Wireless magneto-ionics: voltage control of magnetism by bipolar electrochemistry

Modulation of magnetic properties through voltage-driven ion motion and redox processes, i.e., magneto-ionics, is a unique approach to control magnetism with electric field for low-power memory and spintronic applications. So far, magneto-ionics has been achieved through direct electrical connections to the actuated material. Here we evidence that an alternative way to reach such control exists in a wireless manner. Induced polarization in the conducting material immersed in the electrolyte, without direct wire contact, promotes wireless bipolar electrochemistry, an alternative pathway to achieve voltage-driven control of magnetism based on the same electrochemical processes involved in direct-contact magneto-ionics. A significant tunability of magnetization is accomplished for cobalt nitride thin films, including transitions between paramagnetic and ferromagnetic states. Such effects can be either volatile or non-volatile depending on the electrochemical cell configuration. These results represent a fundamental breakthrough that may inspire future device designs for applications in bioelectronics, catalysis, neuromorphic computing, or wireless communications.


Introduction
Precise control over selected electronic, magnetic, chemical and/or structural properties of materials is required for a wide range of applications such as batteries 1 , fuel cells 2 and information storage or computing 3,4 .Recently, the use of room-temperature ionic transport to modify magnetic properties (i.e., magneto-ionics) has gained much interest in the pursuit of voltage control of magnetism for energy-efficient spintronics.Under application of external voltage, the electrochemistry fundamental aspects behind the formation of electric double layers (EDLs) at the interface between the solid target material and an adjacent electrolyte, as well as the redox changes occurring at the ionic/electronic conducting material, are central to enable ionic migration.Due to their very narrow thickness (< 0.5 nm), EDLs lead to very strong electric fields under gate voltages of a few Volt 5 .In semiconductors, this causes accumulation of ions at their channelled surface 6,7 .When electrochemical reactions occur, discharge takes place through charge transfer, due to the mixed ionic-electronic conductivity, eventually modifying the entire material structure.Magnetoionics has been explored using electrolyte gating in either transistor-or capacitor-like device configurations 8 as a means to toggle several parameters, such as perpendicular magnetic anisotropy 9 , magnetization 10,11 , exchange bias 12 and domain wall motion 13 .As an extreme case, magneto-ionics can trigger room-temperature reversible paramagnetic-to-ferromagnetic transitions (ON-OFF ferromagnetism), in, for example, electrolyte-gated oxide and nitride films of cobalt 14 .Remarkably, nitrogen ion transport tends to occur uniformly creating a plane-wave-like migration front which is highly beneficial to boost cyclability 14 .
In magneto-ionic devices, electrodes are usually grown adjacent and in direct contact to the target material.Bias voltages to induce ion diffusion are generated by directly connecting these electrodes to a power supply using electrically conductive wires.This standard way to apply voltage is suitable for applications where a physical connection to the actuated material is not a drawback.However, in many cases, such as biomedical stimulation, microfluidics, magnonics or remotely actuated magnetic micro/nanoelectromechanical systems, it might be desirable to induce magneto-ionic effects in a wireless manner.
Interestingly, it has been shown that the surface of electrically conducting objects immersed in liquid electrolytes can become polarized under the action of external electric fields yielding to electrochemical processes.This phenomenon is called "bipolar electrochemistry" (BPE) since it leads to the formation of a dipole with induced anode and cathode poles in the immersed object, along the electric field direction and opposing the external field, where electrochemical reactions may occur for certain potentials without any direct wire connection.In recent years, BPE has gained renewed attention for electrosynthesis of novel materials 15,16 , and for its potential in applications such as sensing, screening and biological actuation 15,17,18 , involving processes not only at the surface but also within the material if intercalation/deintercalation and ionic motion does exist.The wireless voltage-control of physical properties, and specifically magnetism, through BPE has not been explored yet, although it would be of highly relevant interest that offer unexplored perspectives and new engineering options.
In this work, we demonstrate the ON-OFF switching of ferromagnetism in cobalt nitride (CoN) via wireless magneto-ionics.Compared to standard electrolyte-gating methods, here the magneto-ionic material is not directly wired to external power sources.Instead, an electric dipole is induced wirelessly on the target material under the action of an electric field created by external electrodes immersed in the electrolyte medium, leading to chemical processes at the induced poles (i.e., BPE).Modulation of magnetism can be made temporary (volatile) or permanent (non-volatile) depending on the device configuration with respect to the external field.Redox gradients induced in the immersed magneto-ionic material in the horizontal configuration lead to dynamical redox/ionic processes that result in temporary ferromagnetism.Conversely, the vertical configuration (where the magneto-ionic sample is placed parallel to the driving electrodes) develops chemical changes that turn into a permanent ferromagnetic signal.
Beyond magneto-ionics, the reported approach could be used to tune other voltage-dependent physical/chemical properties of materials such as superconductivity 19,20 or metal-insulator transitions 21 , and it is likely to open new avenues in iontronics and wireless magnetoelectric devices.

Wireless magneto-ionics in horizontal configuration
Fig. 1a illustrates a home-made bipolar electrochemical cell setup with horizontal configuration, where a pair of parallel vertical Pt plates are used as external driving electrodes that generate the electric field, and a 50-nm CoN thin film grown on top of Au (60 nm)/Ti (20 nm)/Si acts as the immersed conducting material where polarization effects are to be induced without any electrical wiring.In this experimental scheme, the films are immersed in the electrolyte solution (0.1 M KI in propylene carbonate, PC) and aligned horizontally between the Pt electrodes.Photographs of the experimental setup for the horizontal and vertical configurations are shown in Sections 1 and 2 of the Supplementary Information.Voltages applied to the Pt electrodes generate potentials at opposite poles of the CoN layer, which may yield to capacitive effects (EDL, see Fig. 1a) and, if sufficiently high, to chemical reactions at the induced anode and cathode poles of the CoN sample.Voltammetry scans performed ex-situ and through direct contact to the sample indeed show that the CoN coating undergoes chemical reduction, pointing to the formation of species with several Co:N stoichiometries (Section 3, Supplementary Information).Such redox processes are likely to occur here also through wireless induction of poles.Note that neither visible chemical reactions nor magnetic properties changes are observed upon immersion of the CoN in the electrolyte with the absence of external applied voltages (Section 4, Supplementary Information).
Fig. 1b depicts the potential profiles for electrolyte voltage drop, the distortion created for the immersed conducting material, and the resulting induced potentials at the poles.The interfacial potential difference between the CoN and the electrolyte solution, which is the driving force of electrochemical reactions, varies in this configuration along the lateral length of the actuated film 22,23 .As revealed by COMSOL simulations (Section 5, Supplementary Information), the induced potentials are the highest at the edges of the sample, where anodic and cathodic poles form.Consequently, electrochemical processes are always observed there first.A direct measurement of the induced dipole is not possible through direct contact, since the dipole discharges, but it has been evaluated previously using an indirect approximation 24 , which agrees with the simulation shown here through finite element methods (see Figure S5).Room-temperature hysteresis loops measured by vibrating sample magnetometry (VSM) for the sample before and after applying an external driving voltage of 15 V for 5 min in the horizontal configuration are presented in Fig. 1c.While a paramagnetic state ("OFF" ferromagnetism) is evidenced from the virtually zero net magnetization for the pristine film, a clear ferromagnetic hysteresis loop ("ON" ferromagnetism) with a maximum magnetization, MS, of 53 emu cm -3 builds up for the wireless voltage-actuated samples.
Notably, consecutive hysteresis loops measurements show that MS significantly drops in magnitude over time at ambient conditions.MS reduces by more than 50% within 12 hours and decreases below 10 emu cm -3 after around 24 hours (a value which is <0.7% the saturation magnetization of metallic Co), evidencing that this process is virtually volatile.This magnetization depletion is most likely related to the existence of a charge gradient within the material that facilitates redistribution of charges and ions concentrations from the reduced cathode towards the rest of the sample.Such kind of gradients have been observed before in several BPE systems, for the same configuration 16,25,26 .The hysteresis loop measurements on the individual negative and positive poles and the central part, split from a sample after the bipolar treatment, confirm a significant gradient in the corresponding MS with larger MS towards the positive pole (Section 6, Supplementary Information).On the basis of these results, we hypothesize that, upon the removal of the external driving voltage, the charge gradient created along the sample and the corresponding redox and ionic changes promote internal diffusion of ions that equalize electronic oxidation states, resulting in an internal electrochemical discharge relaxation that restores electroneutrality 25 .
Fig. 1 Demonstration of wireless magneto-ionics in CoN films using an experimental setup with horizontal configuration.a, Schematic illustration of the electrochemical cell and a simplified sketch depicting the induced poles formed and the voltage-controlled nitrogen ion motion at the film-electrolyte interface.The two edges of the bipolar magneto-ionic sample, indicated by blue dashed lines, correspond to the anodic (left) and cathodic (right) poles, whereas the blue dashed line depicted at the centre of the sample corresponds to an initial zero-charge position where the interfacial potential difference is zero with respect to the electrolyte.Note that, for clarity, the sketch depicts only the reactions at the cathodic pole (a correlated I − oxidation to I2 occurs simultaneously at the solution near the induced anode).b, Schematics of the potential profile for the electrolyte and the distortion occurring when a conducting material is immersed in the electrolyte.The deviation from the original electrolyte profile corresponds to the induced poles, opposing the external field, at the extremes of the material.c, Room-temperature hysteresis loops for the CoN films subject to an external driving voltage of 15 V for 5 min using the setup shown in a.The evolution of the saturation magnetization over time indicates the volatility of the magneto-ionic effect for such configuration.
Despite the fact that the generation of a net ferromagnetic response through the wireless method was proven, subsequent structural characterization of the treated samples was hindered due to the time evolution of the gradient observed in MS and therefore the derived chemistry, and the impossibility to perform all experiments in the same time period and therefore, that is, due to the volatile nature of the induced effects in the horizontal BPE treatments.It was then envisaged that the use of a vertical configuration, where all regions of the surface are equidistant from the external Pt parallel electrodes (and therefore subject to the same induced pole charging) might be a good strategy to achieve non-volatile (i.e., permanent) magnetoionic effects.

Wireless magneto-ionics in vertical configuration
Figure 2a shows that the magneto-ionic effect can also be achieved using a vertical BPE configuration.
Here, the films are immersed in the liquid electrolyte (0.1 M KI in PC) and placed parallel to the Pt electrodes.An induced negative potential is generated on the CoN film, which is facing the driving Pt electrode charged positively (while a positive pole is created on the Pt sheet attached to the back of the sample and facing the negative driving Pt electrode).Such negative potential promotes the formation of the EDL at the negative pole and nitrogen ionic motion towards the interface with the electrolyte.In contrast to the horizontal BPE cell, here the interfacial potential difference is uniform along the film surface.As before, the ionic movement is correlated with an electrochemical redox reaction, namely a change in Co oxidation state, as it will be described below.In turn, the electrolyte surrounding the positive pole also undergoes a chemical reaction, where I − is oxidized at the induced anode, forming I2 or I 3 − , leading to the observed orange colour in the electrolyte (Section 2, Supplementary Information).Simultaneous secondary reactions are possible in both poles at the largest applied potentials.For example, under the application of 15 V for more than 5 min, it is observed that bubbles start to appear on the induced cathode of the bipolar electrode surface, which could be related to the formation of N2 gas after reduction of CoN, or to the complex reduction of the propylene carbonate electrolyte to aliphatic species 27 .Since the connected Pt cathode also shows gas evolution the most probable reaction is derived from the solvent.
Voltage-driven changes in the magnetic properties are revealed by VSM measurements.Fig. 2b shows room-temperature hysteresis loops for the as-grown sample and for samples after the vertical BPE treatment under a constant applied voltage of 15 V during various time spans.While no ferromagnetic signal is detected for the pristine film, a clear hysteresis loop with maximum magnetization, MS, of 20 emu cm -3 appears upon 15 V biasing for 2.5 min.By extending the voltage application time, the hysteresis loops become more square-shaped, and MS progressively increases, reaching 145 emu cm -3 after 20 min (Fig. 2c).The dependence of the induced MS as a function of the driving voltage is shown in Fig. 2d for a fixed actuation time of 15 min.For 5 V and 10 V, hysteresis loops just start to emerge gradually, and MS remains lower than 25 emu cm -3 .A clear increase of MS is observed at 15 V, where MS reaches 130 emu cm -3 , suggesting that at this potential suitable conditions for the reaction are reached.MS exceeds 200 emu cm -3 for a driving voltage of 20 V, the maximum external potential applied in the present study.With time and voltage, the variation of coercivity (HC) shows no clear trends (values ranging from around 50 to 200 Oe), whereas the evolution of squareness (ratio between remanent saturation MR and saturation magnetization MS: MR/MS (%)) tends to increase with both time and voltage (going from 7 to 66 %), as shown in Table S1 in the Supplementary Information.This is consistent with an increased amount of ferromagnetic counterpart (i.e., the evolution with time and voltage of MS), with a better defined in-plane magnetic anisotropy.While the ferromagnetic behaviour becomes more pronounced for larger potentials, secondary reactions eventually detach the CoN coating above 20 V. The hysteresis loops as a function of driving voltage for an application time of 5 min are shown in Section 2, Supplementary Information.The evolution of MS as a function of biasing voltage for actuation times of 5 and 15 min is summarized in Fig. 2e.Between 10 and 15 V, a pronounced slope change occurs, evidencing that 15 V is above the onset voltage for magnetoionics in this system.The maximum MS, 210 emu cm -3 , obtained at the 20 V/15 min conditions is smaller than that in previous studies on CoN films actuated through direct electrical connections (MS ≈ 637 emu cm -3 ) 14 .This difference is partially because, during the BPE experiment, the voltage induced at the film is much smaller than the externally applied driving voltage (e.g., the film is under 1 V when the Pt electrodes are biased at 15 V in the vertical BPE devices, as discussed in Section 5 of the Supplementary Information).
Other contributing factors are the larger potentials used in the reported direct-wiring case, where the gating voltage was 50 V, and the use of different electrolyte media 28 .
In contrast to the horizontal BPE, the resulting ferromagnetism using the vertical configuration is stable over time, as shown in Section 2 of the Supplementary Information.This allows for subsequent examination of the structural properties in these films.The volatility of the effect in the horizontal configuration was the occurrence of a redox/ionic lateral gradient within the zone where potentials are sufficiently high.Such gradient is likely to be present also for the vertical configuration (perpendicular to the sample), but with much smaller magnitude since the film is very thin in the field direction and now the entire outer surface of the film (and not only the edges) contributes to the redox process.Finally, the reversibility characterization of the magnetic change (Section 7, Supplementary Information) suggests the existence of quasireversibility for short treatment times (5 min), evidencing that a large fraction of the sample recovers within such time scale, possibly through redistribution of the remaining N ions.The dashed lines in d and e are guides to the eye.
Microstructural and compositional characterizations were conducted by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and electron energy loss spectroscopy (EELS) on cross-section lamellae of the as-grown and voltage-actuated (treated at 15 V for 15 min in the bipolar vertical configuration) samples (Fig. 3).Both the untreated and treated CoN films show a fully dense structure and a flat surface.In contrast to the pristine film, where homogeneous distributions of Co and N elements are found, the treated film shows a two-layered morphological feature, with a nitrogendepleted top layer that can be resolved from the elemental mapping.This evidences a redistribution of nitrogen ions and a uniform nitrogen ion migration front along the film perpendicular direction.The propagation of this front is accompanied by nitrogen ions release to the electrolyte.This effect, which is responsible for the gradual increase of ferromagnetic signal, is driven by the potential gradient occurring during the bipolar electrochemical process.Such planar ion migration front was also observed in electrolyte-gated magneto-ionic nitrides using conventional wired electrodes 14,29,30 .
Based on previous literature on cobalt nitride with variable nitrogen concentration 31 , the X-ray photoemission spectroscopy (XPS) study (Section 8, Supplementary Information) shows a relative change on the N/Co ratio and on the N components for the treated sample, thus corroborating the voltage-induced nitrogen ion release to the electrolyte.The EELS spectrum acquired from the bottom sublayer of the treated film closely resembles that of the pristine sample (Fig. 3c).For the top sublayer, however, the spectrum changes considerably with a significant increment in the relative intensity of Co L3 white line compared to that of the as-prepared sample.The intensity ratio, L3/L2, is enhanced from around 2.6 for the latter to nearly 3.1 for the top sublayer (Fig. 3d).This indicates a decrease of Co valence states in the most affected top layer of the treated sample 32 , resulting from the reduction process in the CoN film.
Aside from metallic cobalt, other Co-N interstitial compounds such as Co3N and Co4N (Co 2+ /Co 3+ mixed valences with lower N/Co ratios) reportedly exhibit ferromagnetic behaviour at room temperature 33,34 .
Therefore, the electrochemical redox reactions on the bipolar electrode involving ion deintercalation may give rise to reduced CoN1-x or Co magnetic species, which account for the observed voltage-induced magnetization changes.To gain more insight into the voltage-driven phase transitions, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements were performed.Fig. 4a shows the Co L2,3edge XAS spectra of the as-grown film and the films treated under driving voltages of 15 V and 20 V for 15 min in the vertical configuration.The spectrum for the pristine sample strongly resembles that of lowspin Co 3+ XAS line shapes, which possess a prominent peak around 779 eV with a high-energy shoulder structure at the L3 edge.This ascertains the primary Co 3+ valence state.Interestingly, the spectra of the actuated films exhibit three noticeable features (A, B, and C on the right panels of Fig. 4).Specifically, in contrast to the spectrum of the pristine state, an additional peak is detected at around 776 eV for the treated samples (Fig. 4, feature A).This is the characteristic pre-peak of Co 2+ .Additionally, there is an energy shift of the L3-edge maximum towards lower energies upon voltage application (Fig. 4, feature B).For example, the L3-edge maximum shifts from 779 eV for the pristine state to 778 eV for the sample actuated under 20 V. Similar behaviour is also visible for the Co L2-edge (feature C in Fig. 4).The shifting is related to the partial reduction of Co 3+ to lower oxidation states.Further, we find that the XMCD signal is practically zero for the as-grown sample (Fig. 4b), as expected.The treated samples, however, possess gradually increased XMCD intensities, resulting from a net magnetic moment from Co or CoN1-x species.This agrees with the previous magnetometry results.Taken together, our XAS-XMCD studies corroborate the Co valence transition from Co 3+ to Co 2+ and occurrence of metallic Co, which are responsible for the wirelessly induced magnetic response.

Conclusions and outlook
This work constitutes the first experimental demonstration of wireless magneto-ionic effects in thin films via bipolar electrochemistry.The study is performed using CoN films immersed in a liquid electrolyte.
Evidence for nitrogen ion motion and the associated electronic structure transitions has been obtained through microscopic and spectroscopic methods.In the absence of direct wired connections to the actuated material, voltage-induced transitions from paramagnetic to ferromagnetic states are driven by the formation of induced charged poles in the material in response to the externally applied voltage and the concomitant discharging processes (i.e., capacitive and faradaic components).We demonstrate that the ion-motion driven magnetic switching behaviour through BPE can be volatile or non-volatile depending on the chosen experimental configuration with regard to the imposed external electric field: for the vertical setup, the uniform charging and corresponding redox processes on the CoN film give rise to sizable magnetization changes with high stability, whereas volatile magneto-ionic effects are observed for the horizontal BPE configuration, presumably due to the spatial gradient in charge and in oxidation state.This work offers a new paradigm for magnetoelectric actuation, that may extend the use of magneto-ionics to widespread areas beyond memories relying on electrically interconnected bits of data.This may comprise applications where electric wiring is a drawback, such as electrostimulation in medical treatments, microfluidics, magnonics or remotely actuated magnetic microrobots, amongst others.Furthermore, the actuation protocols presented here could be extrapolated to other materials, as a means to control other physical properties in a wireless manner, such as superconductivity, memristors or insulator-metal transitions, thus further spanning the range of applications and the technological impact of the obtained results.

Methods
Sample preparation.Room-temperature magnetron sputtering deposition of the thin films was performed in an AJA International ATC 2400 Sputtering System.For the horizontal BPE samples, 50-nm CoN films were deposited using a 50% N2/50% Ar atmosphere on [100]-oriented Si substrates (8 mm × 8 mm lateral size, 0.5 mm thick), precoated with 20 nm of Ti and 60 nm of Au.The total pressure was set at a value of 3.0 × 10 -3 Torr for the sputtering.For the vertical BPE samples, a change in lateral dimensions was adopted: layered structures of Ti (20 nm)/Au (60 nm)/Si of lateral dimension of 8 mm × 20 mm were used as the substrates for depositing the 50 nm-thick CoN films, using the same sputtering conditions as for the horizontal BPE films.The depositions were done while masking the Au layer to fix the lateral size of the magneto-ionic layers to 8 mm × 6 mm, permitting the full immersion of CoN in the electrolyte in our setup.

Bipolar electrochemistry experiments. BPE experiments were carried out in homemade electrochemical
cells, as schematically depicted in Fig. 1a and Fig. 2a.The electrolyte solution was 0.1 M KI in PC.
Anhydrous PC was used a solvent to prevent CoN hydrolysis in an aqueous media, which would yield cobalt oxides.The presence of KI offers the required conductivity of the electrolyte and it also supports the complementary oxidation reaction at the driving anode and the induced anode at the bipolar electrodes.The optimal electrolyte concentrations were screened previously through a set of experiments with correlated magnetization measurements.Empirically, it was found that very small water contents improve the process, in a relatively narrow range between 200 and 800 ppm, and therefore, each experiment was performed with a parallel evaluation of the water content using the Karl Fisher method (see Section 3, Supplementary Information).The driving electrodes were a pair of parallel planar Pt plates (Goodfellow 99.95%, COMSOL Simulations.Electrostatic simulations of the BPE cell were carried out in each configuration with the software platform COMSOL Multiphysics, using the electrostatics module, to evaluate the initial dipole induced at the borders of immersed bipolar CoN electrode.For the horizontal configuration, it is a 3D calculation with a geometry identical to the experimental cell.The mesh size was optimized for each calculation until convergence of the physical solution of loads and potentials in the cell was achieved.The

Section 1. Horizontal bipolar electrochemistry cells
Representative horizontal bipolar electrochemical cells can be visualized in Fig. S1.After applying 10 V for 5 min, no appreciable signs of CoN reduction are observed on its cathodic pole (the right end of the sample in this case).Prolonging the voltage actuation time creates a few nitrogen bubbles or PC reduction products on the cathodic pole, as seen in Fig S1b-c (please note that the negative CoN pole is close to the external Pt positive pole).Significant changes occur when the driving voltage is further increased.Due to the creation of sufficiently large induced potential difference, the bubbling creeps towards the surface of the cathodic poles for bipolar electrochemical experiments above 10 V (Fig. S1d-e).
Meanwhile, the bubbles get denser and denser in response to enhanced external voltages.Iodide

Section 2. Vertical BPE devices and the non-volatility of induced magneto-ionics
Representative optical images showing the BPE device configuration and evolution of electrochemical/magneto-ionic processes are presented in Fig. S2.One can observe the electrochemical reactions on the driving electrodes (Fig. S2b and Fig S2c).While no bubbles on the CoN bipolar electrode are visible under 10 V, when the driving voltage increases to 15 V, small bubbles start to appear on the CoN BPE cathode pole in addition to the Pt driving cathode (see Fig. S2c).At this potential, the saturation magnetization (MS) increases considerably (green curve in Fig. S2d).Further MS increase is observed at 20

Section 3. Linear sweep voltammetry measurements
The possible redox processes on CoN ca n be studied by linear sweep voltammetry via direct contact of the sample.In this case, a three-electrode cell was used with a Pt sheet (15 mm × 35 mm in size) as a counter electrode, a Pt wire as a pseudo-reference electrode, and CoN as a working electrode.First, an attempt was made to characterize the electrochemical response of the PC electrolyte with 0.1M KI containing different amounts of water using a glassy carbon electrode 3 mm in diameter (Biologic) (Fig. S3a).Then, the electrochemical response of the CoN layer on Au/Ti/Si was studied and compared with the bare Au/Ti/Si.Linear scans were run from the rest potential down to -3 V vs Pt (Pt vs. Ag/AgCl approx.0 V) with a speed of 5 mV s -1 (Fig. S3b).The response of the electrolyte, using a glassy carbon as a working electrode, shows the presence of three reduction waves.The lowest intensity wave is at -0.7 V vs. Pt, followed by a wave at -2.42 V vs. Pt and, finally, a third wave of larger intensity at -2.86 V vs. Pt (see Fig. S3a).In principle, several reduction signals are expected for the electrolyte used: from the solvent PC itself, from iodine given the existence of KI that in an atmospheric medium could have formed I2 (or I 3 − ), and from H2O dissolved by environmental absorption (its content determined by the Karl Fischer method is around 147 ppm).Subsequently, the same study was performed in a PC + 0.1 M KI solution with 10 times larger water content (red curve).The corresponding voltammetry curve shows an intensity enhancement for all the waves, evidencing that water plays a global role in the process, and is not a mere individual component.The largest relative enhancement is for the largest intensity wave at -2.8 V vs Pt.In addition, as water content increases, all the reduction waves are anodically displaced by 0.1 V, facilitating reduction processes.It is evident, therefore, that a greater presence of water substantially modifies the solvent and increases the ease of reduction (decreases the overpotential) in all processes.
Once the behavior of the electrolyte was established, the cathodic response of CoN on Au/Ti/Si was studied in PC with 0.1M of KI (Fig S3b).Three waves are observed, in addition to the -0.7 V vs Pt: the first at -1.42 V vs Pt, with lower intensity than the second, at -1.74 V vs Pt and the third -1.93 V vs Pt, the last two overlapping.Since both I2 and water would be reduced at more positive potentials than CoN, it is considered that these last two waves correspond to the reduction of CoN.
Importantly, the underlying Au conducting layer does not contribute once the CoN is deposited on it.No clear reoxidation waves are observed for CoN reduction within the same potential window, suggesting that the reverse process may require an additional overpotential.The existence of two overlapping waves suggests that the reduction process of CoN could occur in several stages, but it also evidences that the precursor layer of CoN could contain several stoichiometries, CoNx, or several microstructures.This last possibility is in perfect agreement with the observations made by XPS that suggest the formation of two phases with different N/Co stoichiometry ratios.able to be reinserted back to the sample.The chemistry and kinetics associated with this process needs further investigation, but it is important to point out that the wireless magnetoionic effect can be made largely reversible.S2.
Both N 1s spectra have a peak maximum located at approximately 398 eV together with a small shoulder at slightly larger binding energy (≈ 400 eV).It has been reported that, in Co-N compounds, due to the charge transfer between Co and N atoms, the N 1s XPS peak tends to shift to lower binding energies as the nitrogen content increases 1 .Thus, we have deconvoluted the spectra assuming two different contributions or phases (A and B) of Co-N with dissimilar N/Co ratio.Phase A, which has a higher N/Co ratio, corresponds to the primary peak at 398 eV, and Phase B, with a lower N/Co ratio, would be contributing mainly to the shoulder peak at 399-400 eV (see Fig. S4).After identifying each phase, we have calculated the area of each component for the two spectra, and the results are listed in Table S2.While there is an overall N/Co atomic ration lower in the treated sample (0.07) than in the as prepared sample (0.11), there are different proportions of the N 1s signal within each sample.For the as-grown sample, the ratio between Phase B and Phase A is 48%, which is smaller than that for the treated sample  Co-N phases in both samples: "Phase A" (in bluish), which has a higher N/Co ratio at lower binding energy and "Phase B" (in brown) with a lower N/Co ratio at higher binding energy.Solid black lines correspond to experimental data, whereas dash-dotted red lines refer to the fitted data.

Section 9. Current profile for the connected Pt electrode circuit
The current observed through the connected Pt electrodes during the experiments always followed an exponential decrease, as expected in any electrochemical process.I − and solvent reactions occur at the driving electrodes, and the extent of the reaction decreases with time.Please note that the processes occurring at the unwired bipolar electrode are not reflected in these curves, and that, by intrinsic definition, those currents cannot be registered through a wired connection.

Fig. 2
Fig. 2 Wireless magneto-ionic control of ON-OFF ferromagnetism at room temperature using a vertical bipolar electrochemistry cell.a, Schematic illustration of the electrochemical cell and a simplified sketch depicting the generation of the EDL and voltage-controlled nitrogen ionic motion at the film-electrolyte interface.Note that an electronically conducting path is created externally (shown as a bracket in the figure) between the Au underlayer supporting the CoN and a Pt sheet placed at the backside that acts as induced anode, to assure the creation of the induced dipole.Since Au is deposited on a Si wafer, no electric contact holds without such connection.b, Room-temperature magnetic hysteresis loops for the as-grown and voltage-actuated films under a 15 V biasing voltage for various durations.c, Saturation magnetization (MS) vs. biasing duration upon applying 15 V. d, Roomtemperature hysteresis loops as a function of the magnitude of the driving voltage for the same biasing duration of 15 min for the pristine (as-sputtered) and voltage-actuated films.e, MS versus external driving voltage at two different application times of 5 min (open red circles) and 15 min (open blue triangles).
Fig. 3e-f presents the high-resolution TEM images of the regions close to the film surface.The corresponding fast Fourier transform (FFT) is shown in Fig. 3g-h.The cubic CoN (200) and(111) reflections appear rather broad in the FFT pattern, which reveals the low-crystallinity nature of the as-grown film.For the voltage-actuated film, besides signals from CoN, the spots corresponding to an interplanar spacing of 2.38 Å (red circles in Fig.3h) agree well with the (110) reflection of orthorhombic Co2N (Pnnm) phase.Furthermore, the blue circles in Fig.3h(interplanar spacing of 1.91 Å) reveal the existence of hexagonal Co and/or Co3N, in agreement with EELS.The coexistence of several phases with different stoichiometries intergrown during reduction was already suggested by linear seep voltammetry measurements (Section 3, Supplementary Information) that evidenced a second reduction wave after the initial reduction.These measurements, carried with direct electrical contact to the CoN sample, suggest that analogous electrochemical processes occur during wired and unwired magneto-ionic actuation.

Fig. 3
Fig. 3 Transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) characterisation of the as-grown and as-treated CoN films.a, b, High-angle annular dark-field (HADDF) scanning TEM images and the corresponding elemental EELS mappings of the areas enclosed in orange of the as-prepared film, a, and the one subjected to 15 V for 15 min, b.Red and green colours correspond to Co and N elements, respectively, for the EELS analysis.c, EELS spectra acquired from the as-prepared film (black) and the two different regions of the treated film, the bottom layer (red) and the top layer of lean nitrogen (blue).The spectra are normalized to the L2 line.d, The calculated intensity

Fig. 4
Fig. 4 Wireless magneto-ionic control of magnetism probed by soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD).Co L2,3-edge XAS spectra, a, and XMCD, b, of the as-sputtered and actuated (applying driving voltages of 15 V and 20 V for 15 min) CoN films, represented by black, red and blue lines, respectively.The reference spectra of low-spin (LS) Co 3+ , highspin (HS) Co 2+ , and metallic Co are shown in the bottom panel of a.The three panels on the right show enlarged views of the dashed rectangular areas, A, B, and C, in a.All XAS spectra are vertically shifted for clarity.The vertical dashed lines mark the appearance of reduced Co 2+ peak.The black downward arrows denote the shift of Co L3 maximum.
of 15 mm × 35 mm size) that ensured a uniform electric field across the cell.External driving voltages were applied through a power supply (Agilent B2902A) across the connected Pt electrodes immersed in the bipolar electrochemical cells.The voltage values presented throughout the manuscript always refer to external driving voltage values.After a careful optimization of distances between electrodes and sample, the Pt electrodes were distanced 20 mm from each other for the vertical BPE alignment and 25 mm for the horizontal device configuration.Thus, the applied field correspond to V/20 and V/25 Volt/mm in vertical and horizontal configurations respectively.For the horizontal BPE devices, the samples were simply placed into a shallow pool containing a 25 mL electrolyte solution and were aligned at the centre of the Pt plates.Millimetre graded papers were put underneath the electrochemical cell to ensure the samples were positioned correctly in the centre and to ensure the reproducibly in all experiments.For the vertical BPE experiments, the samples were vertically inserted in the electrolyte with the help of homemade Teflon supports halfway between the driving Pt electrodes.In order to avoid undesired electrochemical processes and to increase the effective potential on the CoN electrode, the back of the immersed CoN sample (Pt) is electronically connected to the Au underlayer through a U-shaped Pt foil.Iodide ions form I2 (or I 3 − ) during the process, corresponding to the yellow-orange colours observed in the pictures (Sections 1 and 2, Supplementary Information).I vs. t curves for the connected Pt driving electrodes during the bipolar experiments are shown in the Supplementary Information, Section 9. Reversibility tests were performed applying 15 V between driving Pt electrodes for five minutes, allowing the bipolar electrode to discharge, and applying an opposite voltage of the same magnitude.
, I - oxidation to I2 (or I 3 − ) is observed at the wired Pt anode, and in smaller amount at the induced anode (left side of the sample) on the immersed unwired sample.During reduction the black CoN transforms into an apparently bright metallic cobalt (right side of sample).

Fig
Fig S1 a-e, Representative optical images of the horizontal BPE cells.From a to e, they correspond to the BPE conditions of 10 V / 5 min, 10 V / 10 min, 10 V / 20 min, 15 V / 5 min, and 20 V / 5 min, respectively (The values before and after the slash symbols refer to the magnitudes of the driving voltage and application time, respectively).Note that graph papers were put underneath the glass cells to allow a reproducible arrangement of the sample and the electrodes.

V.
In addition, Fig 2e shows that consecutive hysteresis loops measurements (even after several hours from bipolar electrochemical experiments) do not reveal any degradation of the induced magnetic signal within the studied time intervals.

Fig. S2
Fig.S2 a-c, Representative optical images of the vertical bipolar electrochemical cells.From a to c, they correspond to the initial state (no driving voltage applied), and application of external potential of 10 V / 5 min and at 15 V / 5 min, respectively.(The values before and after the slash symbols refer to the magnitudes of the driving voltage and application time, respectively).d, Room-temperature hysteresis loops at various driving voltages for 5 min actuation time.e, Room-temperature hysteresis loops for the as-prepared state and the sample treated at 15 V for 15 min.For the later, the loops were repeatedly recorded during >24 h (for clarity, only the first loop and the ones recorded after 6, 12 and 24 h are shown).

Fig. S3
Fig. S3 a, Cathodic linear sweep voltammetry of PC with 0.1M KI and different water contents, recorded on a glassy carbon (GC) electrode vs. Pt.b, Cathodic linear sweep voltammetry of the CoN/Au/Ti/Si in a PC electrolyte with 0.1M KI.Note that no waves were observed or were very flat at potentials above 0 V vs reference.

Section 4 .
Blank sample CoN immersed within the electrolyte Immersed CoN samples in the electrolyte for a period of 15 minutes show no change in saturation magnetization, MS, as shown from the hysteresis loops in Fig S4, evidencing the absence of a direct reaction of CoN either with I − or I2, implying that all observations reported correspond to the electrochemical treatment.The corresponding XPS survey shows no evidence of I 3d signals either.

Fig. S4
Fig. S4 Room-temperature hysteresis loops for the as-grown CoN films with (in red) or without (in black) submerging in the liquid electrolyte.The submersion duration is 15 minutes.Note the low magnetic moment signal in the Y-axis for both cases.

Fig
Fig S7 Room-temperature hysteresis loops for the as-grown (in black), treated (in red) and recovered (in blue) samples.The actuation is conducted under a condition of 15 V / 5 min, and the recovery is done firstly by discharging (ground) and then applying −15 V / 5 min (reversing the polarity of driving voltage).Both are done in the vertical BPE configuration.

(
55%).These results indicate that there is a smaller fraction of Phase A and larger fraction of Phase B in the treated sample.Along with the decrease of the N/Co ratio, showing that the amount of nitrogen decreases upon voltage treatment, i.e., cobalt becomes more reduced, in agreement with other characterization techniques, the comparison of phase A and B implies that the decrease corresponds mostly to phase A.

Fig. S8
Fig.S8a, General survey spectra for the as-grown sample and treated sample at a driving voltage of 15 V for 15 min.The N 1s X-ray photoelectron spectra for the as-grown sample, b, and treated sample at a driving voltage of 15 V for 15 min, c.Spectra have been fitted considering the presence of two Co-N phases in both samples: "Phase A" (in bluish), which has a higher N/Co ratio at lower binding energy and "Phase B" (in brown) with a lower N/Co ratio at higher binding energy.Solid black lines correspond to experimental data, whereas dash-dotted red lines refer to the fitted data.

Fig. S9 Representative
Fig. S9 Representative Current versus time plot, through the Pt connected driving electrodes during the bipolar experiment.

Table S1 .
Dependence of the coercivity and the squareness ration (MR/MS) on the driving voltage and actuation time.

Table S2
Atomic ratio N/Co and Calculated areas of Phase A and Phase B, along with the ratios between the two areas, for the XPS core-level N 1s spectra of the as-grown film and the film actuated at 15 V for 15 min in vertical BPE cell.