Boosting Room‐Temperature Magneto‐Ionics in a Non‐Magnetic Oxide Semiconductor

Voltage control of magnetism through electric field‐induced oxygen motion (magneto‐ionics) could represent a significant breakthrough in the pursuit for new strategies to enhance energy efficiency in magnetically actuated devices. Boosting the induced changes in magnetization, magneto‐ionic rates and cyclability continue to be key challenges to turn magneto‐ionics into real applications. Here, it is demonstrated that room‐temperature magneto‐ionic effects in electrolyte‐gated paramagnetic Co3O4 films can be largely increased both in terms of generated magnetization (6 times larger) and speed (35 times faster) if the electric field is applied using an electrochemical capacitor configuration (utilizing an underlying conducting buffer layer) instead of placing the electric contacts at the side of the semiconductor (electric‐double‐layer transistor‐like configuration). This is due to the greater uniformity and strength of the electric field in the capacitor design. These results are appealing to widen the use of ion migration in technological applications such as neuromorphic computing or iontronics in general.


Introduction
Current computers rely on Von Neumann's structural design in which the central processing unit and memory constitute different sub-devices bridged by the communication bus. This is The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202003704. two and five orders of magnitude lower than that required in complementary metal oxide semiconductor (CMOS) technology (≈10 −1 fJ per bit) and magnetic-based devices like magnetoresistive random access memories or hard disk drives (≈10 2 fJ per bit), respectively. [28] Typically, magneto-ionic systems consist of layered heterostructures in which ferromagnetic metals, such as Co [22] or Fe, [28] are grown adjacent to GdO x or HfO 2 layers, which act as ion reservoirs and, thereby, accepting or donating oxygen ions depending on the voltage polarity. Room-temperature ionic motion of oxygen is slow, involving times between 10 2 -10 3 s (10 −2 -10 −3 Hz in frequency rate) to switch the magnetic state, such as the magnetic anisotropy easy axis from out-of-plane to in-plane and vice versa in ultra-thin Co layers by voltage-driven oxygen migration from a GdO x reservoir. [22] By heterostructure miniaturization (from 55 nm down to 18 nm in thickness, i.e., around 3 times reduction), times ≈10 s have been achieved. [22] Therefore, alongside the applied voltage, these solid electrolytes usually require of high temperatures since ion migration is a thermally activated process. [22][23][24]29,30] In these magnetoionic systems, the pristine ferromagnetic layer suffers from pronounced structural and compositional changes, leading to irreversibility [23] and, thus, poor cyclability. [22] Recently, via a proton-based approach, excellent endurance and 10 −1 s (10 Hz) room-temperature operation has been shown feasible in spite of certain instability since hydrogen retention is limited. [29] Moreover, voltage-induced changes of magnetization have also been achieved by the insertion/removal of ions other than oxygen, such as Li [35,36] or F. [37] An alternative approach is the use of structural oxygen (selfcontained in the magnetic material of interest), hence avoiding the need of external oxygen sources. [26] This has been shown in electrolyte-gated paramagnetic Co 3 O 4 films, in which roomtemperature voltage-controlled on-off ferromagnetism has been achieved by electric switching of the oxidation state of cobalt (i.e., voltage-driven reduction/oxidation), taking advantage of the defect-assisted voltage-driven migration of structural oxygen. [26] Even though this route still yields slow room-temperature magneto-ionic motion, it shows outstanding stability and promising cyclability since the target is already oxidized.
Herein, we demonstrate that, without degrading cyclability, room-temperature magneto-ionic motion in electrolyte-gated, paramagnetic, and fairly thick Co 3 O 4 films (thicknesses above 100 nm) can be enhanced in terms of both generated magnetization (6 times larger) and speed (35 times faster) by using an electrochemical capacitor configuration (i.e., with a suitable conducting buffer layer grown underneath the oxide film) rather than just making the contacts on top or at the sides of the semiconducting layer (configuration analogous to an electricdouble-layer transistor, [38,39] without an underlying metallic seed layer), as in some previous works from the literature. [26,[40][41][42][43] The presence of this underlying conducting metallic layer in the capacitor configuration largely enhances the uniformity and strength of the electric field generated across the oxide film when voltage is applied. Our results, showing the importance of properly optimizing device design to apply electric field, could extend the use of oxygen magneto-ionics in new types of MEMS devices, energy storage systems (batteries), iontronics [43] and, specifically, in brain-inspired computing, [39] which demand endurance and moderate speed of operation. [44] Figure 1a,b show the two types of film structures (Co 3 O 4 (130 nm)/SiO 2 (20 nm)/(100)-oriented Si substrate -transistor-like configuration-and Co 3 O 4 (130 nm)/TiN (170 nm)/ (100)-oriented Si substrate -capacitor configuration-) investigated in this work, aimed at unraveling the role of the design of electric field actuation to apply electric field in the magnetoionic response of Co 3 O 4 . In contrast to SiO 2 , the TiN buffer layer is conducting. [45] Electrolyte-gating is used to generate the electric field while performing in-plane vibrating sample magnetometry (VSM), that is, magnetoelectric measurements. A Pt wire is used as counter electrode/gate electrode (see Experimental Section for further details). The as-prepared Co 3 O 4 sample in the transistor-like configuration shows residual ferromagnetic behavior (<2 emu cm −3 ), whereas the as-prepared Co 3 O 4 film in the electrochemical capacitor configuration exhibits some traces of ferromagnetic signal <10 emu cm −3 (Figure 1c-e, and Figure S1, Supporting Information). This mild ferromagnetism could be ascribed to local deviations of Co 3 O 4 stoichiometry, in particular at the TiN-Co 3 O 4 interface since the deposition of Co 3 O 4 is carried out at 200 °C and the thermal stability of TiN is lower than that of SiO 2 . Anyhow, this pristine ferromagnetism of the Co 3 O 4 film in capacitor configuration is minor, especially when compared to the amount of magnetic moment generated upon voltage treatment. The as-prepared state of the films is ruled by an overall paramagnetic behavior at room-temperature. [26] To investigate magneto-ionics in each configuration, the samples were subjected to −50 V for several hours and magnetic hysteresis loops of 25 min of duration were continuously recorded. After subjecting each sample to −50 V for 25 min (i.e., upon the first hysteresis loop is recorded), the measurements show a clear hysteretic behavior, evidencing the emergence of ferromagnetism. The capacitor configuration shows a remarkable increase in magnetization upon sweeping the first quadrant of the first hysteresis cycle, which doubles once the measurement reaches the fourth quadrant of the first loop. A much more gradual increase of the magnetization is observed in the transistor-like configuration (see Figure S2, Supporting Information, to observe the evolution of the magnetization with time for the first hysteresis loop of both configurations). Figure 1e shows the saturation magnetization (M S ) as a function of time (see Figure S3, Supporting Information, for information on M S quantification). The magnetic moment scales monotonically with time for each configuration, but with a sixfold larger increase between the transistor-like and capacitor configurations in the total magnetization (118.5 to 699.2 emu cm −3 , respectively) reached after magneto-ionic motion has stabilized. Furthermore, the time scale for ferromagnetism generation ("on" state) in the capacitor configuration is significantly faster than in the transistor-like structure. To compare properly the rate of "on" switching, this magnetization increase is determined by a linear fit of the M S versus t plot evaluated during the first minutes of voltage application (wherein M S in the capacitor configuration fully saturates). The rates are 33.1 and 1170.8 emu cm −3 h −1 , showing that the use of a conducting buffer (capacitor) layer enhances ion migration by a factor 35 with respect to the insulating buffer layer (transistor-like).

Results
Looking at the M (magnetization)-H (applied magnetic field) loops (Figure 1c,d), there are also marked shape differences. The capacitor configuration exhibits more square-shaped cycles (i.e., it has a more "easy axis" character) than the transistorlike configuration. To examine the shape of the M-H loops, the squareness, defined as the ratio between the remnant magnetization (M R ) and M S (M R /M S ), and the slope of the hysteresis loop at the coercive field (H C ) normalized to M S (dM/dH ) have been calculated for both the descending and ascending branches of the measured hysteresis loops (Figure 1f). The capacitor configuration exhibits higher M R /M S ratios and slopes at H C (narrower distribution of coercive fields) throughout the time the voltage was applied, in concordance with more square-shaped hysteresis loops. [46] To further examine the nature of the electric field experienced by the Co 3 O 4 samples, COMSOL simulations were performed to model the initial voltage distributions for each configuration upon electrolyte-gating (see Experimental Section for further simulation details). In Figure 1g,h, electric contact to the working electrodes (Co 3 O 4 and TiN for the transistor-like and capacitor configurations, respectively) is made at the top of the left plane which represents the samples, whereas the right plane corresponds to the counter electrode (i.e., Pt wire).
Clear differences can be seen in the equipotential lines for the transistor-like ( Figure 1g) and capacitor ( Figure 1h) configurations. In the transistor-like structure, the dielectric nature of SiO 2 and limited electric conductivity of Co 3 O 4 [47] manifest in a macroscopic, non-homogeneous voltage distribution along the vertical extent of the Co 3 O 4 film, showing a weaker and less uniform applied electric field as the distance from the electric contact is increased. Conversely, in the capacitor configuration, the conducing nature of TiN results in a nearly uniform voltage distribution along the vertical cross-section of the sample, which gives rise to a larger and better-defined electric field along the direction perpendicular to the Co 3 O 4 film plane. In contrast to the transistor-like configuration, the whole Co 3 O 4 film is straightaway activated here for magneto-ionic motion ( Figure S4, Supporting Information).
To assess the degree of structural and compositional change that Co 3 O 4 undergoes with voltage for the two investigated configurations, cross section lamellae of the pristine and treated Co 3 O 4 films were prepared and characterized by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and electron energy loss spectroscopy (EELS), respectively (Figure 2).
The morphology of the pristine sample grown on TiN ( Figure 2a) shows regular, columnar-shaped grains as it happens in Co 3 O 4 deposited by atomic layer deposition on SiO 2 . [26] This morphology remains rather unaltered after treating the Co 3 O 4 film deposited on SiO 2 with −50 V for 80 min (Figure 2e). On the contrary, the Co 3 O 4 morphology in the capacitor configuration treated at −50 V for 80 min shows no columnar grains consistent with a more nanostructured Co 3 O 4 phase (Figure 2c and Figure S5, Supporting Information).
To locally quantify the Co/O distribution, Co and O EELS mappings were conducted for the as-grown films and the samples treated at −50 V for 80 min for both transistor-like and capacitor configurations ( Figure 2). Co (red) and O (blue) are homogeneously distributed in the as-grown sample with capacitor configuration (Figure 2d) and nearly homogeneously distributed in the treated sample with the transistor-like configuration (Figure 2e), which sharply contrasts with the sample treated under −50 V in the capacitor configuration ( Figure 2f). The corresponding Co (red) and O (blue) EELS mappings reveal the presence of Co-rich and O-rich areas due to voltagedriven ion migration. In contrast to the sample grown on SiO 2 , electrolyte-gating of the Co 3 O 4 sample grown on TiN results in bubbling, evidencing that, on top of oxygen redistribution within the film, [26] oxygen might be also released into the liquid medium, acting as an oxygen sink. Upon negative biases, oxygen ions, negatively charged, effectively move towards the positively charged counter electrode/gate electrode. Upon traversing the liquid electrolyte and reaching the counter electrode, oxygen ions may form O 2 , causing bubbling, which can be visible to the eye.
As can be seen in Figure 2, a thin layer (a few nm in thickness) is present at the TiN/Co 3 O 4 interface, likely caused by interface reaction while growing the Co 3 O 4 at 200 °C on TiN. By HAADF-STEM, this layer appears darker evidencing its lighter nature compared to the contiguous phases, thus compatible with a TiO 2 -based phase which is the lightest among the possible phases that might form at the TiN/Co 3 O 4 : TiO 2 +CoN+Co 2 N, as predicted using the Materials API (MAPI software). [48] Further structural characterization was carried out by θ/2θ X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and variable energy positron annihilation spectroscopy (VEPAS). The XRD patterns of the asprepared samples are consistent with a textured Co 3 O 4 phase along (1 1 1), (2 2 2) and (3 1 1) planes. Upon electrolyte-gating at −50 V for 80 min, the intensity of the (1 1 1) and (2 2 2) planes strongly decreases while that of the (3 1 1) planes reduces only slightly. For the Co 3 O 4 sample grown on TiN, the peak corresponding to (1 1 1) planes fully vanishes after the application of this negative voltage (Figure 3a). Furthermore, as seen in the detailed XRD view of Figure 3b, the capacitor configuration shows the emergence of a new peak after gating at −200 V for 80 min, which is consistent with the diffraction from (0 0 2) planes of hexagonal close-packed Co (HCP-Co).
Moreover, high resolution transmission electron microscopy (HRTEM) was performed in the cross-section of a Co 3 O 4 film grown on TiN and treated at −50 V (Figure 3c). The inset shows the fast Fourier transform of the area marked with a red rectangle, which results in three well-defined spots highlighted in red circles and numbered 1, 2 and 3. The corresponding interplanar distances are 1.991, 1.920 and 2.535 Å, respectively. The interplanar distance of 1.920 Å is unambiguously ascribed to (1 0 1) HCP-Co (ICDD JCPDF 00-005-0727), whereas 1.991 Å could be associated with either (0 0 2) HCP-Co or (4 0 0) Co 3 O 4 (ICDD JCPDF 00-009-0418). However, since the as-prepared Co 3 O 4 films do not exhibit traces of (4 0 0) planes and (0 0 2) HCP-Co is observed by XRD, the interplanar distance of 1.991 Å is likely to belong to HCP-Co. Finally, 2.535 Å is consistent with an O-deficient (3 1 1) Co 3 O 4 (ICDD JCPDF 00-009-0418) phase. [26] To examine the microstructure of the as-prepared films at atomic level, variable energy positron annihilation spectroscopy (VEPAS) was performed (Figure 3d). Both low and high electron momentum fraction (S and W, respectively) as a function of positron implantation energy, E p , virtually overlap up to the first 50 nm in depth. The differences at further depths of the film are essentially due to the different chemical nature of the buffer layer. This indicates that the as-prepared films grown on both substrates have similar amount and type of defects, independently of the substrate they are grown on.
The onset voltage for magneto-ionic motion and cyclability has also been investigated for both configurations (Figure 4).
To determine the onset voltage, the gating was monotonically decreased in steps of −2 V to observe when the system started to display ferromagnetic behavior. In contrast to the measurements presented in Figure 1 in which consecutive hysteresis loops are acquired while gating the sample, now the film is subjected to a constant applied magnetic field (specifically, 5 kOe, which is above the anisotropy field of the induced ferromagnetic phase) and the magnetization continuously measured while varying the applied voltage. Afterwards, the voltage polarity was reversed to test the cyclability of the magneto-ionic effect. The Co 3 O 4 film grown on TiN exhibits an onset voltage of −4 V and requires of +50 V to fully recover the pristine paramagnetic state. Conversely, the transistor-like configuration shows an onset voltage of −10 V and requires of +10 V to recover the initial state, in agreement with previously reported results on the same configuration but with a thicker SiO 2 buffer layer. [26] While the onset/recovery process is repeatable, to perform cyclability tests the applied biases were increased in order to enhance the magneto-ionic signal-to-noise ratio and, thus, better observe endurance. 30 cycles were performed using voltage pulses of −20 V/+40 V and −20 V/+200 V for the transistor-like and capacitor configurations, respectively (Figure 4c). The change in magnetization relative to the background magnetization is observed to be repeatable in time scale and magnetization quality, suggesting suitable reproducibility for long term use. The magnetoelectric-voltage coefficient according to ref. [31], which is given by α c,V = ΔM/|ΔV|, has been calculated for both configurations upon inducing magneto-ionics under the conditions of Figure 4c

Discussion
The role of the the electric field configuration used during magnetoelectric actuation (electric double layer transistor-like vs capacitor-condenser-like) in the magneto-ionic behavior of Co 3 O 4 thin films has been investigated. As seen in Figure 1, upon electrolytegating at −50 V, the use of an underlying conducting (capacitor) rather than an insulating (transistor-like) buffer layer boosts magneto-ionics in terms of both long-term generated magnetization (sixfold relative increase) and initial magneto-ionic motion (35 times faster). The achieved room temperature magneto-ionic rate for the capacitor configuration (≈10 3 emu cm −3 h −1 ) is of the same order of magnitude than, for instance, that one reached in Li-ion intercalation-based magneto-ionics exhibited by particulate composites gated using a liquid electrolyte, [35] and it is faster than F-ion (de)intercalation in an all-solid-state system based on La 2−2x Sr 1+2x Mn 2 O 7 . [37] Furthermore, the times involved for cyclability are of the same order of magnitude than those corresponding to layered heterostructures containing solid state electrolytes for voltage-driven oxygen motion. [22] The different behaviors of film configuration are already revealed by the M-H hysteresis loops which show pronounced differences in shape. The capacitor configuration results in more square-shaped loops with larger squareness, narrower distribution of coercive fields, and higher slopes at H C , evidencing a more "easy-axis" nature than the transistor-like configuration. This is consistent with the generation of more uniform ferromagnetic regions in the Co 3 O 4 film (with better defined shape anisotropy) when a conducting buffer layer is grown, in agreement with the COMSOL simulations of Figure 1g,h. In the transistor-like configuration, the interplay between the dielectric nature of SiO 2 and the limited electric conductivity of Co 3 O 4 manifests in a non-homogeneous voltage distribution along the Co 3 O 4 film, resulting in a non-uniform electric field which decreases as the distance from the electric contact increases. Conversely, in the capacitor configuration, the conducing nature of TiN results in a nearly uniform electric field along the vertical cross-section of the sample, which gives rise to a larger and better-defined electric field along the direction perpendicular to the Co 3 O 4 film plane. In contrast to the transistor-like configuration, the Co 3 O 4 film on TiN is fully and homogeneously activated alongside its complete vertical extent. This results in a well-defined path with a nearly full perpendicular electric field component for magneto-ionic motion in the conducting configuration. On the contrary, in the transistor-like configuration, the strength and speed of magneto-ionic motion is hindered due to the limited electric conductivity of Co 3 O 4 . This is in concordance with a broader size distribution of ferromagnetic regions in the electrolyte-gated Co 3 O 4 film on SiO 2 , in agreement with the magnetometry results which indicate lower squareness values and a broader distribution of coercive fields. This is also evidenced by the evolution of coercivity with time for the consecutive loops taken while electrolyte-gating both configurations at −50 V ( Figure S6, Supporting Information). Whereas the transistor-like configuration results in a monotonic increase of H C with time, the electrochemical capacitor configuration shows a maximum at the very beginning. This maximal behavior resembles the typical dependence of coercivity with particle size in magnetic systems, consistent with a scenario in which a more homogeneous generation of ferromagnetic regions occurs, uniformly evolving in size, likely starting from a superparamagnetic behavior, followed by a single domain state (maximum of H C ) and ending with a multi-domain state. [46] The effect of electric field configuration on the compositional and structural properties of Co 3 O 4 is clearly observed in Figure 2. The morphology of the pristine samples shows regular, columnar-shaped grains (Figure 2a and Figure S5, Supporting Information) and homogeneous composition (Figure 2d). This largely remains upon treating the Co 3 O 4 film deposited on SiO 2 with −50 V for 80 min (Figure 2b,e). Conversely, the morphology of the film in the capacitor configuration treated at −50 V for 80 min shows almost no columnar grains and a highly nanostructured Co 3 O 4 phase (Figure 2c and Figure S5, Supporting Information) with Co and O segregation (Figure 2f). This indicates that this configuration can electrically modulate ion migration at much higher strengths. This is further confirmed by XRD and HRTEM which show traces of metallic Co only for the capacitor configuration ( Figure 3). As can be seen in Figure 3a, for the Co 3 O 4 film in capacitor configuration upon voltage treatment, the intensities of (1 1 1) and (2 2 2) XRD peaks of Co 3 O 4 do not decrease proportionally (the (1 1 1) peak vanishes, while the (2 2 2) peak is still present). This could be consistent with the appearance of other phases, such as Co 2 O 3 (ICDD JCPDF 00-002-0770) or some other amounts of non-stoichiometric Co oxide phases, that could result in a peak around 38.4 degrees. Moreover, the intensity of the (3 1 1) peak slightly decreases only. This strongly suggests that, on top of texture changes, phase transformations may also occur, giving rise to peaks overlapping the pristine peaks of Co 3 O 4 and, thus, altering the initial relative intensities. This is in concordance with the presence of rock salt CoO (ICDD JCPDF 00-001-1025) as a product of Co 3 O 4 reduction: specifically, the (1 1 1) peak of CoO which should be present at around 36.7°, overlapping the (3 1 1) peak of Co 3 O 4 , which is located at 36.5°. Even though the as-prepared Co 3 O 4 films show similar crystallographic features regardless of the buffer layer, further structural characterization to examine the local microstructure of the as-prepared samples was carried out by VEPAS. As seen in Figure 3d, both low and high electron momentum fraction (S and W, respectively) as a function of positron implantation energy, E p , virtually overlap, indicating that Co 3 O 4 grown on either SiO 2 or TiN exhibits analogous defect environment, ruling out minor microstructure differences in Co 3 O 4 as the origin of the observed magneto-ionic effects.
As seen in Figure 4, the minimum voltage bias required to perform an onset/recovery cycle is asymmetric for the Co 3 O 4 Figure 4. a,b) Onset/recovery behavior of the transistor-like and capacitor configurations, respectively. c) Cyclability for both sample configurations (−20 V/+40 V and −20 V/+200 V pulses for the transistor-like and capacitor configurations, respectively). The data are shifted in ΔM-axis to make them distinguishable among configurations. Cyclability was carried out under the application of 5 kOe to ensure being above the anisotropy field and, thus, in saturation. film grown on TiN (−4 V/+50 V), while it is symmetric for the Co 3 O 4 film grown on SiO 2 (−10 V/+10 V). The onset bias is significantly larger for the transistor-like configuration (−4 V (conducting TiN) vs −10 V (insulating SiO 2 )). In contrast to the transistor-like configuration, the use of a conducting buffer layer activates the whole Co 3 O 4 sample, resulting in a more intense and better defined perpendicular electric field and, thus, in enhanced magneto-ionic effect and motion. Bubbling is noticeably observable only in the electrochemical capacitor configuration, evidencing that, on top of oxygen redistribution, [26] oxygen may be released into the liquid electrolyte, which might act as an oxygen sink, and as an oxygen reservoir due to the oxygen solubility in propylene carbonate. [47] The voltage asymmetry in the capacitor configuration can be linked to O 2 bubbling since propylene carbonate reaches O supersaturation and the oxygen forming bubbles cannot be recovered. Moreover, partial degradation of propylene carbonate cannot be ruled out as an additional origin of bubbling since it may result among others in propylene gas. [49] Cycling ( Figure 4) further corroborates the faster magnetoionic rates of the electrochemical capacitor configuration, particularly during the generation of the "on" states. Time span in the transistor-like configuration has been enlarged to reach a suitable signal-to-noise ratio, as a consequence of the slower magneto-ionic kinetics. For the capacitor configuration, cyclability is lost when lower voltages (higher in absolute value but negatively biased) are applied (e.g., −50 V) due to strong irreversible bubbling.

Conclusion
The role of the electric field configuration (determined by the electrical properties of the substrate/buffer layer) in the magneto-ionic behavior of Co 3 O 4 thin films has been investigated. Polycrystalline 130 nm-thick Co 3 O 4 films have been grown by atomic layer deposition on either insulating SiO 2 or conducting TiN buffer layers. The use of an electrochemical capacitor configuration rather than contacting the semiconducting layer using a transistor-like configuration boosts magneto-ionics in terms of both generated magnetization (sixfold increase: from 118.5 (Co 3 O 4 /SiO 2 ) to 699.2 emu cm −3 (Co 3 O 4 /TiN)) and magneto-ionic rates (35 times faster: from 33.1 (Co 3 O 4 /SiO 2 ) to 1170.8 emu cm −3 h −1 (Co 3 O 4 /TiN)). The room temperature magneto-ionic motion for the capacitor configuration is comparable to the speeds achieved, for example, in Li-ion intercalation-based magneto-ionics shown by particulate systems gated using a liquid electrolyte, [35] and in oxygen magneto-ionics of layered heterostructures containing solid state electrolytes. [22] Remarkably, even though the voltage required is larger for our system, our results are promising in terms of speed since this work deals with both relatively thick films (rather than nanoparticles [35] which exhibit a much higher surface-to-volume ratio or ultra-thin ferromagnetic films) [22] and the use of oxygen ions (larger than Li ions). [35] Actually, the magneto-ionic speed of our system could be strongly increased by miniaturization and/or by a local magneto-ionic actuation to modulate, for instance, the magnetic properties, such as magnetic domains, at the nanoscale rather than the whole magnetization of a thin film. [50] Upon gating, transmission electron microscopy and electron energy loss spectroscopy show the emergence of Co-rich areas at a greater intensity for the Co 3 O 4 grown on an electrically conducting substrate. Magnetization measurements also show a marked increase in the squareness ratio and a decrease in the switching field distribution of the hysteresis loops from Co 3 O 4 deposited in the capacitor configuration, evidencing the generation of more uniform ferromagnetic regions. This dissimilar behavior between the use of either an insulating or a conducting substrate arises from the intensity and uniformity of the electric field, which are maximized when using a conducting substrate while preserving stability and endurance. These results demonstrate the importance of the specific device design in order to optimize the strength and speed of the magneto-ionic effect. Our results prompt the way to make oxygen magneto-ionics feasible for practical applications in fields like neuromorphic and stochastic computing or magnetic MEMS, where high frequencies are not necessarily required for device engineering. [39]

Experimental Section
Sample Preparation: 130 nm thick Co 3 O 4 films were grown on two substrates: i) thermally oxidized non-doped Si wafers (SiO 2 (20 nm)/(1 0 0)-oriented Si (0.5 mm)), and ii) non-doped Si wafers coated with a TiN buffer layer (TiN (170 nm)/(1 0 0)-oriented Si (0.5 mm)). The deposition of Co 3 O 4 was carried out by plasma enhanced atomic layer deposition as described in refs. [26,51,52]. The TiN buffer layer was grown by reactive sputtering using the conditions reported in ref. [37]. TiN was selected because of its conductivity and thermal stability allowing proper growth of the Co 3 O 4 film which was carried out at 200 °C by plasma enhanced atomic layer deposition.
Magnetoelectric Characterization: Magnetic measurements under electrolyte gating (i.e., magnetoelectric characterization) were carried out at room-temperature in a vibrating sample magnetometer from Micro Sense (LOT-Quantum Design), with a maximum applied magnetic field of 2 T. Two different configurations were implemented, an electrochemical capacitor one and an electric double layer transistorlike design, where electric contacts are just made on top and at the sides of the semiconducting layer. The sample was mounted in a homemade electrolytic cell filled with anhydrous propylene carbonate with Na + solvated species (5-25 ppm), and the magnetic properties were measured along the film plane after applying different voltages, using an external Agilent B2902A power supply, between the sample and the counter-electrode in a similar fashion of that presented in refs. [16,26,34]. The Na + solvated species in the electrolyte are aimed at reacting with any traces of water. [16] The magnetic signal was normalized to the area of the sample exposed to the electrolyte during the voltage application process. All hysteresis loops were background-corrected and the correction was carried out at high fields (i.e., fields always far above saturation fields) to eliminate linear contributions (paramagnetic and diamagnetic signals).
Structural and Compositional Measurements: θ/2θ X-ray diffraction (XRD) patterns were recorded on a Philips X'Pert Powder diffractometer with a PIXcel 1D detector using Cu K α radiation.
High resolution transmission electron microscopy (HRTEM), highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and electron energy loss spectroscopy (EELS) were performed on a TECNAI F20 HRTEM/STEM microscope operated at 200 kV. Cross-sectional lamellae were prepared by focused ion beam and placed onto a Cu transmission electron microscopy grid.
Variable energy positron annihilation spectroscopy (VEPAS) [53,54] was used to investigate depth-resolved open volume defects at the Slow-Positron System of Rossendorf (SPONSOR) beamline, which provides monoenergetic but variable energy positron beam.