Article https://doi.org/10.1038/s41467-023-42206-5 Wireless magneto-ionics: voltage control of magnetism by bipolar electrochemistry Received: 2 March 2023 Zheng Ma 1,6, Laura Fuentes-Rodriguez 2,3,6 , Zhengwei Tan 1 , Eva Pellicer 1 , Llibertat Abad 3, Javier Herrero-Martín 4 , Enric Menéndez 1 , Accepted: 28 September 2023 Nieves Casañ-Pastor 2 & Jordi Sort 1,5 Check for updates Modulation of magnetic properties through voltage-driven ion motion and redox processes, i.e., magneto-ionics, is a unique approach to control mag- netism with electric field for low-power memory and spintronic applications. 1234567890():,; 1234567890():,; So far, magneto-ionics has been achieved through direct electrical connec- tions 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 transi- tions between paramagnetic and ferromagnetic states. Such effects can be either volatile or non-volatile depending on the electrochemical cell config- uration. These results represent a fundamental breakthrough that may inspire future device designs for applications in bioelectronics, catalysis, neuro- morphic computing, or wireless communications. Precise control over selected electronic, magnetic, chemical and/or ions at their channelled surface6,7. When electrochemical reactions structural properties of materials is required for a wide range of occur, discharge takes place through charge transfer, due to the applications such as batteries1, fuel cells2 and information storage or mixed ionic-electronic conductivity, eventually modifying the entire computing3,4. Recently, the use of room-temperature ionic transport material structure. Magneto-ionics has been explored using to modify magnetic properties (i.e., magneto-ionics) has gained electrolyte gating in either transistor- or capacitor-like device much interest in the pursuit of voltage control of magnetism for configurations8 as a means to toggle several parameters, such as energy-efficient spintronics. Under application of external voltage, perpendicular magnetic anisotropy9, magnetization10,11, exchange the electrochemistry fundamental aspects behind the formation of bias12 and domain wall motion13. As an extreme case, magneto-ionics electric double layer (EDL) at the interface between the solid target can trigger room-temperature reversible paramagnetic-to- material and an adjacent electrolyte, as well as the redox changes ferromagnetic transitions (ON-OFF ferromagnetism), in, for occurring at the ionic/electronic conducting material, are central to example, electrolyte-gated oxide and nitride films of cobalt14. enable ionic migration. Due to their very narrow thickness Remarkably, nitrogen ion transport tends to occur uniformly (<0.5 nm), EDLs lead to very strong electric fields under gate vol- creating a plane-wave-like migration front which is highly beneficial tages of a few Volt5. In semiconductors, this causes accumulation of to boost cyclability14. 1 Departament de Física, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spain. 2Institut de Ciència de Materials de Barcelona, CSIC, Campus UAB, 08193 Bellaterra, Barcelona, Spain. 3Institut de Microelectrònica de Barcelona-Centre Nacional de Microelectrònica, CSIC, Campus UAB, 08193 Bellaterra, Barcelona, Spain. 4ALBA Synchrotron Light Source, 08290 Cerdanyola del Vallès, Spain. 5Institució Catalana de Recerca i Estudis Avançats (ICREA), Pg. Lluís Companys 23, Barcelona 08010, Spain. 6These authors contributed equally: Zheng Ma, Laura Fuentes-Rodriguez. e-mail: enric.menendez@uab.cat; nieves@icmab.es; jordi.sort@uab.cat Nature Communications | (2023)14:6486 1 Article https://doi.org/10.1038/s41467-023-42206-5 In magneto-ionic devices, electrodes are usually grown adjacent reduction, pointing to the formation of species with several Co:N and in direct contact to the target material. Bias voltages to induce ion stoichiometries (Section 3, Supplementary Information). Such redox diffusion are generated by directly connecting these electrodes to a processes are likely to occur here also through wireless induction of power supply using electrically conductive wires. This standard way to poles. Note that neither visible chemical reactions nor magnetic apply voltage is suitable for applications where a physical connection properties changes are observed upon immersion of the CoN in the to the actuated material is not a drawback. However, in many cases, electrolyte with the absence of external applied voltages (Section 4, such as biomedical stimulation, microfluidics, magnonics or remotely Supplementary Information). actuated magnetic micro/nano-electromechanical systems, it might be Figure 1b depicts the potential profiles for electrolyte voltage desirable to induce magneto-ionic effects in a wireless manner. drop, the distortion created for the immersed conducting material, Interestingly, it has been shown that the surface of electrically and the resulting induced potentials at the poles. The interfacial conducting objects immersed in liquid electrolytes can become potential difference between the CoN and the electrolyte solution, polarized under the action of external electric fields yielding to elec- which is the driving force of electrochemical reactions, varies in this trochemical processes. This phenomenon is called “bipolar electro- configuration along the lateral length of the actuated film22,23. As chemistry” (BPE) since it leads to the formation of a dipole with revealed by COMSOL simulations (Section 5, Supplementary Infor- induced anode and cathode poles in the immersed object, along the mation), the induced potentials are the highest at the edges of the electric field direction and opposing the external field, where elec- sample, where anodic and cathodic poles form. Consequently, elec- trochemical reactions may occur for certain potentials without any trochemical processes are always observed there first. Unfortunately, a direct wire connection. In recent years, BPE has gained renewed direct measurement of the induced dipole is not possible through attention for electrosynthesis of novel materials15,16, and for its direct contact, since the dipole discharges, but it was evaluated pre- potential in applications such as sensing, screening and biological viously using an indirect approximation24, and the results agree with actuation15,17,18, involving processes not only at the surface but also the simulation shown here through finite element methods (see within the material if intercalation/deintercalation and ionic motion Fig. S5). This indirect approximation consisted of connecting one pole does exist. The wireless voltage-control of physical properties, and to one edge of the sample while leaving the other measuring pole in specifically magnetism, through BPE has not been explored yet, the nearby electrolyte, which prevents discharge. Although the ionic although it would be of highly relevant interest that offer unexplored mobility and diffusion generate noise, the distortion of the potential perspectives and new engineering options. profile across the sample could be still observed in aqueous media24. In this work, we demonstrate the ON-OFF switching of ferro- The values previously obtained were in qualitative agreement with magnetism in cobalt nitride (CoN) via wireless magneto-ionics. Com- electrostatic simulations, thereby confirming the validity of such pared to standard electrolyte-gating methods, here the magneto-ionic simulations. The same type of experimental evaluation is not feasible material is not directly wired to external power sources. Instead, an using the PC electrolyte within the potential window available and for electric dipole is induced wirelessly on the target material under the the dimensions of the samples prepared here, and therefore the action of an electric field created by external electrodes immersed in COMSOL calculations are shown instead (see Fig. S5). the electrolyte medium, leading to chemical processes at the induced Room-temperature hysteresis loops measured ex situ by vibrating poles (i.e., BPE). Modulation of magnetism can be made temporary sample magnetometry (VSM) for the sample before and after applying (volatile) or permanent (non-volatile) depending on the device con- an external driving voltage of 15 V for 5 min in the horizontal config- figuration with respect to the external field. Redox gradients induced uration are presented in Fig. 1c. While a paramagnetic state (“OFF” in the immersed magneto-ionic material in the horizontal configura- ferromagnetism) is evidenced from the virtually zero net magnetiza- tion lead to dynamical redox/ionic processes that result in temporary tion for the pristine film, a clear ferromagnetic hysteresis loop (“ON” ferromagnetism. Conversely, the vertical configuration (where the ferromagnetism) with a maximum magnetization, MS, of 53 emu cm–3 magneto-ionic sample is placed parallel to the driving electrodes) builds up for the wireless voltage-actuated samples. Notably, con- develops chemical changes that turn into a permanent ferromagnetic secutive hysteresis loops measurements show that MS significantly signal. Beyond magneto-ionics, the reported approach could be used drops in magnitude over time at ambient conditions. MS reduces by to tune other voltage-dependent physical/chemical properties of more than 50% within 12 hours and decreases below 10 emu cm–3 after materials such as superconductivity19,20 or metal-insulator transitions21, around 24 hours (a value which is <0.7% the saturation magnetization and it is likely to open new avenues in iontronics and wireless mag- of metallic Co), evidencing that this process is virtually volatile. This netoelectric devices. magnetization depletion is most likely related to the existence of a charge gradient within the material that facilitates redistribution of Results charges and ions concentrations from the reduced cathode towards Wireless magneto-ionics in horizontal configuration the rest of the sample. Such kind of gradients have been observed Figure 1a illustrates a home-made bipolar electrochemical cell setup before in several BPE systems, for the same configuration16,25,26. The with horizontal configuration, where a pair of parallel vertical Pt plates hysteresis loop measurements on the individual negative and positive are used as external driving electrodes that generate the electric field, poles and the central part, split from a sample after the bipolar treat- and a 50-nm CoN thin film grown on top of Au (60 nm)/Ti (20 nm)/Si ment, confirm a significant gradient in the corresponding MS with acts as the immersed conducting material where polarization effects larger MS towards the positive pole (Section 6, Supplementary Infor- are to be induced without any electrical wiring. In this experimental mation). On the basis of these results, we hypothesize that, upon the scheme, the films are immersed in the electrolyte solution (0.1 M KI in removal of the external positive Pt pole, the charge gradient created propylene carbonate, PC) and aligned horizontally between the Pt along the sample and the corresponding redox and ionic changes electrodes. Photographs of the experimental setup for the horizontal promote internal diffusion of ions that equalize electronic oxidation and vertical configurations are shown in Sections 1 and 2 of the Sup- states, resulting in an internal electrochemical discharge relaxation plementary Information. Voltages applied to the Pt electrodes gen- that restores electroneutrality25. erate potentials at opposite poles of the CoN layer, which may yield to Despite the fact that the generation of a net ferromagnetic capacitive effects (EDL, see Fig. 1a) and, if sufficiently high, to chemical response through the wireless method was proven, subsequent reactions at the induced anode and cathode poles of the CoN sample. structural characterization of the treated samples was hindered due to Voltammetry scans performed ex-situ and through direct contact to the time evolution of the gradient observed in MS and therefore the the sample indeed show that the CoN coating undergoes chemical derived chemistry, and the impossibility to perform all experiments in Nature Communications | (2023)14:6486 2 Article https://doi.org/10.1038/s41467-023-42206-5 60 15 V / 5 min Magnetization (emu cm 3) 40 20 0 As-grown -20 1st loop After 6 h -40 After 12 h After 24 h -60 -8 -6 -4 -2 0 2 4 6 8 Applied Field (kOe) Fig. 1 | Demonstration of wireless magneto-ionics in CoN films using an cathodic pole (a correlated I oxidation to I2 occurs simultaneously at the solution experimental setup with horizontal configuration. a Schematic illustration of near the induced anode). b Schematics of the potential profile for the electrolyte the electrochemical cell and a simplified sketch depicting the induced poles formed and the distortion occurring when a conducting material is immersed in the elec- and the voltage-controlled nitrogen ion motion at the film-electrolyte interface. trolyte. The deviation from the original electrolyte profile corresponds to the The two edges of the bipolar magneto-ionic sample, indicated by blue dashed lines, induced poles, opposing the external field, at the extremes of the material. c Room- correspond to the anodic (left) and cathodic (right) poles, whereas the blue dashed temperature hysteresis loops for the CoN films subject to an external driving vol- line depicted at the centre of the sample corresponds to an initial zero-charge tage of 15 V for 5 min using the setup shown in a. The evolution of the saturation position where the interfacial potential difference is zero with respect to the magnetization over time indicates the volatility of the magneto-ionic effect for such electrolyte. Note that, for clarity, the sketch depicts only the reactions at the configuration. the same time period and therefore, that is, due to the volatile nature sample and facing the negative driving Pt electrode). Such negative of the induced effects in the horizontal BPE treatments. It was then potential promotes the formation of the EDL at the negative pole and envisaged that the use of a vertical configuration, where all regions of nitrogen ionic motion towards the interface with the electrolyte, along the surface are equidistant from the external Pt parallel electrodes with redox changes. In contrast to the horizontal BPE cell, here the (and therefore subject to the same induced pole charging) might be a interfacial potential difference is uniform along the film surface. As good strategy to achieve non-volatile (i.e., permanent) magneto-ionic before, the ionic movement is correlated with an electrochemical effects. redox reaction, namely a change in Co oxidation state, as it will be described below. In turn, the electrolyte surrounding the positive pole Wireless magneto-ionics in vertical configuration also undergoes a chemical reaction, where I is oxidized at the induced Figure 2a shows that the magneto-ionic effect can also be achieved anode, forming I2 or I3 , leading to the observed orange colour in the using a vertical BPE configuration. Here, the films are immersed in the electrolyte (Section 2, Supplementary Information). Simultaneous liquid electrolyte (0.1 M KI in PC) and placed parallel to the Pt elec- secondary reactions are possible in both poles at the largest applied trodes. An induced negative potential is generated on the CoN film, potentials. For example, under the application of 15 V for more than which is facing the driving Pt electrode charged positively (while a 5 min, it is observed that bubbles start to appear on the induced positive pole is created on the Pt sheet attached to the back of the cathode of the bipolar electrode surface, which could be related to the Nature Communications | (2023)14:6486 3 Article https://doi.org/10.1038/s41467-023-42206-5 a b 200 As-grown c 250 15 V 2.5 min 15 V Magnetization (emu cm–3) 150 5 min 200 100 10 min MS (emu cm–3) 15 min 50 20 min 150 0 -50 100 -100 50 -150 -200 0 -4 -3 -2 -1 0 1 2 3 4 0 5 10 15 20 Applied Field (kOe) Time (min) d 200 As-grown e 250 15 min 5V Magnetization (emu cm–3) 150 15 min 10 V 200 5 min 100 15 V MS (emu cm–3) 20 V 50 150 0 -50 100 -100 50 -150 -200 0 -4 -3 -2 -1 0 1 2 3 4 0 5 10 15 20 Applied Field (kOe) External Driving Voltage (V) Fig. 2 | Wireless magneto-ionic control of ON-OFF ferromagnetism at room without such connection. b Room-temperature magnetic hysteresis loops for the temperature using a vertical bipolar electrochemistry cell. a Schematic illus- as-grown and voltage-actuated films under a 15 V biasing voltage for various tration of the electrochemical cell and a simplified sketch depicting the generation durations. c Saturation magnetization (MS) vs. biasing duration upon applying 15 V. of the EDL and voltage-controlled nitrogen ionic motion at the film-electrolyte d Room-temperature hysteresis loops as a function of the magnitude of the driving interface. Note that an electronically conducting path is created externally (shown voltage for the same biasing duration of 15 min for the pristine (as-sputtered) and as a bracket in the figure) between the Au underlayer supporting the CoN and a Pt voltage-actuated films. e MS versus external driving voltage at two different appli- sheet placed at the backside that acts as induced anode, to assure the creation of cation times of 5 min (open red circles) and 15 min (open blue triangles). The the induced dipole. Since Au is deposited on a Si wafer, no electric contact holds dashed lines in d and e are guides to the eye. Nature Communications | (2023)14:6486 4 Article https://doi.org/10.1038/s41467-023-42206-5 formation of N2 gas after reduction of CoN, or to the complex reduc- magnitude since the film is very thin in the field direction and now the tion of the propylene carbonate electrolyte to aliphatic species27. Since entire outer surface of the film (and not only the edges) contributes to the connected Pt cathode also shows gas evolution the most probable the redox process. Finally, the reversibility characterization of the reaction is derived from the solvent. Alternative secondary reactions, magnetic change (Section 7, Supplementary Information) suggests the like Co metal oxidized by I2 yielding soluble green CoI2, do not seem to existence of quasi- reversibility for short treatment times (5 min), evi- take place, since no Co depletion occurs (i.e., no bluish colour is dencing that a large fraction of the sample recovers within such time observed and MS increases). A control experiment was carried out to scale, possibly through redistribution of the remaining N ions. verify this using metallic cobalt powder in a I2-containing PC electro- Microstructural and compositional characterizations were con- lyte (see Section 4, Supplementary Information). No iodine signal is ducted by high-angle annular dark-field scanning transmission elec- observed either in the solid material, so no additional phases con- tron microscopy (HAADF-STEM) and electron energy loss taining iodine form, in agreement with previous works using this spectroscopy (EELS) on cross-section lamellae of the as-grown and electrolyte28. Open circuit potentials and initial voltammetry potentials voltage-actuated (treated at 15 V for 15 min in the bipolar vertical are very stable and constant, evidencing no spontaneous direct reac- configuration) samples (Fig. 3). Both the untreated and treated CoN tions in the setting. films show a fully dense structure and a flat surface. In contrast to the Voltage-driven changes in the magnetic properties are revealed by pristine film, where homogeneous distributions of Co and N elements VSM measurements. Figure 2b shows room-temperature hysteresis are found, the treated film shows a two-layered morphological feature, loops for the as-grown sample and for samples after the vertical BPE with a nitrogen-depleted top layer that can be resolved from the ele- treatment under a constant applied voltage of 15 V during various time mental mapping. This evidences a redistribution of nitrogen ions and a spans. While no ferromagnetic signal is detected for the pristine film, a uniform nitrogen ion migration front along the film perpendicular clear hysteresis loop with maximum magnetization, MS, of direction. The propagation of this front is accompanied by nitrogen 20 emu cm–3 appears upon 15 V biasing for 2.5 min. By extending the ions release to the electrolyte. This effect, which is responsible for the voltage application time, the hysteresis loops become more square- gradual increase of ferromagnetic signal, is driven by the potential shaped, and MS progressively increases, reaching 145 emu cm–3 after gradient occurring during the bipolar electrochemical process. Such 20 min (Fig. 2c). The dependence of the induced MS as a function of the planar ion migration front was also observed in electrolyte-gated driving voltage is shown in Fig. 2d for a fixed actuation time of 15 min. magneto-ionic nitrides using conventional wired electrodes14,29,30. For 5 V and 10 V, hysteresis loops just start to emerge gradually, and MS Based on previous literature on cobalt nitride with variable remains lower than 25 emu cm–3. A clear increase of MS is observed at nitrogen concentration31, the X-ray photoemission spectroscopy (XPS) 15 V, where MS reaches 130 emu cm–3, suggesting that at this potential study (Section 8, Supplementary Information) shows a relative change suitable conditions for the reaction are reached. MS exceeds on the N/Co ratio and on the N components for the treated sample, 200 emu cm–3 for a driving voltage of 20 V, the maximum external thus corroborating the voltage-induced nitrogen ion release to the potential applied in the present study. With time and voltage, the electrolyte. The EELS spectrum acquired from the bottom sublayer of variation of coercivity (HC) shows no clear trends (values ranging from the treated film closely resembles that of the pristine sample (Fig. 3c). around 50 to 200 Oe), whereas the evolution of squareness ratio (ratio For the top sublayer, however, the spectrum changes considerably between remanent saturation MR and saturation magnetization MS: with a significant increment in the relative intensity of Co L3 white line MR/MS (%)) tends to increase with both time and voltage (going from 7 compared to that of the as-prepared sample. The intensity ratio, L3/L2, to 66 %), as shown in Table S1 in the Supplementary Information. This is enhanced from around 2.6 for the latter to nearly 3.1 for the top is consistent with an increased amount of ferromagnetic counterpart sublayer (Fig. 3d). This indicates a decrease of Co valence states in the (i.e., the evolution with time and voltage of MS), with a better defined most affected top layer of the treated sample32, resulting from the in-plane magnetic anisotropy. While the ferromagnetic behaviour reduction process in the CoN film. becomes more pronounced for larger potentials, secondary reactions Aside from metallic cobalt, other Co–N interstitial compounds eventually detach the CoN coating above 20 V. The hysteresis loops as such as Co3N and Co4N (Co2+/Co3+ mixed valences with lower N/Co a function of driving voltage for an application time of 5 min are shown ratios) reportedly exhibit ferromagnetic behaviour at room in Section 2, Supplementary Information. The evolution of MS as a temperature33,34. Therefore, the electrochemical redox reactions on function of biasing voltage for actuation times of 5 and 15 min is the bipolar electrode involving ion deintercalation may give rise to summarized in Fig. 2e. Between 10 and 15 V, a pronounced slope reduced CoN1–x or Co magnetic species, which account for the change occurs, evidencing that 15 V is above the onset voltage for observed voltage-induced magnetization changes. Figure 3e, f pre- magneto-ionics in this system. The maximum MS, 210 emu cm–3, sents the high-resolution TEM images of the regions close to the film obtained at the 20 V/15 min conditions is smaller than that in previous surface. The corresponding fast Fourier transform (FFT) is shown in studies on CoN films actuated through direct electrical connections Fig. 3g, h. The cubic CoN (200) and (111) reflections appear rather (MS ≈ 637 emu cm–3)14. This difference is partially because, during the broad in the FFT pattern, which reveals the low-crystallinity nature of BPE experiment, the voltage induced at the film is much smaller than the as-grown film. For the voltage-actuated film, besides signals from the externally applied driving voltage (e.g., the film is under 1 V when CoN, the spots corresponding to an interplanar spacing of 2.38 Å (red the Pt electrodes are biased at 15 V in the vertical BPE devices, as dis- circles in Fig. 3h) agree well with the (110) reflection of orthorhombic cussed in Section 5 of the Supplementary Information). Other con- Co2N (Pnnm) phase. Furthermore, the blue circles in Fig. 3h (inter- tributing factors are the larger potentials used in the reported direct- planar spacing of 1.91 Å) reveal the existence of hexagonal Co and/or wiring case, where the gating voltage was 50 V, and the use of different Co3N, in agreement with EELS. The coexistence of several phases with electrolyte media28. different stoichiometries intergrown during reduction was already In contrast to the horizontal BPE, the resulting ferromagnetism suggested by linear seep voltammetry measurements (Section 3, using the vertical configuration is stable over time, as shown in Section Supplementary Information) that evidenced a second reduction wave 2 of the Supplementary Information. This allows for subsequent after the initial reduction. These measurements, carried with direct examination of the structural properties in these films. The volatility of electrical contact to the CoN sample, suggest that analogous electro- the effect in the horizontal configuration was the occurrence of a chemical processes occur during wired and unwired magneto-ionic redox/ionic lateral gradient within the zone where potentials are suf- actuation. ficiently high. Such gradient is likely to be present also for the vertical To gain more insight into the voltage-driven phase transitions, configuration (perpendicular to the sample), but with much smaller X-ray absorption spectroscopy (XAS) and X-ray magnetic circular Nature Communications | (2023)14:6486 5 Article https://doi.org/10.1038/s41467-023-42206-5 5 nm 10 As-grown 3.1 As-treated_bottom As-treated_top L3 Intensity ( 106 counts) As-treated_top Intensity Ratio, L3/L2 8 3.0 L2 6 2.9 2.8 4 2.7 As-treated_bottom 2 As-grown 2.6 0 750 770 790 810 830 Energy Loss (eV) Fig. 3 | Transmission electron microscopy (TEM) and electron energy loss lines, L3/L2. e, f High-resolution TEM (HRTEM) images of the as-grown film, e, and spectroscopy (EELS) characterisation of the as-grown and as-treated CoN films. the film actuated at 15 V for 15 min. f The corresponding fast Fourier transform a, b High-angle annular dark-field (HADDF) scanning TEM images and the corre- patterns are shown in g and h, respectively. The broad rings in yellow and pink sponding elemental EELS mappings of the areas enclosed in orange of the as- correspond to the (200) and (111) reflections of cubic CoN. The spots in green prepared film, a, and the one subjected to 15 V for 15 min. b Red and green colours match the CoN (220) reflection. The spots corresponding to a lattice spacing of correspond to Co and N elements, respectively, for the EELS analysis. c EELS spectra 2.38 Å (red circles) agree well with the (110) reflection of orthorhombic Co2N acquired from the as-prepared film (black) and the two different regions of the (Pnnm) phase. The blue circles, corresponding to an interplanar spacing of 1.91 Å, treated film, the bottom layer (red) and the top layer of lean nitrogen (blue). The are consistent with the hexagonal Co (101) or hexagonal Co3N (201) orientation. spectra are normalized to the L2 line. d The calculated intensity ratios of the white dichroism (XMCD) measurements were performed. Figure 4a shows and C on the right panels of Fig. 4). Specifically, in contrast to the the Co L2,3-edge XAS spectra of the as-grown film and the films treated spectrum of the pristine state, an additional peak is detected at around under driving voltages of 15 V and 20 V for 15 min in the vertical con- 776 eV for the treated samples (Fig. 4, feature A). This is the char- figuration. The spectrum for the pristine sample strongly resembles acteristic pre-peak of Co2+. Additionally, there is an energy shift of the that of low-spin Co3+ XAS line shapes, which possess a prominent peak L3-edge maximum towards lower energies upon voltage application around 779 eV with a high-energy shoulder structure at the L3 edge. (Fig. 4, feature B). For example, the L3-edge maximum shifts from This ascertains the primary Co3+ valence state. Interestingly, the 779 eV for the pristine state to 778 eV for the sample actuated under spectra of the actuated films exhibit three noticeable features (A, B, 20 V. Similar behaviour is also visible for the Co L2-edge (feature C in Nature Communications | (2023)14:6486 6 Article https://doi.org/10.1038/s41467-023-42206-5 0.08 As-grown 15 V / 15 min 20 V / 15 min 0.04 XAS Intensity (arb. units) 0.00 LS Co3+ HS Co2+ Co metal 775 780 785 790 795 800 Photon Energy (eV) 0.002 XMCD (arb. units) 0.000 -0.002 775 780 785 790 795 800 Photon Energy (eV) Fig. 4 | Wireless magneto-ionic control of magnetism probed by soft X-ray Co3+, high-spin (HS) Co2+, and metallic Co are shown in the bottom panel of a. The absorption spectroscopy (XAS) and X-ray magnetic circular dichroism three panels on the right show enlarged views of the dashed rectangular areas, A, B, (XMCD). Co L2,3-edge XAS spectra, a, and XMCD, b, of the as-sputtered and actu- and C, in a. All XAS spectra are vertically shifted for clarity. The vertical dashed lines ated (applying driving voltages of 15 V and 20 V for 15 min) CoN films, represented mark the appearance of reduced Co2+ peak. The black downward arrows denote the by black, red and blue lines, respectively. The reference spectra of low-spin (LS) shift of Co L3 maximum. Fig. 4). The shifting is related to the partial reduction of Co3+ to lower uniform charging and corresponding redox processes on the CoN film oxidation states. Further, we find that the XMCD signal is practically give rise to sizable magnetization changes with high stability, whereas zero for the as-grown sample (Fig. 4b), as expected. The treated volatile magneto-ionic effects are observed for the horizontal BPE samples, however, possess gradually increased XMCD intensities, configuration, presumably due to the spatial gradient in charge and in resulting from a net magnetic moment from Co or CoN1-x species. This oxidation state. This work offers a paradigm for magnetoelectric agrees with the previous magnetometry results. Taken together, our actuation, that may extend the use of magneto-ionics to widespread XAS-XMCD studies corroborate the Co valence transition from Co3+ to areas beyond memories relying on electrically interconnected bits of Co2+ and occurrence of metallic Co, which are responsible for the data. This may comprise applications where electric wiring is a draw- wirelessly induced magnetic response. back, such as electrostimulation in medical treatments, microfluidics, magnonics or remotely actuated magnetic microrobots, amongst Outlook others. Furthermore, the actuation protocols presented here could be This work constitutes the experimental demonstration of wireless extrapolated to other materials, as a means to control other physical magneto-ionic effects in thin films via bipolar electrochemistry. The properties in a wireless manner, such as superconductivity, memris- study is performed using CoN films immersed in a liquid electrolyte. tors or insulator-metal transitions, thus further spanning the range of Evidence for nitrogen ion motion and the associated electronic applications and the technological impact of the obtained results. structure transitions has been obtained through microscopic and spectroscopic methods. In the absence of direct wired connections to Methods the actuated material, voltage-induced transitions from paramagnetic Sample preparation to ferromagnetic states are driven by the formation of induced Room-temperature magnetron sputtering deposition of the thin charged poles in the material in response to the externally applied films was performed in an AJA International ATC 2400 Sputtering voltage and the concomitant discharging processes (i.e., capacitive System. For the horizontal BPE samples, 50-nm CoN films were and faradaic components). We demonstrate that the ion-motion dri- deposited using a 50% N2/50% Ar atmosphere on [100]-oriented Si ven magnetic switching behaviour through BPE can be volatile or non- substrates (8 mm × 8 mm lateral size, 0.5 mm thick), precoated with volatile depending on the chosen experimental configuration with 20 nm of Ti and 60 nm of Au. The total pressure was set at a value of regard to the imposed external electric field: for the vertical setup, the 3.0 × 10–3 Torr for the sputtering. For the vertical BPE samples, a Nature Communications | (2023)14:6486 7 Article https://doi.org/10.1038/s41467-023-42206-5 change in lateral dimensions was adopted: layered structures of Ti dimensions, given the small thickness of the bipolar electrode (20 nm)/Au (60 nm)/Si of lateral dimension of 8 mm × 20 mm were (immersed sample) in the direction of the electric field. Therefore, an used as the substrates for depositing the 50 nm-thick CoN films, approximation was made where the immersed CoN is 1 mm in using the same sputtering conditions as for the horizontal BPE films. thickness is the only material taken into account. The conductivities The depositions were done while masking the Au layer to fix the used in the simulation were taken from the literature, i.e., 2.4 × lateral size of the magneto-ionic layers to 8 mm × 6 mm, permitting 101 S m–1 for CoN30, 4.6 × 10–2 S m–1 for an anhydrous PC electrolyte the full immersion of CoN in the electrolyte in our setup. with dissolved salts35,36. Bipolar electrochemistry experiments Vibrating sample magnetometry BPE experiments were carried out in homemade electrochemical Magnetic hysteresis loops were recorded ex situ at room temperature cells, as schematically depicted in Figs. 1a and 2a. The electrolyte using a VSM from Micro Sense (LOT-Quantum Design). All measure- solution was 0.1 M KI in PC. Anhydrous PC was used a solvent to ments were carried out with the applied magnetic field parallel to the prevent CoN hydrolysis in an aqueous media, which would yield film plane. The maximum magnetic field was 20 kOe. The hysteresis cobalt oxides. The presence of KI offers the required conductivity of loops were background-corrected to subtract the linear diamagnetic/ the electrolyte and it also supports the complementary oxidation paramagnetic contributions at high fields, where ferromagnetic com- reaction at the driving anode and the induced anode at the bipolar ponent signals get fully saturated. Please note that the vibrating fea- electrodes. The optimal electrolyte concentrations were screened tures of the VSM and the required dimensions of samples prevent in previously through a set of experiments with correlated magnetiza- operando conditions. tion measurements. Empirically, it was found that very small water contents improve the process, in a relatively narrow range between Transmission electron microscopy 200 and 800 ppm, and therefore, each experiment was performed TEM and EELS measurements were performed on an FEI Tecnai G2 F20 with a parallel evaluation of the water content using the Karl Fisher microscope with field emission gun operating at 200 kV. Cross- method (see Section 3, Supplementary Information). The driving sectional thin lamellae of the samples were cut by focused ion beam electrodes were a pair of parallel planar Pt plates (Goodfellow after the deposition of Pt protective layers and were subsequently 99.95%, rectangular of 15 mm × 35 mm size) that ensured a uniform placed onto a Cu TEM grid. electric field across the cell. External driving voltages were applied through a power supply (Agilent B2902A) across the connected Pt X-ray photoelectron spectroscopy electrodes immersed in the bipolar electrochemical cells. The vol- XPS analysis were carried out using a PHI 5500 Multitechnique System tage values presented throughout the manuscript always refer to spectrometer, equipped with a monochromatic Al Kα X-ray source external driving voltage values. After a careful optimization of dis- (1486.6 eV) at a power of 350 W, from Physical Electronics. Background tances between electrodes and sample, the Pt electrodes were dis- subtraction and peak fitting were performed with MagicPlot software. tanced 20 mm from each other for the vertical BPE alignment and The XPS spectra were corrected using the C 1 s line at 284.5 eV. 25 mm for the horizontal device configuration. Thus, the applied fields correspond to ΔV/20 and ΔV/25 Volt/mm in vertical and hor- Polarization dependent X-ray spectroscopy izontal configurations respectively. For the horizontal BPE devices, Room-temperature soft XAS and XMCD measurements were carried the samples were simply placed into a shallow pool containing a out at the BOREAS beamline of the ALBA synchrotron (Barcelona, 25 mL electrolyte solution and were aligned at the centre of the Pt Spain)37, under ultra-high vacuum conditions (base pressure below 10−7 plates. Millimetre graded papers were put underneath the electro- Torr). The Co L2,3 X-ray absorption was measured using circularly chemical cell to ensure the samples were positioned correctly in the polarized light with photon helicity parallel (μ+) or antiparallel (μ−) with centre and to ensure the reproducibility in all experiments. For the respect to a magnetic field of 1 Tesla, along the beam incidence vertical BPE experiments, the samples were vertically inserted in the direction, in a sequence of μ+μ−μ−μ+μ−μ+μ+μ− to disentangle the XMCD. electrolyte with the help of home-made Teflon supports halfway The average XAS and XMCD spectra were calculated as the sum and between the driving Pt electrodes. In order to avoid undesired elec- the difference, respectively, of the XAS spectra measured with two trochemical processes and to increase the effective potential on the opposite helicities. CoN electrode, the back of the immersed CoN sample (Pt) is elec- tronically connected to the Au underlayer through a U-shaped Pt foil. Data availability Iodide ions form I2 (or I3 ) during the process, corresponding to the All data that support the findings of this study are present in the Article yellow-orange colours observed in the pictures (Sections 1 and 2, and/or its Supplementary Information. 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ALBA Synchrotron with the collaboration of ALBA staff. J.S. thanks the Phys. C. Supercond. 235–240, 563–564 (1994). Spanish “Fábrica Nacional de Moneda y Timbre” (FNMT) for fruitful dis- 21. Jeong, J. et al. Suppression of Metal-Insulator Transition in VO2 by cussions. E.M. is a Serra Húnter Fellow. electric field-induced oxygen vacancy formation. Science 339, 1402–1405 (2013). Author contributions 22. Fosdick, S. E., Knust, K. N., Scida, K. & Crooks, R. M. Bipolar elec- J.S., N.C-P., E.P. and E.M. conceived and supervised the study. Z.M. trochemistry. Angew. Chem. - Int. Ed. 52, 10438–10456 (2013). prepared the samples with input from Z.T., and L.A. helped with the 23. Mavré, F. et al. Bipolar electrodes: a useful tool for concentration, fabrication of the samples with desired dimensions. Z.M., N.C-P and L.F- separation, and detection of analytes in microelectrochemical R. performed the bipolar electrochemistry experiments, with N.C-P systems. Anal. Chem. 82, 8766–8774 (2010). conceptions. Z.M. and J.S. performed and analysed the VSM measure- 24. Fuentes-Rodríguez, L. et al. Dramatic drop in cell resistance through ments. Z.M. and E.M. performed TEM and EELS measurements and induced dipoles and bipolar electrochemistry. J. Electrochem. Soc. analysed the results. Z.M. and E.P. performed and analysed the XPS 169, 016508 (2022). spectroscopy. J.H-M. and Z.M. performed the XAS/XMCD experiments 25. Eden, A. et al. Discharging behavior of confined bipolar electrodes: and analysed the data. L.F-R. and N.C-P. performed linear sweep vol- coupled electrokinetic and electrochemical dynamics. Electro- tammetry measurements. L.F-R. and L.A. conducted the COMSOL chim. Acta 330, 135275 (2020). simulations. Z.M. wrote the manuscript with inputs from J.S., E.M., 26. Fattah, Z., Garrigue, P., Lapeyre, V., Kuhn, A. & Bouffier, L. Con- E.P., N.C-P. and L.F-R. All authors contributed to discussions and editing. trolled orientation of asymmetric copper deposits on carbon microobjects by bipolar electrochemistry. J. Phys. Chem. C. 116, Competing interests 22021–22027 (2012). The authors declare no competing interests. Nature Communications | (2023)14:6486 9 Article https://doi.org/10.1038/s41467-023-42206-5 Additional information Open Access This article is licensed under a Creative Commons Supplementary information The online version contains Attribution 4.0 International License, which permits use, sharing, supplementary material available at adaptation, distribution and reproduction in any medium or format, as https://doi.org/10.1038/s41467-023-42206-5. long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if Correspondence and requests for materials should be addressed to changes were made. 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