confirm- mobility in SrCoO2.5 to produce limited but measurable interfacial ing the generation of a well-defined patterned magnetic composite interactions, an indication that properly tuned dielectrics can have a (due to the dissimilar magneto-ionic strengths in the non-masked and huge impact on ionic migration. Generally speaking, progress via masked regions of the Co3O4 film). These stripes are even more pro- hydrogen boosting, as seen in GdOx,123 and battery-like intercalation nounced under higher applied field (450 and 450 Oe), shown in Fig. can be promising,130,134 while in the specific case of nitrogen magneto- 5(d), with excellent demarcation of the exposed Co3O4. To aid in visu- ionics solid state structures will require exploring possible nitrogen res- alizing the evolution of the magnetic domains under an applied field, a ervoir sources161 (such as zirconium oxide-, tantalum oxide-, or movie is provided in the supplementary material. Along the same mayenite-based nitrides, or GdNx) that can assist the insertion and vein, nitrogen magneto-ionics has also been demonstrated on 125 nm extraction of nitrogen ions from transition metal nitride layers. thick polycrystalline FeN reactively sputtered onto a Ti/Cu substrate, In terms of durability, state-of-the-art magneto-ionic materials subsequently masked using electron beam lithography patterning to have shown high quality repeatability well over 1000 cycles,81,124 while obtain dots of 2 lm in radius with a pitch of 10 lm. Figure 5(e) shows O2 systems, including structural ion, have demonstrated 100 cycles the resulting FeN dots, which exhibit a clear ferromagnetic-on state or less. This can partly be attributed to the deformation or changes when the dots were biased at 50 V for 75 min, with hysteresis loops introduced when inducing ionic motion beyond the interface and may measured by VSM after biasing was complete [Fig. 5(f)] the first dem- point toward three-dimensional, high surface-to-volume ratio struc- onstration of magneto-ionics in a patterned nitrogen magneto-ionic tures focusing on interfacial effects being a more practical option. target. However, tailoring ionic pathways by, for example, ion irradiation of Magneto-ionics research in the past few recent years has opened the target films may allow for more facile insertion and extraction of a plethora of possible materials chemistry, structure types, and mag- ions, improving durability and ionic rates. This can be done not only netic states to be explored. Perhaps the most glaring involves opera- with He, Ar, or Xe ions (inert ions) but also by the same magneto- tional speeds, durability, industrial deployment, and establishing ionic ion species, such as O2 or N3. This process is known to allow functionalities (such as plasticity in neuromorphic computing). Many for tuning of structural defects and may affect ionic pathways and the systems suffer from slow room-temperature ionic response electric field generated inside the target material.131 The structural ion (102–103 s) and are typically boosted via heating to induce ionic approach allows for tuning ion vacancies or enriching regions of the motion (at the expense of energy efficiency) in the range of 101 s or target material and consequently modifying magneto-ionic perfor- below,82,84,143,157 which is much slower than current transistor flip mance, as recently demonstrated by Jensen et al. in Gd/NiCoO.162 speeds (ⱗ1010 s), although proton-based systems are pushing switch- Indeed, structural ion materials are still far from being fully optimized. ing times down to the order of 1 ms or lower.81,123,124,135 In the struc- The threshold voltage needed to actuate nonvolatile on-off behavior is tural ion systems discussed so far in this Perspective, switching speeds 64 V in CoN and 68 V in Co3O4, comparable with voltages applied and cyclability must be addressed before they can be considered com- in other systems as can be found in a recent Perspective by Gu petitive with other magneto-ionic approaches. Switching states in et al.,163 although 50 V was needed to extract nitrogen up to a depth structural ion systems require 102–103 s at room temperature, well of 40 nm in relatively thick films. However, while several studied sys- behind Hþ or heat-assisted O2 systems. Due to the reduction of tems have required an asymmetric voltage to recover the pristine state energy efficiency when applying heat treatment to improve ionic and, in many cases, the phase transformations are irreversible, our Appl. Phys. Lett. 120, 070501 (2022); doi: 10.1063/5.0079762 120, 070501-8 C Author(s) 2022 V Applied Physics Letters PERSPECTIVE scitation.org/journal/apl work has demonstrated that symmetric and recoverable states can be AUTHOR DECLARATIONS achieved in transition metal-nitride systems, suggesting that there is Conflict of Interest still more to learn about these systems. In particular, recent works The authors declare no conflicts of interest. have shown that DC voltage pulsing at frequencies in the range of 1–100 Hz results in cumulative magnet-ionic effects,97,158 allowing for DATA AVAILABILITY the frequency-dependent tuning of magnetic parameters, a path The data that support the findings of this study are available toward the neuromorphic applications of magneto-ionics, still in its from the corresponding authors upon reasonable request. infancy.18,93 The observed dynamics under DC pulsing include a cumulative increase and partial recovery of magnetization (analogous to potentiation and depression), threshold activation, and spike-rate- REFERENCES 1 dependent (i.e., frequency-dependent) magnetic plasticity for learning C. Mead, Proc. IEEE 78, 1629 (1990). 2 functionalities, giving access to functions seen in the biological syn- Y. LeCun, Y. Bengio, and G. Hinton, Nature 521, 436 (2015). 3 A. Krizhevsky, I. Sutskever, and G. E. 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