Materials Horizons This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online COMMUNICATION View Journal | View Issue Frequency-dependent stimulated and post-stimulated voltage control of magnetism Open Access Article. Published on 28 October 2022. Downloaded on 2/22/2023 2:11:57 PM. Cite this: Mater. Horiz., 2023, 10, 88 in transition metal nitrides: towards brain-inspired magneto-ionics† Received 30th August 2022, Accepted 12th October 2022 Zhengwei Tan, a Julius de Rojas,a Sofia Martins, a Aitor Lopeandia,ab DOI: 10.1039/d2mh01087a Alberto Quintana,c Matteo Cialone,d Javier Herrero-Martı́n,e Johan Meersschaut, f André Vantomme, g José L. Costa-Krämer, h rsc.li/materials-horizons Jordi Sort *ai and Enric Menéndez *a Magneto-ionics, which deals with the change of magnetic proper- ties through voltage-driven ion migration, is expected to be one of New concepts the emerging technologies to develop energy-efficient spintronics. We propose an approach to control the magnetization evolution of While a precise modulation of magnetism is achieved when voltage electrolyte-gated CoN-based heterostructures both during (stimulated) is applied, much more uncontrolled is the spontaneous evolution of and after (post-stimulated) voltage actuation by means of electric-field- driven transport of N ions. Specifically, upon voltage removal (i.e., once magneto-ionic systems upon removing the electric stimuli (i.e., voltage stimuli are off), magnetization in our magneto-ionic systems can post-stimulated behavior). Here, we demonstrate a voltage- be either reduced or increased, without any further energy input, controllable N ion accumulation effect at the outer surface of depending on the interplay between film thickness and the prior CoN films adjacent to a liquid electrolyte, which allows for the voltage actuation protocols. The latter offers a new logical function that control of magneto-ionic properties both during and after voltage enables, for instance, the possibility to mimic neuromorphic learning under deep sleep. Such functionality cannot be emulated with existing pulse actuation (i.e., stimulated and post-stimulated behavior, types of neuromorphic materials (e.g., resistive switching or spintronic respectively). This effect, which takes place when the CoN film materials). Here, post-stimulated magneto-ionic control is possible by a thickness is below 50 nm and the voltage pulse frequency is at least voltage-driven N ion accumulation effect at the outer surface of CoN films 100 Hz, is based on the trade-off between generation (voltage ON) adjacent to a liquid electrolyte, which is generated for sufficiently thin and partial depletion (voltage OFF) of ferromagnetism in CoN by CoN films and high voltage actuation frequencies, thus offering a new concept, at the materials level, for advanced brain-inspired computing magneto-ionics. This novel effect may open opportunities for new functions. neuromorphic computing functions, such as post-stimulated neural learning under deep sleep. Introduction Magneto-ionics, relying on the control of magnetic properties through voltage-driven ion (O2 ,1–5 H+,6–8 Li+,9 F 10 or N3 11–14) a Departament de Fı́sica, Universitat Autònoma de Barcelona, E-08193 Cerdanyola migration, holds potential for being one of the technologies to del Vallès, Spain. E-mail: jordi.sort@uab.cat, enric.menendez@uab.cat b Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, develop low-power memory applications. In recent years, sig- Campus UAB, Cerdanyola del Vallès, E-08193 Barcelona, Spain nificant progress has been made in improving ion motion rates c Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, and endurance, which are the two main bottlenecks to exploit Bellaterra, E-08193 Barcelona, Spain the full potential of magneto-ionics at room temperature. This d CNR-SPIN Genova, Corso F. M. Perrone 24, 16152 Genova, Italy e has been achieved by proper selection of the moving ion ALBA Synchrotron Light Source, 08290 Cerdanyola del Vallès, Spain f IMEC, Kapeldreef 75, B-3001 Leuven, Belgium species, target materials, heterostructure designs, electrolytes g Quantum Solid State Physics, KU Leuven, Celestijnenlaan 200 D, B-3001 Leuven, and voltage actuation protocols. These advances have made it Belgium possible to propose magneto-ionic materials as suitable candi- h IMN-Instituto de Micro y Nanotecnologı́a (CNM-CSIC), Isaac Newton 8, PTM, dates for new spintronic applications,15,16 as well as for new 28760 Tres Cantos, Madrid, Spain i computation paradigms, such as neuromorphic systems.17,18 The Institució Catalana de Recerca i Estudis Avançats (ICREA), Pg. Lluı́s Companys 23, E-08010 Barcelona, Spain precise modulation of magnetism by magneto-ionics has been † Electronic supplementary information (ESI) available. See DOI: https://doi.org/ already successful to emulate the main synaptic functions: 10.1039/d2mh01087a activation threshold, plasticity (potentiation and depression), 88 | Mater. Horiz., 2023, 10, 88–96 This journal is © The Royal Society of Chemistry 2023 View Article Online Communication Materials Horizons spike-magnitude dependence, spike-timing-dependent plasticity, oxygen magneto-ionics. Prior to CoN deposition, a portion of spike-rate-dependent plasticity and forgetting.17,18 Plasticity is the Cu/Ti/Si substrate was masked to make the required electric linked to learning and memory, and, in particular, spike-rate- contact on Cu for in situ magnetoelectric experiments. The dependent plasticity is considered the most accurate model to distance between the target and the substrate was approxi- This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. describe learning and memory.19–24 mately 10 cm and the sputtering rate was about 1 Å s 1. CoN Among materials to mimic synapses, magneto-ionic systems was grown under a total pressure of 8  10 3 Torr with an stand out from phase-change compounds,25 resistive-switching atmosphere of N2 : Ar =1 : 1. oxides20,26–28 or spintronic multilayers21,23 basically due to the potential improvement of energy efficiency, envisaging ultra- Magneto-ionic characterization low power hardware with minimized Joule heating effects.29–31 In-plane magnetization was measured at room temperature Open Access Article. Published on 28 October 2022. Downloaded on 2/22/2023 2:11:57 PM. In addition, magneto-ionics could exploit the extra degree of using a commercial vibrating sample magnetometer (VSM) freedom provided by the vector nature of magnetization which from Micro Sense (LOT-Quantum Design), with a maximum may allow synapses to be directly programmed with positive and applied magnetic field of 2 T. The CoN samples were negative synaptic weights without need of additional electronics.17 electrolyte-gated (with an external Agilent B2902A power sup- However, while the weight (e.g., resistance,20,22,26,28 magneti- ply). A Pt wire was used as counter electrode, whereas the zation21,23) update in most current non-volatile technologies can working electrode was the investigated CoN/Cu/Ti/Si film. be manipulated by changing the input electric signal (value and (i.e., home-made electrochemical capacitor geometry). Negative sign), the post-stimulated (without input) states are usually not voltages here mean that negative charges accumulate at the controllable due to system-inherent relaxation effects.32,33 This working electrode (and vice versa for positive voltages). Anhy- post-stimulus uncontrollability greatly limits the emulation of drous propylene carbonate with Na+ and OH solvated species some important brain-inspired functions, such as maintaining (5–25 ppm) was used as electrolyte. Metallic sodium was learning efficiency even during deep sleep.34 Therefore, procedures immersed in the electrolyte to react with any traces of water. capable of controlling post-stimulated evolution while maintaining The magnetization (M) arises from normalizing the magnetic good tunability in modulation of weight update are highly desired. signal to the sample volume mounted in the electrolyte. Note Here, we propose a new approach to control the magnetiza- that the linear background of the hysteresis loops (stemming tion evolution both in stimulated and post-stimulated states from paramagnetic and diamagnetic contributions) was based on voltage-driven ferromagnetism in CoN-based hetero- removed using the signal at high fields (i.e., fields always far structures through the transport of N3 ions, with a cumulative above saturation fields). 10 2 s ion motion response. By application of successive voltage pulses, information processing, memory (retention), recovering or self-evolution by maturity (i.e., controllable updat- Structural and compositional measurements ing even when a voltage is no longer applied) can be emulated y/2y X-ray diffraction (XRD) patterns were measured on a in a controlled manner. Such control is possible by tuning the Materials Research Diffractometer (MRD) from Malvern PANa- CoN film thickness (which determines ion motion rates) and lytical company, equipped with a PIXcel1D detector, using the pulse frequency. The observed effects result from the trade- Cu Ka radiation. High-resolution transmission electron micro- off between generation (voltage ON) and partial depletion scopy (HRTEM), high-angle annular dark-field scanning trans- (voltage OFF) of magnetization. Besides boosting energy effi- mission electron microscopy (HAADF-STEM), and electron ciency, this approach offers an important additional logical energy loss spectroscopy (EELS) were performed on a TECNAI function: after voltage has been applied, the device can be F20 HRTEM/STEM microscope operated at 200 kV. Cross sec- either programmed to learn or forget without any further tional lamellae were prepared by focused ion beam and placed energy input, thus mimicking synaptic functions under deep onto a Cu transmission electron microscopy grid. X-ray absorp- sleep, when processing of information can continue without tion spectroscopy (XAS) at the Co L2,3 edges was performed any external signal input. using the BL29-BOREAS beamline at ALBA Synchrotron Light Source (Barcelona, Spain).35 The spectra were measured in total electron yield (TEY), under ultra-high vacuum conditions Experimental (10 7 Torr) and at room temperature (300 K). For the elastic recoil detection (ERD) analysis, an impinging ion beam of Sample preparation 9.6 MeV 79Br5+ particles was used. The sample was tilted to a CoN films with thicknesses ranging from 5 to 200 nm were grazing incidence angle of 201 between the ion beam and the grown using reactive sputtering, as described earlier.11 A 20 nm surface of the film. The elastic collision between the impinging Ti and 60 nm Cu buffer layers were first prepared by sputtering nuclei and target atoms can result in recoiling target species. on B-doped, highly conducting [100]-oriented Si wafers (60 nm The mass, energy and probability of the atoms emitted from the Cu/20 nm Ti/0.5 mm [100]-oriented Si). The CoN films were sample in forward direction were recorded by means of a multi- grown in a home-made triode sputtering system with a base dispersive detector telescope.36 ERD analysis yields a quantita- pressure as low as 10 8 Torr to provide an ultra-high vacuum, tive elemental depth profile of the sample, including that of oxygen-free growth condition, and exclude interferences of light elements. This journal is © The Royal Society of Chemistry 2023 Mater. Horiz., 2023, 10, 88–96 | 89 View Article Online Materials Horizons Communication Results and discussion loop turns out to be highly dependent on the CoN film thick- Enhancing ion motion by film thickness reduction: enabling ness. Since the largest generation of magnetization takes place magneto-ionics for brain-inspired computing during the first 5 min (i.e., during the descending branch of the first hysteresis loop), saturation magnetization measurements This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. To investigate magneto-ionics, CoN films with a thickness as a function of time were recorded while electrolyte-gating at ranging from 5 to 200 nm were electrolyte-gated in an electro- 25 V to properly determine the ion motion rates at the chemical capacitor configuration using a Pt wire as counter beginning of the magneto-ionic effect from saturation magne- electrode (as sketched in Fig. 1a),3,11 while performing in-plane tization changes (Fig. 1c and d). For this, an external magnetic vibrating sample magnetometry (VSM). As shown in Fig. 1b, an field of 10 kOe, above the anisotropy field of the generated anhydrous polar liquid electrolyte (propylene carbonate, PC, ferromagnetic Co, was applied to ensure magnetic saturation. Open Access Article. Published on 28 October 2022. Downloaded on 2/22/2023 2:11:57 PM. with Na+ and OH solvated species)37,38 was employed to allow At the initial stages of voltage actuation, the rate of magnetiza- the formation of an electric double layer (EDL)39 and to serve as tion generation increases as the CoN film thickness decreases. a N ion reservoir.11 This generation of magnetization reaches saturation (i.e., show- The as-grown CoN films are highly nanostructured (Fig. S1, ing a steady MS) faster for thin than for thick films. However, ESI†) and textured along (111) (Fig. S2, ESI†). As seen in Fig. S3 the achieved steady value of MS increases with film thickness (ESI†), the X-ray absorption spectra of 5 nm- and 25 nm-thick (Table S1, ESI†) in agreement with the results of consecutive CoN films show virtually the same shape, suggesting that the magnetic hysteresis loops recorded under the same voltage CoN stoichiometry is generally preserved regardless of film actuation (Fig. S4, ESI†). In addition, the steady coercivities thickness. As seen in Fig. S4 (ESI†), the as-grown films exhibit HC and squareness MR/MS (%) follow a dissimilar trend with virtually no ferromagnetism (MS o 10 emu cm 3) in agreement thickness reduction. While coercivity decreases with film with the paramagnetic character of polycrystalline CoN.11 Con- reduction, squareness increases, suggesting that the generated secutive hysteresis loops (each lasting 30 min) were recorded ferromagnetic Co grows more uniformly, enhancing exchange while electrolyte-gating to track the evolution of magnetic interactions among Co clusters which are in detriment of properties with time and, thus, characterize the magneto- coercivity.40 Fig. 1d shows the time evolution of growth rates ionic response. Irrespectively of the film’s thickness, all pristine of generated magnetization for all CoN films obtained by CoN films exhibit magneto-ionic effects after being subjected to differentiating MS with respect to time t: dMS/dt. This allows a DC voltage of 25 V, confirming voltage-driven transport of determining initial ion motion rates. An around 7.4-fold N3 ions (Fig. S4 and Table S1, ESI†).11 As also seen in Fig. S4 enhancement of the highest ion motion rate is achieved by (ESI†), the timescale to achieve a steady magnetic hysteresis reducing the thickness from 200 nm to 5 nm, showing that of Fig. 1 Role of CoN film thickness in ion motion. (a) Schematic representation of the voltage actuation on CoN films by electrolyte-gating. (b) Sketch of the electric double layer (EDL) formed at the surface of the CoN films while electrolyte-gating at DV o 0. (c) Saturation magnetization (MS) as a function of time t for all the investigated CoN films under electrolyte-gating at 25 V while applying an external in-plane magnetic field of 10 kOe. (d) Derivative of the MS (t) dependences: dMS/dt of the results in panel (c). 90 | Mater. Horiz., 2023, 10, 88–96 This journal is © The Royal Society of Chemistry 2023 View Article Online Communication Materials Horizons 0.8 emu cm 3 s 1 and 5.9 emu cm 3 s 1, respectively. The latter decays, in agreement with the partial magnetization depletion represents the largest ion motion speed achieved so far in (i.e., recovery) that occurs in these systems, which shares the self-contained magneto-ionic approach (see Table S2, resemblance with a nitriding process of metallic Co.11 From a ESI†).11–13 Considering that 1 s is the time resolution of the synaptic-like viewpoint and taking saturation magnetization as This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. employed magnetometer, all CoN films clearly exhibit sub-1s synaptic weight, this allows mimicking plasticity (i.e., change in magneto-ionic response since the generated magnetic moment magnetization with time). Specifically, potentiation (i.e., mag- at 1 s is always larger than the highest sensitivity of the VSM netization increase, voltage on), representing learning, and (125 nemu), a value which implies changes in magnetic depression (i.e., magnetization reduction, voltage off), reprodu- moment above the resolution of the setup (Fig. S5 and cing forgetting17 can be mimicked. Fig. 2b also shows the Table S1, ESI†). In Table S1 (ESI†), the DM in 1 s increases evolution of MS vs. t upon subjecting the 200 nm CoN films Open Access Article. Published on 28 October 2022. Downloaded on 2/22/2023 2:11:57 PM. greatly as the thickness decreases, indicating a considerable to voltage pulses of different frequency where the generated improvement of the trigger efficiency of the magneto-ionic magnetization scales with frequency. As f increases from 1 to motion because of thickness reduction. This suggests feasibil- 100 Hz, the MS values at 960 s increase one order of magnitude, ity of our approach as a framework for synaptic-like materials from 17.6 to 168.2 emu cm 3 (see also Fig. S6, ESI†). Moreover, since synapse functions require responses to repeated 10 2 s the steady MS values and its recovery are also frequency stimuli.41 The leap in ion motion for thinner films is mainly dependent, emulating spike-rate-dependent plasticity. Resem- associated with the enhancement of the applied electric field bling human brain, repeated spiking (i.e., high frequency due to film thickness reduction. In a first approximation, the voltage pulsing) reinforces retention of information and thus system can be considered as a condenser-like structure with learning.43,44 Human learning also takes place by transforming parallel plates (i.e., parallel plate capacitor), as shown in ‘short-term memory’ (STM) into ‘long-term memory’ (LTM) Fig. 1b. Given a certain potential V, the electric field E is through the increase in frequency of the voltage stimuli.28 inversely proportional to active dielectric thickness (the total STM refers to the temporal storage of input information in thickness of the CoN film plus the EDL).42 Hence, the thinner the hippocampus, which lasts only a few seconds. By increasing the film, the thinner the active dielectric thickness and the the frequency of training and stimulation, information more intense the electric field, providing a higher driving force obtained from outside can be continuously transferred to and to overcome the energy barrier for ion diffusion and to accel- stored in the cerebral cortex for hours or even years (LTM). As erate transport of nitrogen ions. depicted in Fig. 2b, this transition can be mimicked by voltage- driven generation of ferromagnetism in CoN through voltage Dynamic (pulsed DC actuation) characterization of the pulse waves of different frequency. This learning emulation magneto-ionic response: tuning the learning performance and capacity raises from the pulsed DC actuation which results in a emulation of ‘self-learning by maturity’ function (with no trade-off between generation (voltage ON) and partial deple- external voltage input) tion/recovery (voltage OFF) of magnetization. With increased Since communication among neurons is frequency-encoded, a frequency, generation of magnetization prevails over dynamic dynamic characterization of the magneto-ionic response is recovery, resulting in larger MS values. As seen in Fig. S1b crucial to determine its capability to mimic synaptic functions. (ESI†), nitrogen migration when applying pulsed DC voltage Therefore, for magneto-ionic characterization, electrolyte- actuation takes place via planar-migration fronts, similarly to gating via electric voltage pulses was performed. Static constant CoN films subjected to DC voltage actuation,11 suggesting that voltages (DC) were used as reference. Fig. 2a shows schemati- the ion motion mechanisms are independent from voltage cally the voltage protocol used for the dynamic characteriza- actuation characteristics. tion: voltages square waves (from 0 V to 25 V) of period T (and Conversely, the 25 nm-thick CoN films offer a completely corresponding frequency of f =1/T) and pulse duration of T/2 different scenario, where trade-off between generation (voltage (resulting in a duty cycle of 50%). The period was varied from ON) and partial depletion (voltage OFF) of magnetization is 102 s to 10 2 s to obtain actuation frequencies from 10 2 Hz to strongly altered. Specifically, for DC and pulsed actuation at 102 Hz. CoN films with thicknesses of 25 nm and 200 nm were high frequencies (i.e., 1, 10 and 100 Hz), once the voltage chosen as representative heterostructures to characterize the actuation is switched off (i.e., after t = 620 s in Fig. 2c), the magneto-ionic response during pulsed DC voltage actuation, saturation magnetization increases (Fig. 2c), rather than due to their significantly different ion motion speed. Both films decreasing (as for the 200 nm-thick film), hence emulating a were electrolyte-gated at different frequencies for gating times self-learning process with no external stimulus. This suggests of around 550 s and 900 s, respectively, to ensure reaching a that, once voltage is switched off, an additional driving force is saturated magnetization state. While gating, time-dependent set in place in the thinner films to pump ions towards the magnetization measurements under an applied magnetic field electrolyte, which screens and, in fact, overcomes recovery. This of 10 kOe were recorded by VSM. self-learning effect is particularly obvious after non-pulsed DC As can be seen in Fig. 2b, which corresponds to the voltage actuation (blue curve in Fig. 2c). Given the high motion magneto-ionic response of the 200 nm film, MS gradually rates of N3 ions across the 25 nm-thick CoN films (much larger increases up to 248 emu cm 3 upon treatment at 25 V DC than for thicker films –see Table S1, ESI†) and the limited N3 voltage for 900 s. Then, after turning off the voltage, MS slightly solubility in the liquid electrolyte,45,46 a N3 ion accumulation This journal is © The Royal Society of Chemistry 2023 Mater. Horiz., 2023, 10, 88–96 | 91 View Article Online Materials Horizons Communication This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 28 October 2022. Downloaded on 2/22/2023 2:11:57 PM. Fig. 2 Pulsed DC (dynamic) voltage actuation. (a) Schematic representation of the DC and pulsed DC voltage actuations. (b) Time evolution of the saturation magnetization (MS vs. t) while electrolyte-gating at 25 V with DC and pulsed DC (at f from 1 to 100 Hz) voltages for the 200 nm CoN film. STM and LTM represent the short-term memory and the long-term memory, respectively. (c) Time evolution of the saturation magnetization (MS vs. t) while electrolyte-gating at 25 V with DC and pulsed DC voltages (at f from 0.01 Hz to 100 Hz) for the 25 nm CoN film. (d) Zoom (A) of panel (c). (e) Zoom (B) of panel (c). effect at the interface between the magneto-ionic film and the starts (after 170 s for 50 nm and after 695 s for 25 nm thick liquid reservoir is envisaged. Due to the N3 concentration films). This effect becomes less pronounced when the actuation gradient, these ions will be presumably released to the electro- frequency is decreased. In such a case, dynamic recovery occurs lyte once voltage is switched off (following Fick’s laws47), leading already during the DV = 0 V intervals of the pulse voltage to the increase of MS. Note that whereas the frequency influence actuation, precluding the ion accumulation effect at the inter- on M is clear, the larger M at higher frequency may also be due face between the CoN films and the electrolyte (Fig. 2c). Indeed, to the larger number of applied pulses, which results in more when using a frequency of 0.01 Hz, recovery can be clearly nitrogen ion movement and, thus, higher amounts of Co observed upon voltage actuation (Fig. 2c and d), during the sub- formed behind. segments with DV = 0 V. Fig. 2d and e show two zooms of the MS Remarkably, compared with 25 nm-thick CoN films, the vs. t dependencies for voltage actuations at a frequency of 50 nm-thick ones display a weaker increase of MS when 0.01 Hz. In zoom (A), a rise followed by a continuous depletion the voltage actuation is turned off (Fig. S7, ESI†) whereas the of MS can be clearly seen (Fig. 2d). Zoom (B) indicates that the 200 nm-thick films do not show this effect at all (Fig. 2b), system exhibits an effective cyclability due to the trade-off indicating that the voltage-driven ion motion rate across the between generation and magnetization recovery (Fig. 2e). Par- film is sufficiently slow in that case. Thus, the ion motion rate tial cyclability can be reproduced without the need of biases of is a critical parameter to observe ion accumulation. As seen in opposite polarity, providing an energy-efficient and endurable Fig. S7 (ESI†), this voltage-free magnetization generation is way to cycle the system, since repeatedly switching voltage limited in time since, after a while, magnetization recovery polarities is often linked to degradation. Remarkably, this ion 92 | Mater. Horiz., 2023, 10, 88–96 This journal is © The Royal Society of Chemistry 2023 View Article Online Communication Materials Horizons accumulation effect can be tuned by controlling just two external parameters: the CoN film thickness, which largely determines the ion motion speed, and the frequency of the pulsed voltage actuation. This demonstrates that it is possible This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. to tune learning and forgetting at will by solely modifying the voltage actuation frequency since this parameter allows tuning the interplay between magnetization generation, recovery and, thus, ion accumulation at the CoN film/reservoir interface. Existing emulation of learning in artificial intelligence heavily relies on external inputs, which makes it difficult to simulate Open Access Article. Published on 28 October 2022. Downloaded on 2/22/2023 2:11:57 PM. some synaptic functions under deep sleep, when information retention can continue even after stopping the input signal. In fact, the possibility of learning even without stimulus is analo- gous to a ‘learning by maturity’ process, i.e., the possibility to remember fine details after resting. During sleep, biological synapses relax but maintain the brain’s neuroplasticity and the ability to learn.34 Recently, incorporation of slow-wave sleep periods has been shown advantageous also for artificial neuro- morphic computing algorithms, to enhance the dynamical computation stability.48 Remarkably, the reported effect could also be understood as a useful method to simulate overstimula- tion and neuronal malfunction.49 Microscopic mechanism behind the magneto-ionic emula- tion of the self-learning neuromorphic function Fig. 3a sche- matically shows the MS vs. t evolution for systems displaying and lacking the ion accumulation effect, respectively. If we define MS0 as the steady saturation magnetization achieved while actuating with voltage and MS1 the saturation magnetiza- tion reached within a short period of time after removing the voltage, the sign of the difference between MS0 and MS1 will Fig. 3 Frequency dependence of the ion accumulation effect. (a) Sche- represent the existence or absence of the ion accumulation matic representation of the typical MS vs. t dependences of heterostruc- effect: MS0–MS0 4 0 (no accumulation) and MS0–MS0 o 0 tures with thin CoN films (thicknesses r 50 nm) showing (DC and pulsed (accumulation). The ion accumulation effect at the surface of DC at a frequency Z 1 Hz: blue dashed line) or not showing (pulsed DC at a the CoN layer can be understood as the interplay between frequency o 1 Hz: red dots) an ion accumulation interfacial effect. (b) (c) microscopic mechanisms for the existence and absence of ion accumula- several dynamic processes at the atomic level. Highlighted tion, respectively. nE, nRed, nA, nRec represent the speed of N ion migration in among them are: (1) conventional electric-field-induced N3 CoN, redistribution of PC molecules, migration of accumulated N ions, and ion migration across CoN towards the electrolyte11 (represented N ion recovery, respectively. For simplicity, N denotes N3 in the picture. by a velocity vE), (2) redistribution (i.e., intermixing) between solvated N3 ions (identified as N-PC) and N-free PC molecules driven by the gradient50–52 in number of PC molecules carrying PC, i.e., absence of N-free PC molecules) and forcing N3 ions to N3 along the direction perpendicular to the interface (the accumulate at the surface of the CoN film. After switching the velocity of this redistribution process is represented by vRed), voltage off, the concentration gradient between PC molecules (3) diffusion of N3 ions accumulated at the interface (vA), and tends to balance, further redistributing regular PC molecules (4) N3 ion recovery from the electrolyte to the CoN film (vRec).11 and PC molecules saturated with N3 . In this way, some regular Fig. 3b schematically depicts the generation of magnetization PC molecules are able to reach the interface, providing more in a thin CoN from an atomistic viewpoint that shows ion sites to incorporate N and, thus, to dissolve it. At the same time, accumulation effects. When a negative voltage is applied, N3 the accumulated N3 ions at the upper part of the CoN films ions continuously migrate towards the electrolyte, leaving can diffuse into the electrolyte and inside the CoN films (i.e., behind Co-rich areas in the CoN film responsible for the towards the inner parts of the film, which normally occurs increase in saturation magnetization (stage (i) in Fig. 3b). quite slowly). The former process, i.e., N dissolving in the Concomitantly, N3 dissolution takes above. However, after a electrolyte, is the mechanism behind the increase in MS even while, the generation of saturation magnetization slows down, without voltage (stage (iii) in Fig. 3b). After some time, the N3 tending to saturate (stage (ii) in Fig. 3b). This is ascribed to the diffusion process in the film weakens due to the decreasing fact that the N3 ion migration occurs faster than the redis- concentration gradient and becomes surpassed by the conven- tribution of PC molecules, thus limiting further dissolution of tional ion recovery process, which accounts for the decrease in N3 ions into the electrolyte (due to the limited N solubility in MS (stage (iv) in Fig. 3b). Remarkably, ion accumulation takes This journal is © The Royal Society of Chemistry 2023 Mater. Horiz., 2023, 10, 88–96 | 93 View Article Online Materials Horizons Communication This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 28 October 2022. Downloaded on 2/22/2023 2:11:57 PM. Fig. 4 Direct proof of the ion accumulation effect. (a) Elastic recoil detection measurements revealing the N depth distribution of an as-prepared and an actuated ( 25 V DC voltage for 60 s) 50 nm-thick film, with a zoomed-in view of the area surrounded by the purple dashed box. (b) Cartoon representing the N ion distribution in an as-prepared sample and an actuated sample showing the ion accumulation effect. place only in thin films (thickness r 50 nm), since they exhibit magneto-ionic properties both while and upon removing vol- sufficiently high voltage-driven ion motion (i.e., vE) values when tage pulse actuation (effect off and on, resulting in stimulated actuated with either DC or pulsed DC voltage at high frequen- and post-stimulated behaviour, respectively), expanding the cies ( f Z 1, 10 and 100 Hz). Fig. 3c shows the atomistic potential of magneto-ionics in the emulation of post-stimulated mechanism when the heterostructure does not show ion neuromorphic functions, such as neural learning under deep sleep. accumulation effects, which corresponds to the situation in which the frequency of the pulsed DC actuation is low (e.g., f = 0.01 Hz). Even though weak ion accumulation effects Author contributions are feasible (stage (I)), they tend to rapidly vanish due to the Z. T., J. S. and E. M. designed the experiment and the study. J. L. long voltage interruption between the consecutive pulses, facil- C.-K. synthesized the material. Z. T., S. M., A. Q., J. S. and E. M. itating recovery processes and thus causing a partial magneti- carried out the VSM measurements and analysed the data. Z. T. zation depletion (stages (II) and (III)) during pulsing, as and J. L. C.-K. performed the XRD characterization. Z. T. and experimentally seen in Fig. 2c and e and depicted in the red A. L. prepared the cross-section lamellae for TEM. Z. T., A. L., dotted curve of Fig. 3a. J. S. and E. M. performed the HRTEM, STEM-HAADF and EELS With the goal of obtaining direct evidence of this ion and the corresponding data analysis. Z. T., J. deR., M. C. and accumulation effect at the interface between the film and the J. H.-M. conducted the XAS experiments. J. M. and A. V. carried electrolyte, the N3 depth distribution of a 50 nm-thick sample out the elastic recoil detection (ERD) analysis. All authors upon DC voltage actuation has been assessed by elastic recoil discussed the results and commented on the article. The article detection analysis. As clearly seen in Fig. 4a, the depth dis- was written by Z. T., J. S. and E. M. tribution of N3 for as-prepared CoN films is symmetric but becomes asymmetric during the DC voltage treatment. After applying a short 60 s DC voltage, this distribution is positively Conflicts of interest skewed, confirming that the N3 content at the upper part of There are no conflicts to declare. the film is larger than that at the bottom (i.e., deeper) region and, thus, validating the ion accumulation effect (Fig. 4b). Acknowledgements Conclusion Financial support by the European Research Council (MAGIC- Our work demonstrates a controllable ion accumulation effect SWITCH 2019-Proof of Concept Grant, Agreement No. 875018), at the interface between the CoN films and the adjacent the European Union’s Horizon 2020 research and innovation electrolyte, which is set by a proper selection of the CoN film programme (European Training Network, BeMAGIC ETN/ITN thickness and the pulse frequency of the voltage actuation. This Marie Skłodowska-Curie grant No. 861145; and Integrated effect, which takes place when the CoN film thickness is below Infrastructure, RADIATE, grant No. 824096), the Spanish Gov- 50 nm and the voltage pulse frequency is at least 100 Hz, is ernment (MAT2017-86357-C3-1-R, PID2020-116844RB-C21 and based on the trade-off between generation (voltage ON) and PDC2021-121276-C3), the Generalitat de Catalunya (2017-SGR- partial depletion (voltage OFF) of ferromagnetism in CoN by 292 and 2018-LLAV-00032), the European Regional Develop- magneto-ionics. This effect allows for both the control of ment Fund (MAT2017-86357-C3-1-R and 2018-LLAV-00032) and 94 | Mater. Horiz., 2023, 10, 88–96 This journal is © The Royal Society of Chemistry 2023 View Article Online Communication Materials Horizons the KU Leuven (BOF program) is acknowledged. A. Q. acknowl- 14 Z. Tan, S. Martins, M. Escobar, J. De Rojas, F. Ibrahim, edges financial support from the Spanish Ministry of Science, M. Chshiev, A. Quintana, A. Lopeandia, J. L. Costa-Krämer, Innovation and Universities through the ‘‘Severo Ochoa’’ Pro- E. Menéndez and J. Sort, ACS Appl. Mater. Interfaces, 2022, gramme for Centers of Excellence in R&D (FUNFUTURE 14, 44581. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. CEX2019-000917-S) and the Juan de la Cierva formación con- 15 L. Herrera Diez, R. Kruk, K. Leistner and J. Sort, APL Mater., tract (FJC2019-039780-I). J. S. thanks the Spanish Fábrica 2021, 9, 050401. Nacional de Moneda y Timbre (FNMT) for fruitful discussions. 16 Y. Gu, C. Song, Q. Wang, W. Hu, W. Liu, F. Pan and The XAS measurements were performed at BL29-BOREAS Z. Zhang, APL Mater., 2021, 9, 040904. beamline at ALBA Synchrotron with the collaboration of ALBA 17 R. Mishra, D. Kumar and H. Yang, Phys. Rev. Appl., 2019, staff. We acknowledge service from MiNa Laboratory at IMN, 11, 054065. Open Access Article. Published on 28 October 2022. Downloaded on 2/22/2023 2:11:57 PM. and funding from CM (project S2018/NMT-4291 TEC2SPACE), 18 S. Martins, J. De Rojas, Z. Tan, M. Cialone, A. Lopeandia, MINECO (project CSIC13-4E-1794) and EU (FEDER, FSE). E. M. J. Herrero-Martı́n, J. L. Costa-Krämer, E. Menéndez and is a Serra Húnter Fellow. J. Sort, Nanoscale, 2022, 14, 842. 19 J. Xiong, R. Yang, J. Shaibo, H.-M. Huang, H.-K. He, W. Zhou, X. Guo, J. Xiong, R. Yang, J. Shaibo, H. Huang, H. He, References W. Zhou and X. Guo, Adv. Funct. Mater., 2019, 29, 1807316. 1 U. Bauer, L. Yao, A. J. Tan, P. Agrawal, S. Emori, H. L. Tuller, 20 Z. Wang, S. Joshi, S. E. Savel’ev, H. Jiang, R. Midya, P. Lin, S. van Dijken and G. S. D. Beach, Nat. Mater., 2015, 14, 174. M. Hu, N. Ge, J. P. Strachan, Z. Li, Q. Wu, M. Barnell, 2 D. A. Gilbert, A. J. Grutter, E. Arenholz, K. Liu, B. J. Kirby, G. L. Li, H. L. Xin, R. S. Williams, Q. Xia and J. J. Yang, Nat. J. A. Borchers and B. B. Maranville, Nat. Commun., 2016, 7, 12264. Mater., 2017, 16, 101. 3 J. de Rojas, A. Quintana, A. Lopeandı́a, J. Salguero, J. L. Costa- 21 J. Torrejon, M. Riou, F. A. Araujo, S. Tsunegi, G. Khalsa, Krämer, L. Abad, M. O. Liedke, M. Butterling, A. Wagner, D. Querlioz, P. Bortolotti, V. Cros, K. Yakushiji, A. Fukushima, L. Henderick, J. Dendooven, C. Detavernier, J. Sort and H. Kubota, S. Yuasa, M. D. Stiles and J. Grollier, Nature, 2017, E. Menéndez, Adv. Funct. Mater., 2020, 30, 2003704. 547, 428. 4 A. Quintana, E. Menéndez, M. O. Liedke, M. Butterling, 22 H. Yeon, P. Lin, C. Choi, S. H. Tan, Y. Park, D. Lee, J. Lee, A. Wagner, V. Sireus, P. Torruella, S. Estradé, F. Peiró, F. Xu, B. Gao, H. Wu, H. Qian, Y. Nie, S. Kim and J. Kim, Nat. J. Dendooven, C. Detavernier, P. D. Murray, D. A. Gilbert, Nanotechnol., 2020, 15, 574. K. Liu, E. Pellicer, J. Nogues and J. Sort, ACS Nano, 2018, 23 J. Grollier, D. Querlioz, K. Y. Camsari, K. Everschor-Sitte, 12, 10291. S. Fukami and M. D. Stiles, Nat. Electron., 2020, 3, 360. 5 M. Cialone, A. Nicolenco, S. Robbennolt, E. Menéndez, 24 T. Ohno, T. Hasegawa, T. Tsuruoka, K. Terabe, J. K. Gimzewski G. Rius and J. Sort, Adv. Mater. Interfaces, 2021, 8, 2001143. and M. Aono, Nat. Mater., 2011, 10, 591. 6 A. J. Tan, M. Huang, C. O. Avci, F. Büttner, M. Mann, W. Hu, 25 W. Zhang, R. Mazzarello, M. Wuttig and E. Ma, Nat. Rev. C. Mazzoli, S. Wilkins, H. L. Tuller and G. S. D. Beach, Nat. Mater., 2019, 4, 150. Mater., 2019, 18, 35. 26 F. Zhou, Z. Zhou, J. Chen, T. H. Choy, J. Wang, N. Zhang, 7 M. Gößler, M. Albu, G. Klinser, E. M. Steyskal, H. Krenn and Z. Lin, S. Yu, J. Kang, H.-S. P. Wong and Y. Chai, Nat. R. Würschum, Small, 2019, 15, 1904523. Nanotechnol., 2019, 14, 776. 8 X. Ye, H. K. Singh, H. Zhang, H. Geßwein, M. R. Chellali, 27 M. Prezioso, F. Merrikh-Bayat, B. D. Hoskins, G. C. Adam, R. Witte, A. Molinari, K. Skokov, O. Gutfleisch, H. Hahn and K. K. Likharev and D. B. Strukov, Nature, 2015, 521, 61. R. Kruk, Nat. Commun., 2020, 11, 4849. 28 J. Rao, Z. Fan, L. Hong, S. Cheng, Q. Huang, J. Zhao, X. Xiang, 9 S. Dasgupta, B. Das, M. Knapp, R. A. Brand, H. Ehrenberg, E.-J. Guo, H. Guo, Z. Hou, Y. Chen, X. Lu, G. Zhou, X. Gao and R. Kruk and H. Hahn, Adv. Mater., 2014, 26, 4639. J.-M. Liu, Mater. Today Phys., 2021, 18, 100392. 10 S. Vasala, A. Jakob, K. Wissel, A. I. Waidha, L. Alff and 29 M. Bibes and A. Barthélémy, Nat. Mater., 2008, 7, 425. O. Clemens, Adv. Electron. Mater., 2020, 6, 1900974. 30 A. Molinari, H. Hahn and R. Kruk, Adv. Mater., 2019, 11 J. de Rojas, A. Quintana, A. Lopeandı́a, J. Salguero, B. Muñiz, 31, 1806662. F. Ibrahim, M. Chshiev, A. Nicolenco, M. O. Liedke, 31 M. Nichterwitz, S. Honnali, M. Kutuzau, S. Guo, J. Zehner, M. Butterling, A. Wagner, V. Sireus, L. Abad, C. J. Jensen, K. Nielsch and K. Leistner, APL Mater., 2021, 9, 30903. K. Liu, J. Nogués, J. L. Costa-Krämer, E. Menéndez and J. Sort, 32 J. J. Yang and Q. Xia, Nat. Mater., 2017, 16, 396. Nat. Commun., 2020, 11, 5871. 33 J. Tian, Z. Tan, Z. Fan, D. Zheng, Y. Wang, Z. Chen, F. Sun, 12 J. de Rojas, J. Salguero, F. Ibrahim, M. Chshiev, A. Quintana, D. Chen, M. Qin, M. Zeng, X. Lu, X. Gao and J.-M. Liu, Phys. A. Lopeandia, M. O. Liedke, M. Butterling, E. Hirschmann, Rev. Appl., 2019, 11, 024058. A. Wagner, L. Abad, J. L. Costa-Krämer, E. Menéndez and 34 S. Fattinger, T. T. de Beukelaar, K. L. Ruddy, C. Volk, N. C. Heyse, J. Sort, ACS Appl. Mater. Interfaces, 2021, 13, 30826. J. A. Herbst, R. H. R. Hahnloser, N. Wenderoth and R. Huber, Nat. 13 J. de Rojas, J. Salguero, A. Quintana, A. Lopeandia, M. O. Liedke, Commun., 2017, 8, 15405. M. Butterling, A. G. Attallah, E. Hirschman, A. Wagner, L. Abad, 35 A. Barla, J. Nicolás, D. Cocco, S. M. Valvidares, J. Herrero- J. L. Costa-Krämer, J. Sort and E. Menéndez, Phys. Rev. Appl., Martı́n, P. Gargiani, J. Moldes, C. Ruget, E. Pellegrin and 2021, 16, 034042. S. Ferrer, J. Synchrotron Radiat., 2016, 23, 1507. This journal is © The Royal Society of Chemistry 2023 Mater. Horiz., 2023, 10, 88–96 | 95 View Article Online Materials Horizons Communication 36 J. Meersschaut and W. Vandervorst, Nucl. Instrum. Methods 45 J. Read, K. Mutolo, M. Ervin, W. Behl, J. Wolfenstine, Phys. Res., Sect. B, 2017, 406, 25. A. Driedger and D. Foster, J. Electrochem. Soc., 2003, 37 C. Navarro-Senent, A. Quintana, E. Menéndez, E. Pellicer 150, A1351. and J. Sort, APL Mater., 2019, 7, 030701. 46 W. Chen, M. Chen, M. Yang, E. Zou, H. Li, C. Jia, C. Sun, This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. 38 G. G. Eshetu, G. A. Elia, M. Armand, M. Forsyth, S. Komaba, Q. Ma, G. Chen and H. Qin, Appl. Energy, 2019, 240, 265. T. Rojo and S. Passerini, Adv. Energy Mater., 2020, 10, 2000093. 47 I. M. Sokolov, J. Klafter and A. Blumen, Phys. Today, 2002, 39 M. Weisheit, S. Fähler, A. Marty, Y. Souche, C. Poinsignon 55, 48. and D. Givord, Science, 2007, 315, 349. 48 Y. Watkins, E. Kim, A. Sornborger and G. T. Kenyon. In 40 E. Menéndez, J. Demeter, J. van Eyken, P. Nawrocki, E. Jedryka, Proceedings of the IEEE/CVF Conference on Computer M. Wójcik, J. F. Lopez-Barbera, J. Nogués, A. Vantomme and Vision and Pattern Recognition Workshops, TN, USA, July, Open Access Article. Published on 28 October 2022. Downloaded on 2/22/2023 2:11:57 PM. K. Temst, ACS Appl. Mater. Interfaces, 2013, 5, 4320. 2020. 41 C. C. McIntyre and R. W. Anderson, J. Neurochem., 2016, 139, 338. 49 H. Huang, C. Ge, Z. Liu, H. Zhong, E. Guo, M. He, C. Wang, 42 D. Halley, N. Najjari, H. Majjad, L. Joly, P. Ohresser, G. Yang and K. Jin, J. Semicond., 2021, 42, 103103. F. Scheurer, C. Ulhaq-Bouillet, S. Berciaud, B. Doudin and 50 W. Guo, L. Cao, J. Xia, F.-Q. Nie, W. Ma, J. Xue, Y. Song, D. Zhu, Y. Henry, Nat. Commun., 2014, 5, 3167. Y. Wang and L. Jiang, Adv. Funct. Mater., 2010, 20, 1339. 43 D. Hassabis, D. Kumaran, C. Summerfield and M. Botvinick, 51 A. Ozcan, C. Perego, M. Salvalaglio, M. Parrinello and Neuron, 2017, 95, 245. O. Yazaydin, Chem. Sci., 2017, 8, 3858. 44 D. Marković, A. Mizrahi, D. Querlioz and J. Grollier, Nat. 52 A. P. Thompson, D. M. Ford and G. S. Heffelfinger, J. Chem. Rev. Phys., 2020, 2, 499. Phys., 1998, 109, 6406. 96 | Mater. Horiz., 2023, 10, 88–96 This journal is © The Royal Society of Chemistry 2023