Nanoscale Horizons View Article Online COMMUNICATION View Journal | View Issue Enhancing magneto-ionic effects in cobalt oxide This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. films by electrolyte engineering† Open Access Article. Published on 25 November 2022. Downloaded on 3/2/2023 6:05:00 PM. Cite this: Nanoscale Horiz., 2023, 8, 118 Sofia Martins, *a Zheng Ma, a Xavier Solans-Monfort, b Mariona Sodupe, b Received 19th July 2022, Luis Rodriguez-Santiago, b Enric Menéndez, a Eva Pellicer *a and Accepted 15th November 2022 Jordi Sort ac DOI: 10.1039/d2nh00340f rsc.li/nanoscale-horizons Electric-field-driven ion motion to tailor magnetic properties of materials (magneto-ionics) offers much promise in the pursuit of New concepts voltage-controlled magnetism for highly energy-efficient spintro- Voltage control of magnetism (VCM) through electric field-induced ion nic devices. Electrolyte gating is a relevant means to create intense motion, referred to as magneto-ionics, is a cutting-edge research topic since it holds the promise to revolutionize current strategies to enhance electric fields at the interface between magneto-ionic materials and energy efficiency in magnetically actuated devices (e.g., micro-electro- electrolytes through the so-called electric double layer (EDL). Here, mechanical systems, magnetic logics, spintronics, or neuromorphic improved magneto-ionic performance is achieved in electrolyte- computing). Modifying the film composition/microstructure or the gated cobalt oxide thin films with the addition of inorganic salts working conditions are the typical approaches followed in the literature (potassium iodide, potassium chloride, and calcium tetrafluorobo- to boost magneto-ionics. However, at present, room temperature oxygen magneto-ionics is still too slow for relevant applications. Increased O2 rate) to anhydrous propylene carbonate (PC) electrolyte. Ab initio motion speeds and smaller threshold voltages are highly desirable. Here, molecular dynamics simulations of the EDL structure show that K+ we demonstrate that O2 motion in electrolyte-gated cobalt oxide films is preferentially located on the cobalt oxide surface and KI (when can be significantly enhanced by suitable ‘electrolyte engineering’ in compared to KCl) favors the accumulation of positive charge close films of a few nm in thickness. In particular, inorganic salts (e.g., KI, to the surface. It is demonstrated that room temperature magneto- KCl and (Ca(BF4)2), have been added to anhydrous propylene carbonate (PC) to modulate the ionic strength and, in turn, the electric field at the ionics in cobalt oxide thin films is dramatically enhanced in KI- cobalt oxide/electrolyte interface by means of the built-in electric double containing PC electrolyte at an optimum concentration, leading to layer. For KI-containing PC, a 35-fold increase of the magneto-ionic rate is 11-fold increase of generated magnetization and 35-fold increase observed compared to plain PC and, importantly, under a relatively low of magneto-ionic rate compared to bare PC. bias value (o 1.5 V), much lower than the voltage values needed so far to induce magneto-ionics in this kind of systems. Introduction Advancements in further downsizing the next-generation spin- magnetic properties by an electric field. This can be accom- tronic devices while minimizing their Joule power dissipation plished, to some extent, in ME multiferroics, where ferroelec- have been remarkable in the last decades.1,2 Voltage control of tricity and magnetism coexist and are usually coupled. magnetism (VCM) promises great potential to reduce the Nevertheless, several challenges must be overcome, such as energy demands of spintronics. As such, magnetoelectric the scarcity of intrinsic single-phase ME materials, the weak (ME) effects, related to the coupling between magnetic and coupling between electric polarization and magnetization at electric behaviors in matter, have been proposed as one of the room temperature, and the clamping effects in the case of ME most promising avenues to realize the manipulation of composites (or heterostructured multiferroics). Alternatively, voltage-driven ion motion to tune magnetism (i.e., magneto- a Departament de Fı́sica, Universitat Autònoma de Barcelona, E-08193 Cerdanyola ionics) has recently attracted great attention thanks to its ability del Vallès, Spain. E-mail: CarlaSofia.Martins@uab.cat, Eva.Pellicer@uab.cat to considerably modulate magnetic properties, in a permanent b Departament de Quı́mica, Universitat Autònoma de Barcelona, E-08193 manner.3 In recent years, magneto-ionics has been demon- Cerdanyola del Vallès, Spain c strated in a wide variety of materials, including both, metals Institució Catalana de Recerca i Estudis Avançats (ICREA), Pg. Lluı́s Companys 23, E-08010 Barcelona, Spain and semiconductors.4 † Electronic supplementary information (ESI) available. See DOI: https://doi.org/ Electrolyte-gating through the formation of an electric dou- 10.1039/d2nh00340f ble layer (EDL) has shown to be a very convenient means to 118 | Nanoscale Horiz., 2023, 8, 118–126 This journal is © The Royal Society of Chemistry 2023 View Article Online Communication Nanoscale Horizons tailor the magnetic properties of magneto-ionic systems. These solvate large amounts of salts but it is toxic, while PC is more systems work as electrochemical capacitors (EC), i.e., super- environmentally friendly, offers a wide electrochemical window capacitors, thereby involving the formation of an EDL between and operating temperature, as well as conductivity of the solid material and the electrolyte. At least three types of ECs 1–2  10 8 Ohm 1 cm 1.5,15,16 Indeed, PC is a dipolar protic can be distinguished, namely electrical double layer capacitors solvent with high dielectric constant (64.40 at 25 1C) and a large (EDLC), pseudocapacitors, and a combination of both.5,6 What- dipole moment (4.81 D) which has been considered an ideal ever the type, ECs refer to a system capable to accumulate structureless dielectric to solubilize strong electrolytes electrical charges at the surface of an electrode material in for a variety of applications. Since PC has a relatively large contact with an electrolyte. When voltage is applied to a dielectric constant, dissolved ionic compounds will be exten- This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. magneto-ionic material, the electric field causes changes in sively dissociated. Open Access Article. Published on 25 November 2022. Downloaded on 3/2/2023 6:05:00 PM. its saturation magnetization (MS) or coercivity (HC), among In spite of the achievements with VCM of Co oxide films, other magnetic properties, which can be finely tuned based room temperature magneto-ionics is still too slow for relevant on electrostatic and/or electrochemical mechanisms.7,8 applications. Even though sub-10 s magneto-ionics has recently According to the different models put in place to explain the been demonstrated by reducing the Co oxide thickness down to concept of EDLs formed at the interface between planar solids 5 nm,12 further improvements are required to enable magneto- and aqueous solutions, the simplest theory states that EDLs ionics in real devices. In addition, the applied voltages required form to neutralize the surface charge on the solid material by to observe a remarkable change in the magnetic properties of ions of opposite sign (Helmholtz model). The more elaborated cobalt oxide were rather large, of the order of 50 V. Recently, Gouy–Chapman–Stern model (GCS) is currently widely used to we showed that magneto-ionics depends on the electrical describe EDLs. It assumes the occurrence of two layers, one properties of the target materials.4 Here we demonstrate that fixed layer next to the electrode surface (Stern layer or Helm- not only the electrical properties of cobalt oxide are important, holtz region) of opposite charge, and a diffuse layer, wherein but also the composition of the electrolyte. More specifically, its ions can move freely. The Stern layer, in turn, is divided into the ionic strength also plays a key role in magneto-ionics. In the inner Helmholtz plane (IHP) and the outer Helmholtz plane past years, various salts and additives have been added to (OHP). The former has approximately one single ion thickness. solvents to increase the ionic strength of electrolytes for a Compared to aqueous solutions, studies on the structure of variety of purposes.17–19 As the salt concentration is increased, EDL in organic solvents are more limited. Feng et al. examined more ions can accumulate in the compact layer of the EDL, a system made of tetraethylammonium tetrafluoroborate reducing the presence of free solvent molecules in the vicinity (TEABF4) salt in acetonitrile (CH3CN) and found remarkable of the electrode. As a result, the performance of energy storage contact adsorption of TEABF4 ions to the surface of the elec- systems improves.17,20 This strategy could well work for trode and strong layering and orientational ordering of acet- magneto-ionics, paving the way for higher charge accumulation onitrile molecules until approx. 1.1 nm into the bulk at the surface of the solid material and, hence, more intense electrolyte.9 The EC based on interfacial ion-modulation has electric fields. To date, different electrolytes have been tested attracted much interest because of the giant charge-carrier for voltage-driven ion migration and, in some cases, salts were density accumulation achieved because of the large EDL capa- added to the solvent (e.g. LiPF6 in dimethyl carbonate/ethylene citance. Under electrolyte gating, permanent changes at the carbonate mixture) to observe the effects.7,21,22 In spite of the surface and the bulk of electrode material are possible, when- intensive research carried out in the last decade, studies with ever chemical reactions are involved. complex PC formulations are lacking. Materials such as carbon, metal oxides and nitrides, and Herein, we have investigated the impact of the addition of conducting polymers have been used in an EC.5,6 Metal oxides/ inorganic salts, namely potassium iodide (KI), potassium chlor- nitrides such as Co3O410–12 and CoN13,14 are very appealing for ide (KCl) and calcium tetrafluoroborate (Ca(BF4)2), to anhy- magneto-ionics, since they not only store energy like electro- drous PC on the magneto-ionic effects induced in 15 nm-thick static carbon materials but can also undergo electrochemical Co oxide films. The solubility of KI and KCl in PC at room reactions under voltage actuation, eventually leading to a fully temperature was reported by Peruzzi et al.23 and that of reversible transformation from a paramagnetic state (OFF) to a Ca(BF4)2 in PC was investigated by Forero-Saboya et al.17 (see ferromagnetic state (ON), and vice versa. This is explained by Experimental section). KCl was primarily chosen as it is the oxygen and nitrogen motion, respectively, under voltage actua- electrolyte support of choice in many electrochemical processes tion, the concurrent creation of atomic defects, and the con- taking place in aqueous solutions. KI was selected as it showed sequent generation of magnetic moment arising from metallic higher solubility in PC and simply implied the replacement of Co.10–14 Cl by I . Finally, Ca(BF4)2 was chosen since it is typically The maximum electric field achievable in ECs is limited dissolved in alkyl carbonates like PC in calcium ion batteries.24 by the breakdown voltage of the electrolyte. Four primary Our work reveals that improvements in oxygen ion motion25–27 classes of electrolyte solutions have been used in EC: organic, in cobalt oxide are accomplished by adding these salts to PC. aqueous, polymeric and ionic liquid electrolytes. Among Specifically, the fastest magneto-ionic rate is observed when organic electrolytes, ACN and propylene carbonate (PC) are using PC + 2.5  10 4 M KI electrolyte, which is 35 times larger the most widely used solvents. ACN presents the ability to than the rate observed for bare PC electrolyte under 1.5 V. This journal is © The Royal Society of Chemistry 2023 Nanoscale Horiz., 2023, 8, 118–126 | 119 View Article Online Nanoscale Horizons Communication Moreover, upon biasing Co oxide for one hour, the same PC + magnetic field, where the generated ferromagnetic response is 2.5  10 4 M KI electrolyte results in the largest generation of saturated. The external voltage was applied in situ during the magnetization. The process is highly reversible for all added magnetic measurements. Fig. 1(d) presents the time evolution salts since the generated magnetism can be reduced after of magnetization change (DM t) for the films actuated in bare applying a voltage of opposite polarity (i.e., +1.5 V). The PC and PC containing 2.5  10 5 M KI, KCl and Ca(BF4)2 magneto-ionic response of the cobalt oxide films was further electrolytes. In the first stage of the measurements, DM con- studied at varying concentrations of KI dissolved in PC. Our tinuously increases to around 30 emu cm 3 for the bare PC results show that improved magneto-ionics can be achieved by electrolyte while DV = 1.5 V is applied for 60 min (i.e., conveniently tuning the EDL strength via liquid electrolyte t = 65 min) (see the green shaded region in Fig. 1(d)). This This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. engineering. indicates the appearance of ferromagnetism in the cobalt Open Access Article. Published on 25 November 2022. Downloaded on 3/2/2023 6:05:00 PM. oxide. In the second stage (i.e., after applying +1.5 V for 25 min), DM sharply drops due to the reversal of the bias Results and discussion polarity, and reaches around 10 emu cm 3. The occurrence of a To investigate the magneto-ionic behavior of 15 nm thick Co ferromagnetic-paramagnetic (ON–OFF) transition in the oxide oxide films in different electrolyte solutions, a home-made films is related to voltage-induced oxygen ion motion.10 Several electrolytic cell, sketched in Fig. 1(a), was used which allowed observations can be made from the DM t curves for electro- to externally polarize the material while recording its magnetic lytes with different inorganic salts. First, for the PC + KCl response under in-plane configuration on a vibrating sample electrolyte, DM increases at a similar rate as the bare PC one magnetometer (VSM). As illustrated in Fig. 1(b), an electroki- until t = 20 min, and it then grows faster and approaches netic potential is created at the solid/electrolyte interface under 40 emu cm 3 at t = 65 min, representing a 33% increase no external electric field applied, considering that EDLs form compared to bare PC. Importantly, the initial DM change is whenever an electronic conducting material is placed in an significantly enhanced with the additions of KI and Ca(BF4)2 electrolyte containing mobile ions.3 This is of course a very into PC, as reflected by the larger slopes of the blue and red simplistic cartoon because the cobalt oxide film is sketched as curves for t o 20 min and t o 35 min, respectively. Further- if it acquires negative charge while positive charges gather more, the maximum DM values at t = 65 min are much higher together at the solid/electrolyte interface, which is not necessa- in the electrolytes containing KI and Ca(BF4)2 than for bare PC rily the case. Under electrolyte gating (Fig. 1(c)), specifically and PC + KCl. Remarkably, the maximum DM for PC + 2.5  when cobalt oxide is negatively polarized, positive ions migrate 10 5 M KI electrolyte is twice higher than that for bare PC towards the interface and PC molecules reorganize and rear- (70 emu cm 3 versus 30 emu cm 3). Finally, the sudden range, thus strongly causing a modification of the EDL decrease of DM when applying voltage of opposite polarity is structure. observed for all the electrolytes. In particular, DM drops The magnetic moment of cobalt oxide in different electrolyte from around 70 emu cm 3 to 10 emu cm 3 during the time solutions was measured at room temperature under a 10 kOe interval from t = 65 min to 70 min for the electrolyte made of Fig. 1 Magneto-ionic characterization of Co oxide under electrolyte-gating. (a) Schematics of the electrolyte gate process under voltage actuation of a Co oxide film. Possible structure of the EDL under (b) no applied electric field (electrostatic forces), and (c) external polarization (electrochemical reaction). (d) Time evolution of magnetization (DM) for bare PC and PC-containing KI, KCl and Ca(BF4)2 salts at equal salt concentration (2.5  10 5 M), at 1.5 V for 60 min followed by +1.5 V (as schematically shown in the upper panel). Magnetic measurements were done in-plane while applying an external magnetic field of 10 KOe. 120 | Nanoscale Horiz., 2023, 8, 118–126 This journal is © The Royal Society of Chemistry 2023 View Article Online Communication Nanoscale Horizons PC + 2.5  10 5 M KI. These observations indicate that magneto-ionic effects in cobalt oxide can be substantially improved by electrolyte engineering. It is worth mentioning that the relatively small gating voltage employed in the current study (1.5 V) can produce similar magneto-ionic responses as previous works where we used applied voltages of the order of 25 V or higher for similar Co oxide film thicknesses.11,12 This decrease in the threshold voltage needed to induce changes in the magnetic properties of the Co oxide can be directly linked to This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. the addition of inorganic salts into PC. Interestingly, although Open Access Article. Published on 25 November 2022. Downloaded on 3/2/2023 6:05:00 PM. one would expect KCl and KI to yield similar results upon negatively biasing the cobalt oxide film, DM was far higher for KI. To get atomistic insights on the initial cobalt oxide – propylene carbonate + KX (X = Cl, I) interface, ab initio mole- cular dynamics (AIMD) simulations at 300 K were carried out. Co3O4 was considered for the simulations. Energies were computed at the PBE-D3+U (Ueff = 3.0 eV) without applying any external potential. The statistical analysis was performed on the last 8 (6 depending on the structure) ps of each simulation (see Computational details). We paid attention on the location of KX (X = Cl or I) and thus, two different initial structures were considered for each salt (Fig. S1 of the ESI†). In one case, the two ions were directly interacting with the surface at distances around 2 Å (KXAds). In the other, the salt was fully solvated by PC molecules (KXSol). Fig. 2 reports: (a) one repre- sentative snapshot of the simulation leading to the most favorable conformation for each salt KClAds and KISol AIMD, (b) the variation of the surface–ion distances in the simulations leading to the most favorable situation and (c) the energetics of the four AIMD. The variation of the potassium–surface dis- tance, the anion–surface distance and a representative snap- shot of the four simulations can be found in Fig. S2, S3 and S4 of the ESI.† The AIMD simulations starting with the two ions adsorbed Fig. 2 (a) Representative snapshots of the most favorable KClAds and KISol on the surface evolved without any relevant variation. All ions AIMD simulations; variation along the AIMD simulation of the (b) surface– remain attached at the surface with average ionsurface dis- ion distance (K, Cl and I) at the most favorable conformation and tances of 2.2, 2.1 and 2.4 Å for K+, Cl and I , respectively. In (c) energy. Label code: Co (blue), O (red), C (black), H (white), K (purple), these simulations no significant charge separation is gener- Cl (green), I (pale orange). ated, suggesting that almost no positive charge will be accu- mulated on the Co3O4 surface. The simulations starting with the two ions in the solution (KClSol and KISol) showed a larger layer. In contrast, for KI, there is a large preference (DG = 1.0 eV) reorganization. Regardless of the counter ion, K+ rapidly for the conformation that accumulates positive charge on the becomes adsorbed on the surface with Ksurface average surface. This can be attributed to the largest ionic radius of I distances that are only slightly higher than in the KXAds when compared to Cl that makes all electrostatic interactions simulation (2.5–2.6 Å). In contrast, the anions remain in the (and particularly that with the surface) weaker. While the AIMD solution. The Cl anion moves around the initial structure with simulations are carried out without applying an external potential, an average surface–Cl distance of 6.5 Å. Iodide prefers to move one can expect that upon externally biasing Co3O4 layer with further away from the surface, the resulting surface–anion negative voltages, the structure accumulating positive charge on distances becoming about 9.8 Å. That is, the KXSol simulations the surface would be favored with respect to that with the two ions lead to a structure defining an EDL, with a compact positive close to the surface but without modifying the trends between the charge accumulation on Co3O4. Comparison between the ener- two ions. Consequently, simulations shows that KI favors the getics of the KXAds and KXSol simulations rises the different structure accumulating more positive charge over Co3O4 and, in behavior of the anions. For KCl, the configuration in which the turn, generates a stronger electric field. two ions are directly adsorbed on the surface is slightly In order to further optimize the magneto-ionic response for more stable (DG = 0.2 eV) than the structure defining the double the KI-containing PC electrolyte, the room temperature This journal is © The Royal Society of Chemistry 2023 Nanoscale Horiz., 2023, 8, 118–126 | 121 View Article Online Nanoscale Horizons Communication This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 25 November 2022. Downloaded on 3/2/2023 6:05:00 PM. Fig. 3 Impact of KI concentration in PC on Co oxide magneto-ionics. (a) Magnetization (DM) as a function of time t for PC with different KI concentrations under electrolyte-gating at 1.5 V, while applying an external in-plane magnetic field of 10 KOe, and (b) the respective magneto-ionic rate, dDM/dt. (c) Magnetization (DM) depletion as a function of time t for the two best performing KI concentrations under an opposite bias (+1.5 V), while applying an external in-plane magnetic field of 10 KOe. magnetic moment as a function of biasing time were recorded respectively, of CoO and/or Co3O4 phases.12,28,29 The satellite with a 10 kOe field and varying KI concentrations (from structure at higher binding energies can be ascribed to the 2.5  10 6 M to 2.5  10 3 M). Upon biasing for 1 hour, the shake-up process of the Co2+ species in the high-spin state. DM first increases as the KI concentration increases from Aside from these four peaks, Fig. 5(b) clearly shows two 2.5  10 6 M to 2.5  10 4 M, and then it decreases with shoulders (indicated by the downward arrows) on the lower further addition of KI. At 2.5  10 3 M KI, the oxide film peeled binding energy side of the major peaks for bare PC-treated off, possibly because of the strong electrochemical reactions sample. The best fitting of the spectrum results in two addi- taking place (see the dashed curve in Fig. 3(a)). Indeed, the tional XPS peaks located at 778.3 eV and 793.3 eV, which are the solution acquired a yellow color, indicating the formation of I2 characteristic binding energies of Co 2p3/2 and Co 2p1/2, respec- gas at the Pt counter electrode, and the subsequent formation tively, for metallic Co.2 This unambiguously confirms the of I3 according to the reaction: reduction of cobalt oxides into Co upon negative voltage actuation in bare PC electrolyte. Interestingly, the relative I2(g) + I(aq) = I(aq)3 (1) intensities of these additional peaks and, importantly, at the expense of the oxide Co 2p peaks, become dominant in the case This further proved the occurrence of electrochemical reactions of the film electrolyte-gated in PC + 2.5  10 4 M KI. This at the cathode side, i.e., at the cobalt oxide film side. Note- indicates the formation of significant amounts of metallic Co worthily, PC is not susceptible to attack by halogens anodically and, accordingly, higher DM as seen in ME experiments. More- liberated at a Pt electrode.16 over, it is instructive to point out that there are no observable Remarkably, after applying 1.5 V for 1 hour, the maximum traces of iodine in the survey spectrum on PC + 2.5  10 4 M KI- DM for 2.5  10 4 M KI electrolyte (E280 emu cm 3) is more treated sample (not shown), ruling out the possibility of for- than one order of magnitude larger than that for bare PC mation of magnetic iodine-containing compounds. Overall, the (E25 emu cm 3). The achieved magneto-ionic rate (i.e., the formation of metallic Co upon voltage application can be rate at which the ferromagnetic signal from metallic Co is written as: generated) is drastically enhanced from 5 emu cm 3 min 1 for bare PC to 184 emu cm 3 min 1 for PC + 2.5  10 4 M KI CoO - Co + VÖ (2) (see Fig. 3(b)). Moreover, partial recovery under an opposite Co3O4 - 3Co + 4VÖ (3) bias of +1.5 V was observed for the best performing KI con- centrations of 2.5  10 4 M and 2.5  10 5 M (Fig. 3(c)). Room 2 where VÖ stands for O vacancy. Note that, while voltage- temperature hysteresis loops of the voltage-actuated cobalt driven O2 ion motion is likely to be the main origin for the oxide films confirm the effect of the KI concentration in PC observed magneto-ionic effects, a redox reaction between I (a on the ON–OFF ferromagnetic switching for this system (Fig. 4). strong reducing agent) and Co3+ (with oxidative character) This corroborates that magneto-ionic effects have a strong could also partially occur at the surface of the Co oxide films dependence on the KI concentration. upon voltage actuation. To gain information on the chemical environment of the To further examine the reversibility of the magneto-ionic cobalt oxide film surface, XPS analyses were carried out before effects of the cobalt oxide film in the PC +2.5  10 4 M KI and after ME experiments. The Co 2p core-level XPS spectra of electrolyte, voltage pulses of 1.35 V and +1.35 V (of 10 min as-prepared sample, and samples treated using bare PC and each) were alternated, and clear changes are observed in DM PC + 2.5  10 4 M KI are shown in Fig. 5. For the as-prepared running in parallel to voltage actuation (Fig. 6). Note that up to sample (see Fig. 5(a)), the two major peaks at 780.1 eV and 5 magneto-ionic cycles could be obtained with these applied 795.9 eV match Co 2p3/2 and Co 2p1/2 binding energies, voltage pulses. 122 | Nanoscale Horiz., 2023, 8, 118–126 This journal is © The Royal Society of Chemistry 2023 View Article Online Communication Nanoscale Horizons This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 25 November 2022. Downloaded on 3/2/2023 6:05:00 PM. Fig. 4 Magnetization (M) vs. applied magnetic field (H) loops of the investigated cobalt oxide films actuated using different concentrations of KI dissolved in PC. Panels a–d show hysteresis loops corresponding to as-grown states (in black), after applying DV = 1.5 V for 1 h (in red) and after subsequently applying +1.5 V (in green). All loops were acquired using a vibrating sample magnetometer with magnetic field applied in-plane. Note that the red and green loops were recorded at 0 V just after switching off the negative or positive voltage, respectively. The red loops are open at the high magnetic field region (at positive saturation) because the system exhibits a slight recovery (slight decrease of M with time) once electrolyte gating is stopped. Conclusions magnetic properties are observed is dramatically decreased when salts are added to PC. Changes are partially reversible Here we have shown that magneto-ionic motion in 15 nm-thick under the application of voltage of opposite polarity for 1 hour, Co oxide films can be dramatically enhanced by proper electro- and cyclability was observed under voltage pulses of 10 min. lyte engineering. Faster magneto-ionic rates and larger genera- tion of magnetization are achieved upon the addition of inorganic salts (KI, KCl, and Ca2(BF4)2) to PC. In this way, the Experimental ionic strength of the medium is increased, thereby causing a higher electric field strength at the Co oxide/electrolyte inter- Sample preparation face upon biasing. As a result, oxygen motion within the 15 nm-thick cobalt oxide films were grown by reactive sputter- material is favored, thereby enabling the rapid and more ing at room temperature on B-doped [100]-oriented Si wafers extensive formation of metallic cobalt. In particular, potassium (0.75 mm thick), previously sputtered with an adhesion/seed iodide at a concentration of 2.5  10 4 M brings about a 35-fold layer of 20 nm Ti/60 nm Cu. The depositions were performed increase of magneto-ionic rate and one order of magnitude with an AJA International, Inc. Magnetron Sputtering System larger variation in magnetization than for bare PC under 1.5 V ATC Orion with a base pressure of 5  10 8 Torr. Argon was biasing for 1 h. AIMD simulations suggest that a more effective used as carrier gas for the Co precursor and O2 plasma as EDL would build up for KI compared to KCl. More importantly, oxygen source. The target to substrate distance was around the onset potential at which significant changes in the 10 cm and the sputtering rate about 23 nm min 1. Co oxide was This journal is © The Royal Society of Chemistry 2023 Nanoscale Horiz., 2023, 8, 118–126 | 123 View Article Online Nanoscale Horizons Communication This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 25 November 2022. Downloaded on 3/2/2023 6:05:00 PM. Fig. 6 Magneto-ionic cyclability of the 15 nm-thick cobalt oxide film in PC with 2.5  10 4 M KI under 1.35 V/+1.35 V voltage pulses of 10 min duration each, assessed from in-plane magnetic measurements while applying a magnetic field of 10 kOe. Compositional measurements X-ray photoelectron spectroscopy (XPS). XPS analysis were carried out on both the as-prepared sample and the samples subject to magnetoelectric actuation in PC and 2.5  10 4 M KI- containing PC. A PHI 5500 Multitechnique System spectro- meter from Physical Electronics was used, which was equipped with a monochromatic Al Ka X-ray source (1486.6 eV) at a power Fig. 5 X-ray photoelectron spectra of Co 2p1/2 and Co 2p3/2 for (a) as- of 350 W. The survey and Co 2p core-level XPS spectra were prepared, (b) treated in bare PC and (c) PC + 2.5  10 4 M KI samples. The taken both before and after sputtering the surface with Ar ions downward arrows point to the Co 2p3/2 and Co 2p1/2 peaks of metallic Co for 1 min. All XPS spectra were corrected using the C 1s line at (in blue and purple in (b) and (c)). Additionally, green and orange corre- spond to Co2+, and pink and brown to Co3+. Solid black lines are 284.6 eV. Background subtraction and peak fitting were per- experimental data, whereas solid red lines refer to fitting spectra. formed with XPSPEAK41 software. grown in a 9% O2/91% Ar (% given from flow rates Inductively coupled plasma mass spectrometry (ICP-MS) of the different gases) atmosphere under a total pressure of The concentration of the salts dissolved in PC were determined 3  10 3 Torr. Prior to that, the adhesive (Ti) and seed (Cu) by ICP-MS. Specifically, the K+ concentration (for KI and KCl layers were also grown using the same experimental setup but dissolved in PC) and Ca2+ concentration (for Ca2(BF4)2 dis- in an inert Ar atmosphere, and then partially masked to use the solved in PC) were determined on an Agilent (7900 model) remaining uncoated surface for electrical connection in sub- spectrophotometer. Before analysis, the aliquots were diluted sequent magnetoelectric experiments. in HNO3 1% (v/v). Electrolyte preparation Magnetoelectric characterization The electrolyte solutions were prepared from propylene carbo- Magnetic measurements were carried out in-plane at room nate (from Sigma-Aldrich). PC was initially treated with metallic temperature in a vibrating sample magnetometer from Micro Na to remove traces of water, leading Na+ and OH ions Sense (LOT-Quantum Design) under electrolyte gating using an behind. Later, varying amounts of KI, KCl and Ca(BF4)2 were external power supply (Agilent B2902A). Voltages were applied added in excess to the PC-based solution taking into between the Cu seed-layer beneath the Co oxide film, which account the solubility limits of these salts in PC at room acted as a working electrode. and a platinum wire which served temperature (s(KI) = 0.221  0.020 mol kg 1,23 s(KCl) = as a counter electrode (Fig. 1(a)). An additional platinum wire (3.67  0.40)  10 4 mol kg 1 23 and way larger for Ca(BF4)2 was introduced into the cell to act as a pseudo-reference (ca. 0.1 M)).17 The corresponding supernatants were analyzed electrode. The generated magnetization (M) was studied for by ICP and further diluted with PC-based solution to the final Na-treated PC to which different salts (KI, KCl, Ca(BF4)2 had concentrations considered in this study. been added. 124 | Nanoscale Horiz., 2023, 8, 118–126 This journal is © The Royal Society of Chemistry 2023 View Article Online Communication Nanoscale Horizons Computational details the Red Española de Supercomputación (QHS-2022-2-0029) is The ab initio molecular dynamics (AIMD) were performed at 0 V also acknowledged. potential using the PBE density functional level of theory30 with the Hubbard-like term (U), as defined by Dudarev et al.31 The References Ueff value was fixed to 3.0 eV based on test calculations reported in Table S1 (ESI†). Moreover, the empirical D3 correction32 was 1 Y. Shiota, T. Nozaki, F. Bonell, S. Murakami, T. Shinjo and also included to account for dispersion forces. The ionic cores Y. Suzuki, Nat. Mater., 2012, 11, 39–43. were described with the PAW pseudopotentials and the valence 2 C. Song, B. Cui, F. Li, X. Zhou and F. Pan, Prog. Mater. Sci., 2017, 87, 33–82. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. electrons were represented with a plane-wave basis set with an energy cutoff of 500 eV. The first Brillouin zone was sampled 3 C. Leighton, Nat. Mater., 2019, 18, 13–18. Open Access Article. Published on 25 November 2022. Downloaded on 3/2/2023 6:05:00 PM. with a (2,2,1) Monkhorst–Pack K-point mesh. The simulations 4 J. De Rojas, J. Salguero, A. Quintana, A. Lopeandia, were carried out within the NVT ensemble (T = 300 K), using the M. O. Liedke, M. Butterling, A. G. Attallah, E. Hirschman, Nosé–Hoover thermostat.33,34 The time step was set to 1fs, and A. Wagner, L. Abad, J. L. Costa-Krämer, J. Sort and the simulations were allowed to evolve until 10 ps were E. Menéndez, Phys. Rev. Appl., 2021, 16, 034042. achieved. All calculations were performed with the spin polar- 5 G. Wang, L. Zhang and J. Zhang, Chem. Soc. Rev., 2012, 41, ized formalism as implemented in VASP 5.4 package.35,36 797–828. The Co3O4-propylene interface was represented with a 4- 6 P. Sharma and T. S. Bhatti, Energy Convers. Manage., 2010, layers thick slab model of the (110) facet where 13 propylene 51, 2901–2912. carbonate molecules were added in the vacuum space between 7 C. Navarro-Senent, A. Quintana, E. Menéndez, E. Pellicer images to reproduce the solvent density at room temperature and J. Sort, APL Mater., 2019, 7, 030701. (r = 1.2 g cm 3). Furthermore, a KX (X = Cl or I) unit was 8 M. Nichterwitz, S. Honnali, M. Kutuzau, S. Guo, J. Zehner, included to represent the dissolved salt. Two initial starting K. Nielsch and K. Leistner, APL Mater., 2021, 9, 030903. configurations were considered for each salt (Fig. S2, ESI†): (a) 9 G. Feng, J. Huang, B. G. Sumpter, V. Meunier and R. Qiao, the two ions are initially located on the surface, directly inter- Phys. Chem. Chem. Phys., 2010, 12, 5468–5479. acting with Co3O4, and (b) the ions are solvated by propylene 10 A. Quintana, E. Menéndez, M. O. Liedke, M. Butterling, A. Wagner, carbonate molecules and thus located at about 6 Å from the V. Sireus, P. Torruella, S. Estradé, F. Peiró, J. Dendooven, surface. Further details can be found in the supporting C. Detavernier, P. D. Murray, D. A. Gilbert, K. Liu, E. Pellicer, information. J. Nogues and J. Sort, ACS Nano, 2018, 12, 10291–10300. 11 J. de Rojas, A. Quintana, A. Lopeandı́a, J. Salguero, J. L. Costa- Krämer, L. Abad, M. O. Liedke, M. Butterling, A. Wagner, Author contributions L. Henderick, J. Dendooven, C. Detavernier, J. Sort and E. Menéndez, Adv. Funct. Mater., 2020, 30, 2003704. E. P., J. S. and E. M. had the original idea and led the 12 S. Martins, J. de Rojas, Z. Tan, M. Cialone, A. F. Lopeandı́a, investigation. S. M., E. P. and J. S. designed the experiments. J. Herrero-Martı́n, J. Costa-Kramer, E. Menéndez and J. Sort, S. M. and Z. M. synthesized the cobalt oxide layers. S. M. and Nanoscale, 2022, 14, 842–852. Z. M. carried out the magnetoelectric measurements and S. M., 13 J. de Rojas, A. Quintana, A. Lopeandı́a, J. Salguero, B. Muñiz, E. P., J. S. and E. M. analyzed the data. X. S.-M., M. S. and F. Ibrahim, M. Chshiev, A. Nicolenco, M. O. Liedke, L. R.-S. carried out the AIMD simulations. Z. M performed and M. Butterling, A. Wagner, V. Sireus, L. Abad, C. J. Jensen, analyzed the XPS measurements. All authors discussed the K. Liu, J. Nogués, J. L. Costa-Krämer, E. Menéndez and J. Sort, results and commented on the article. The article was written Nat. Commun., 2020, 11, 5871. by S. M., E. P., J. S and E. M. 14 J. De Rojas, J. Salguero, F. Ibrahim, M. Chshiev, A. 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