www.acsami.org Research Article From Binary to Ternary Transition-Metal Nitrides: A Boost toward Nitrogen Magneto-Ionics Zhengwei Tan, Sofia Martins, Michael Escobar, Julius de Rojas, Fatima Ibrahim, Mairbek Chshiev, Alberto Quintana, Aitor Lopeandia, José L. Costa-Krämer, Enric Menéndez,* and Jordi Sort* Cite This: ACS Appl. Mater. Interfaces 2022, 14, 44581−44590 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. ABSTRACT: Magneto-ionics is an emerging actuation mechanism to control the magnetic properties of materials via voltage-driven ion motion. This effect largely relies on the strength and penetration of the induced electric field into the target material, the Downloaded via 158.109.55.75 on November 11, 2022 at 17:51:24 (UTC). amount of generated ion transport pathways, and the ionic mobility inside the magnetic media. Optimizing all these factors in a simple way is a huge challenge, although highly desirable for technological applications. Here, we demonstrate that the introduction of suitable transition-metal elements to binary nitride compounds can drastically boost magneto-ionics. More specifically, we show that the attained magneto-ionic effects in CoN films (i.e., saturation magnetization, toggling speeds, and cyclability) can be drastically enhanced through 10% substitution of Co by Mn in the thin-film composition. Incorporation of Mn leads to transformation from nanocrystalline into amorphous-like structures, as well as from metallic to semiconducting behaviors, resulting in an increase of N-ion transport channels. Ab initio calculations reveal a lower energy barrier for CoMn−N compared to Co−N that provides a fundamental understanding of the crucial role of Mn addition in the voltage-driven magnetic effects. These results constitute an important step forward toward enhanced voltage control of magnetism via electric field-driven ion motion. KEYWORDS: magnetoelectricity, voltage control of magnetism (VCM), magneto-ionics, transition metal nitride, ion diffusion ■ INTRODUCTION The intentional incorporation of impurities into a host with O2− transport-based magneto-ionics at room temperature is its poor endurance, owing to slow and irreversible chemical/ material, a process known as element substitution,1 is of structural changes that occur when voltage polarity is reversed.15 Smaller ions, such as H+, F−, or Li+, have been fundamental significance in reconstructing nanocrystals,2 utilized to achieve faster and more cyclable voltage-induced modifying electronic characteristics,3 or modulating magnet- changes of the magnetic properties. However, these systems ism,4,5 among others. Controlled element introduction has pose some limitations in terms of compatibility with traditional been proved effective in yielding semiconductor-based hybrid complementary metal-oxide semiconductor (CMOS)-based materials with desirable properties for target applications, such devices.27 Alternatively, nitrogen magneto-ionics have been as solar cells,6 batteries,7 novel transistors,8,9 superconduc- demonstrated by using a cobalt nitride target material (i.e., tors,10 or spintronics devices.4,5,11,12 Recently, unprecedented CoN23), which provides “ready-prepared” lattice sites for ionic control of magnetic properties (including “ON−OFF” switch- diffusion, leading to improved endurance at room temperature, ing of ferromagnetic states13) has been realized using magneto- without sacrificing operating speed and compatibility. In ionic actuation.14 This is an emerging energy-efficient addition, the microstructure of the magneto-ionic target approach to tune magnetism based on electric field-induced materials play an important role in the attained values of ions (e.g., O2−,13−17 Li+,18 F−,19 H+,20−22 or N3−23−25) motion. resistivity and ion transport mechanisms, resulting in a planar- However, element introduction in magneto-ionic systems, that like migration front for ions,24,25 that is highly appealing for may allow for further engineering of magnetic properties with devices when compared to traditional inhomogeneous or voltage, is still unexplored. A typical magneto-ionic system comprises the target material, usually a ferromagnetic (FM) metal or an oxide,26 Received: July 18, 2022 and a solid or liquid electrolyte (adjacent to the target Accepted: September 13, 2022 material), working as dielectric and ion reservoir layers. One of Published: September 21, 2022 the most studied systems is Co/GdOx, in which control of the perpendicular magnetic anisotropy through voltage-driven oxygen migration has been demonstrated.14 One problem © 2022 The Authors. Published by American Chemical Society https://doi.org/10.1021/acsami.2c12847 44581 ACS Appl. Mater. Interfaces 2022, 14, 44581−44590 ACS Applied Materials & Interfaces www.acsami.org Research Article Figure 1. Structural, compositional, and electric transport characterization of as-deposited films. (a) θ/2θ XRD patterns of the as-prepared 100 nm thick CoN and CoMnN films. (b,c) High-resolution TEM images of the cross section of 30 nm thick CoN and CoMnN films. The inset shows the fast Fourier transform of the area marked with a red square. (d) HAADF-STEM micrograph and EELS Co, Mn, and N elemental mappings for as- prepared 30 nm thick CoMnN films. (e) Electrical resistivity (ρ) measured as a function of temperature from 30 to 300 K, for as-deposited 30 nm thick CoN and CoMnN films. cross-sectional ion diffusion channels typically occurring upon magneto-ionic performance may be potentially boosted. To O2−-ion migration.13,16 Magneto-ionics is essentially a dynamic achieve this goal, introduction of certain elements leading to process,17 which involves breaking and recombining metal-O precise structural and compositional engineering at the or metal-N bonds. This approach requires an effective electric nanoscale, together with highly tunable magnetic and electrical field to drive the process, as well as properly tuned magnetic properties, may turn out to be a suitable strategy. Finding a materials to be modulated.28 Although ferromagnetic metals suitable element and introducing it in a controlled manner into show large and stable magnetization, high TC, and, eventually, a host FM target material is thus critical. In metallic alloys, a well-defined perpendicular magnetic anisotropy,29 their efforts have been made to substitute Co, Ni, or Fe by Mn as an direct utilization for magneto-ionics also poses some draw- effective means to alter their magnetic and electrical backs. Among those is the limited electric-field screening behavior.31−34 In magneto-ionics, Mn introduction may also length stemming from their high electric conductivity, which give specific advantages. First, Mn, as an “amorphizing” means that the effects of voltage are strongly limited to the agent,35 could favor the formation of an amorphous phase, outer surface of the metal.30 This limitation can be overcome, which would promote planar-like diffusion fronts for ion to some extent, using semiconductors instead of metals.29 motion (because, otherwise, ion motion preferentially occurs Thus, if semiconducting properties could be imparted to target along grain boundaries). Also, Mn-substitution could introduce materials without sacrificing their magnetic properties, more defects in the crystal lattice and increase electrical 44582 https://doi.org/10.1021/acsami.2c12847 ACS Appl. Mater. Interfaces 2022, 14, 44581−44590 ACS Applied Materials & Interfaces www.acsami.org Research Article resistivity, possibly leading to the modification of electrical shown in the insets, in which clear diffraction spots can be properties, from metallic-like to semi-metallic or semiconduct- found for CoN films [points inside yellow cycles, correspond- ing behavior.33 Incorporation of Mn may trigger hopping ing to an interplanar distance d = 2.13 Å, consistent with a mechanisms and hence change the transport properties, as cubic CoN (200) orientation], whereas no spot is observed for studied in earlier cobalt ferrite systems.33,34 Most importantly, CoMnN films, in agreement with XRD results and evidencing the enhancement of saturation magnetization and perpendic- the role of Mn as a “amorphizing” agent.35 To further ular magnetic anisotropy of host FM target materials with understand the compositional distribution of CoMnN films, moderate Mn introduction has also been confirmed both from HAADF-STEM and EELS were performed on as-deposited experimental and theoretical studies.36−38 Therefore, introduc- CoMnN films, with corresponding mappings shown in Figure ing Mn into FM nitrides (e.g., CoN) might result in synergetic 1d. The Co (red), Mn (blue), and N (green) elements are all effects and could make magneto-ionics more attractive for uniformly distributed in the films, evidencing the homoge- spin-based and other magnetoelectric devices. neous growth process of CoMnN. An amorphous or very In this work, we show multiple benefits that moderate nanocrystalline structure should promote the formation of a substitution of Co by Mn brings to the magneto-ionic larger density of ion diffusion channels under the application of performance of CoN-based heterostructures. With the addition an applied voltage, compared to a well-crystallized structure of Mn, a change from nanocrystalline to amorphous structures with limited amounts of grain boundaries, eventually allowing is observed, accompanied by a transition from metallic-like to for a greater modulation of magnetic properties with an electric semiconducting properties. The former increases N-ion field.23,24 transport channels, as verified by high-angle annular dark- To investigate the role of Mn on electric transport, resistivity field scanning transmission electron microscopy (HAADF- (ρ) as a function of temperature (T, in the range 30−300 K) STEM) and electron energy loss spectroscopy (EELS). The was measured on both as-prepared CoN and CoMnN films. As latter significantly extends the electric field effect that is seen in Figure 1e, the resistivity at room temperature (300 K) normally limited to a few Å in metals.30 Mn incorporation is approximately 172 μΩ cm for CoN films (the value is close leads to a 6.7-fold voltage-driven enhancement of the to that of metallic Co41 and previously reported less- saturation magnetization (MS) and improved high-frequency stoichiometric Co-rich CoN films prepared by a triode cyclability. In addition, ab initio calculations have been system25) and about 406 μΩ cm for CoMnN films. Moreover, performed to estimate the Co−N and CoMn−N energy the resistivity of CoN is observed to monotonically increase barriers using different crystal orientations and N atom insertion paths. In all cases, a reduced energy barrier for (dρ/dT > 0) throughout the whole measured temperature CoMnN is obtained, compared to CoN. These experimental range, which is consistent with metallic behavior.42 This and theoretical findings prove the highly beneficial effect of metallic behavior has also been reported in other transition- Mn-substitution in enhancing magneto-ionics. These results metal nitrides,43 and it is attributed to some degree of could have implications in diverse technological areas, like wavefunction overlap. In CoN, electrons are localized and neuromorphics39,40 or iontronics in general. there is a little overlap between the wavefunction of ions situated on adjacent sites.44 In contrast, the CoMnN films ■ RESULTS AND DISCUSSION Cobalt nitride (CoN) and cobalt manganese nitride show a monotonic decrease (dρ/dT < 0), corresponding to semiconducting behavior,45 due to the increase in drift mobility of charge carriers at higher temperatures. This result (Co0.9Mn0.1N, identified as CoMnN throughout the text), demonstrates that partial substitution of Co by Mn not only both 30 or 100 nm thick, were grown on Cu (10 nm)/Ti (10 increases the resistivity but also turns the film semiconductor. nm)/[100]-oriented Si substrates by reactive magnetron In addition, the variation of carrier densities (n/p) as a sputtering (see the Methods section). Figure 1a shows the X-ray diffraction (XRD) patterns of 100 function of temperature for CoMnN (Figure S1) reveals an nm thick CoN and CoMnN films. A single diffraction peak is overall negative value (by convention meaning predominant observed for the CoN film at a 2θ ≈ 43.84°, which is electron transport, as opposed to positive when holes would consistent with both (111) Cu (JCPDF card no. 00-001-1241) dominate), hence showing that introducing Mn makes or expanded (200) CoN (JCPDF card no. 00-016-0116) electrons the dominant charge carriers while changing the phases. In contrast to CoN, the CoMnN film does not show behavior from metallic-like to semiconductor (Figure 1d). such well-defined peaks but only a broad and weak peak Additionally, the resistivities of N-free Co and CoMn films centered at 2θ ≈ 43.28°, which is also consistent with (111) (Co0.9Mn0.1) were measured to be 100 and 470 μΩ cm, Cu or expanded (200) CoMnN. However, CoMnN clearly respectively. Thus, Mn incorporation increases ρ both for N- exhibits a much lower crystallite size value, evidencing the free and the nitride films. While the introduction of Mn plays a nanostructuring effect caused by Mn. Because the nitride films role like increasing the nitrogen concentration in the films in are 10 times thicker than the Cu buffer layer, the signal arising terms of electrical properties, the interplay between nitrogen from both patterns is mainly attributed to the CoN and content and defects on resistivity still remains to be fully CoMnN phases. Both films are textured along (200) planes elucidated. Namely, the degree of bonding with nitrogen, and nanocrystalline. In the case of the CoMnN, the degree of effects of variable amounts of introduced Mn, possible charge nanostructuring is more pronounced, approaching a highly compensation effects and impurity scattering in these disordered (eventually amorphous-like) structure. polycrystalline films need to be further studied. Anyhow, the This result is confirmed by high-resolution transmission increased resistivity and thus the change of electric behavior electron microscopy (HRTEM) images of the cross-sections of from metallic to semiconducting achieved by the addition of as-prepared CoN and CoMnN films, as demonstrated in Mn enables a deeper penetration of the electric field (Figure Figure 1b,c, respectively. The areas marked with red squares S2), up to few tens of nm,25 enhancing magneto-ionic effects, were chosen for fast Fourier transform (FFT) analyses, as as will be shown below. 44583 https://doi.org/10.1021/acsami.2c12847 ACS Appl. Mater. Interfaces 2022, 14, 44581−44590 ACS Applied Materials & Interfaces www.acsami.org Research Article Figure 2. Magneto-ionic characterization of 30 nm thick CoN and CoMnN films under electrolyte gating. (a) Schematic of the designed structure for electrolyte actuation (left) and sketch of the formation of electric double layer during voltage actuation (right). (b) Time (t) evolution of the saturation magnetization MS for CoN and CoMnN films under −25 V. (c) t evolution of MS when the voltage is monotonically increased in steps of −1 V to determine the onset voltage required to trigger magneto-ionics in CoN and CoMnN films. (d,e) Magneto-ionic cyclability of the CoMnN and CoN films subjected to −25 V/+25 V voltage pulses applied with a periodicity of 20 s. To investigate the effect of Mn introduction on the This means that the tendency to form off-stoichiometric magneto-ionic performance, both 30 nm thick CoN and regions (i.e., Co-rich CoN clusters) near the CoN/Cu(N) CoMnN thin films were electrolyte-gated in a capacitor-like interface is slightly larger than for CoMnN/Cu(N). configuration using a Pt wire as the counter electrode, while Figure 2b shows the variation of MS of CoN and CoMnN performing in-plane magnetic measurements by vibrating films as a function of time (t) during electrolyte gating at −25 sample magnetometry (VSM) at room temperature. Propylene V, while an external magnetic field of 10 kOe (above the carbonate with Na+- and OH−-solvated species was employed anisotropy field of the generated ferromagnetic counterpart) as an aprotic, anhydrous polar liquid electrolyte to serve as a was applied to ensure magnetic saturation. In response to the nitrogen-ion reservoir 23−25 and to apply a uniformly applied voltage, an immediate increase of MS is observed in distributed, out-of-plane electric field through the formation both CoN and CoMnN films, indicating a quick onset for of an electric double layer (EDL)30,46 at the films’ upper nitrogen motion, resulting in the appearance of metallic surface (as shown in Figure 2a). Co(Mn) ferromagnetic phase.23 Interestingly, while CoN and As seen in Figure S3a, the as-prepared CoN sample exhibits CoMnN show similar increasing trends of MS during the initial some traces of ferromagnetic signal (<21 emu cm−3, which is stages of voltage actuation (i.e., the first 150 s), M S equivalent to ≈1.5% the magnetization of pure Co47), whereas subsequently levels off for CoN films but continues to increase the as-prepared CoMnN sample shows a virtually para- in CoMnN films before reaching a stable value for times close magnetic behavior (<3 emu cm−3). This reduction of residual to 1500 s. As a result, a 6.7-fold enhancement in the obtained ferromagnetism in the as-prepared sample by addition of Mn is steady-state value of MS is achieved in voltage-actuated possibly the result of the reported difference of cohesive energy CoMnN compared to CoN, from 102 to 672 emu cm−3, between Mn−N and Cu−N (≈1.17 eV/atom) being smaller respectively (in agreement with the magnetic hysteresis loops than that between Co−N and Cu−N (≈2.01 eV/atom).48 shown in Figure S3b,c). Note that, as reported earlier,37,38 MS 44584 https://doi.org/10.1021/acsami.2c12847 ACS Appl. Mater. Interfaces 2022, 14, 44581−44590 ACS Applied Materials & Interfaces www.acsami.org Research Article Figure 3. Compositional characterization by HAADF-STEM and EELS of the magnetoelectrically actuated films. HAADF-STEM and elemental EELS mappings corresponding to areas marked in orange for 100 nm thick (a,b) CoN and (c,d) CoMnN films gated at −25 V for 40 min, respectively. Cu and Pt layers serve as working electrodes and protective capping layer for lamellae preparation, respectively. Colors corresponding to each element are noted at the bottom of the figure, that is, Co, Mn, and N are represented by red, blue, and green colors in the EELS elemental mappings, respectively. for CoMn is slightly larger than for pure Co (Figure S3d), but CoMnN is obtained, whereas no signs of magneto-ionic effects this fact by itself alone cannot explain the obvious magneto- are continued after two cycles for CoN films (Figure 2e). ionic enhancement of MS observed in their nitrides and Nonetheless, for longer pulse duration (with voltage switched demonstrates that Mn-substitution boosts magneto-ionics by every 2 min), CoN films show a relatively stable cyclability as some other mechanisms, for example, the dissimilar resistivity well (Figure S3e). This demonstrates that Mn-substitution is variations during the denitrification processes in CoN and not only favorable to enhance the magneto-ionic rates or the CoMnN films. With the goal to investigate the eventual attained MS values but also the cyclability (or endurance) influence of the growing method and to be able to compare through improvement of the recovery process. with previously reported results, 85 nm thick CoN and To further understand the effect of Mn substitution from the CoMnN films were also grown by triode sputtering. This is the perspective of voltage-driven ion transport, cross-sectional thickness of the CoN films investigated in our previous lamellae of 100 nm thick CoN and CoMnN films electrolyte- works.23−25 As shown in Figure S4, the incorporation of Mn gated at −25 V for 40 min were studied by HAADF-STEM also results in a larger generation of magnetization analogously and EELS. As shown in Figure 3a, a moderate denitriding to what happens when comparing the magneto-ionic response process occurs in CoN, which results in sparse, cross-sectional of CoN and CoMnN films grown by magnetron sputtering (i.e., perpendicular-to-film) channels after nitrogen-ion dif- (Figure 2b). It is remarkable that, even though the films grown fusion has taken place, similar to what was reported in other by triode sputtering are thicker than those prepared by magneto-ionic systems like Co3O4 also grown by magnetron magnetron sputtering (85 nm vs 30 nm, respectively), larger sputtering.13,16 This columnar morphology suggests that N-ion magnetization values are attained upon voltage applications in motion takes place, at least to some extent, along these films grown by triode sputtering, both in the CoN and channels generated perpendicular to the film plane during CoMnN systems. This suggests the importance of a more voltage applications. As shown in Figure 3b, EELS mappings nanocrystalline microstructure obtained by triode sputtering in reveal a lack of N and depletion of Co in the channels, in the enhanced nitrogen-ion motion due typically to a larger accordance with the formation nanoporosity in the films (in defect size and density. In any case, our results confirm the role the form of elongated vertical pore channels of 5−10 nm in of Mn as an “amorphizing” agent in improving magneto-ionics, width). The formation of these channels in the upper part of regardless of the growing method. the films is even more evident for the magneto-ionically treated The minimum threshold voltage required to trigger CoMnN films (see Figure 3c,d). The formation of such magneto-ionic effects is of large importance for device channels can have an influence on both the electric properties implementation.24 Thus, the onset voltages were also evaluated (electronic hopping) as well as the time-dependence evolution in both systems by monotonically increasing the external of magneto-ionic effects, although it is quite difficult to voltage in steps of −1 V until MS started to increase, as shown establish the exact current flow paths in this kind of in Figure 2c. The obtained results reveal that the onset voltage nanocrystalline materials. Remarkably, in this case, a planar is approximately −4 V for both CoN and CoMnN films. Figure horizontal front, separating porous and denser areas, is 2c also shows that, in spite of a larger MS, the recovery time for observed. The occurrence of such planar diffusion fronts was CoMnN films is significantly shorter than for CoN films, previously reported in CoN films grown by triode sputtering thereby a better endurance in the case of CoMnN films is (not by magnetron sputtering as in the current work) envisaged. Taking this into account, the cyclability was exhibiting a highly nanocrystalline microstructure and larger investigated by subjecting the CoN and CoMnN films to generated magnetization.23,24 Remarkably, the presence of −25 V/+25 V pulses with a periodicity of 20 s. As shown in metallic Co was confirmed by the fast Fourier transform (FFT) Figure 2d, a very stable and reversible cycling behavior for spots obtained from high-resolution TEM, as shown in Figure 44585 https://doi.org/10.1021/acsami.2c12847 ACS Appl. Mater. Interfaces 2022, 14, 44581−44590 ACS Applied Materials & Interfaces www.acsami.org Research Article Figure 4. Ab initio calculations of Co−N and CoMn−N energy barriers. Schematic of (a) CoMn and Co (001) surfaces and (b) CoMnN and CoN (111) surfaces, respectively, designed for N-atom insertion. For CoMnN surfaces, both Co−Mn path (path 1) and Co−Co path (path 2) have been considered. (c,d) Total energy per atom as a function of distance for the aforementioned surfaces. The total energy is plotted relative to the minimum energy values, as a function of the displacement between the reference CoMn or Co outermost surface atom and the inserted N atom. S5, which is responsible for the MS increase after voltage nitrogen ion is inserted into a cobalt or cobalt manganese treatment (Figure 2b). These observations are consistent with surface with 10 atoms per layer (Figure 4a,b). Two vertical and horizontal line profiles of the N element, as shown orientations, (001) and (111), were considered and the total in Figure S6, which reveal an obvious difference in N content energies per atom were calculated as a function of the distance between the upper and bottom parts of the CoMnN films (in z between the inserted N atom and a Co or CoMn upper contrast to CoN, where the N concentration is more surface (see Figure 4c,d, respectively). Furthermore, for the homogeneous). Additionally, the nanoporosity causes sudden case of CoMnN films, the formation energy of the N insertion jumps of the N content along the horizontal profiles for both into either Co−Mn (path 1) or Co−Co (path 2) were also CoN and CoMnN films. These results reveal that, under the calculated. The position z = 0 Å refers to the outmost layer of same magnetoelectric actuation conditions, a larger amount of Co or CoMn, while its negative sign represents the nitrogen is released to the electrolyte in CoMnN compared to displacement inside of the film (and vice versa for the positive CoN films. This agrees with Figure 2b,c, which show a larger sign). For the Co (001) surface, CoMn (001) surface-path 1 increase of MS in CoMnN. Overall, the HAADF-STEM and and CoMn (001) surface-path 2, the local minimum EELS observations on CoN and CoMnN films illustrate that (maximum) energies are located at around z = 0.4 (−1 Å), Mn substitution significantly increases nitrogen transport 0.4 (−1 Å), and 0.3 (−1.4 Å), respectively, with corresponding channels, leading to an enhanced magneto-ionic behavior energy barriers between the two extrema are calculated to be (thus explaining the shorter recovery time and the improved 126, 104, and 76 meV/atom. Further calculation shows that cyclability). the energy barrier needed to be overcome by N is lower for the The efficiency of magneto-ionics initially depends on the CoMn (001) surface (by 22 meV/atom for path 1 and 50 minimum energy required for breaking and re-forming meV/atom for path 2) compared with the Co (001) surface. magnetic metal-ion bonds. Using the nudged elastic band This reduction of energy barrier by Mn addition is consistent method (NEB)49,50 (see the Methods section), the Co−N and with the larger quantity of channels and nanoporosity due to CoMn−N energy barriers were calculated in a case where a N-ion movement observed in CoMnN compared to CoN 44586 https://doi.org/10.1021/acsami.2c12847 ACS Appl. Mater. Interfaces 2022, 14, 44581−44590 ACS Applied Materials & Interfaces www.acsami.org Research Article films, which brings about a larger MS and faster recovery ionics), such as electrochemical catalysis, batteries, solar cells, process (thus improved cyclability) for CoMnN films. or spintronics. The results for the (111) surfaces, however, are more complex. When a N atom is introduced into the Co surface, the positions of local minimum and maximum energies are z = ■ METHODS Sample Fabrication. CoN and Co0.9Mn0.1N thin films with 1.1 and −0.1 Å, respectively, while the corresponding positions thicknesses of 30 nm or 100 nm were grown at room temperature by for the CoMn-path 1 case are z = 1.1 and −0.4 Å. In turn, the magnetron sputtering in a high-vacuum chamber (with a base calculated energy barriers between the two extrema are 25.4 pressure <8 × 10−8 Torr) on non-doped (100)-oriented Si wafers (0.5 mm thick) previously coated with a 10 nm thick titanium adhesion and 28.9 meV/atom for nitrogen displacement in Co and layer and 10 nm thick copper seed layer. The copper was partly CoMn-path 1 structures, respectively. Note that we find a masked to avoid the nitride film deposition and later serve as working reduced barrier height for Co−N in the present study electrodes. Pure Co and Mn targets were co-sputtered for the compared to ref 23. This is explained by the dependence of deposition of CoMnN and rates were calibrated to obtain a Co/Mn the energy barrier on the concentration of the inserted atoms ratio of 9:1. The growth of CoN and CoMnN was carried out in a into a surface. In ref 23, the ratio of N to Co atoms per surface mixed Ar and N2 atmospheres always using a nitrogen partial pressure layer was 1:1 and this implies the 100% concentration of N of 50% and a total working pressure of 3 × 10−3 Torr. The distance insertion into the surface, whereas here we have a supercell, between the substrate and targets was around 10 cm, and the which is laterally larger to simulate a 10% N insertion into the sputtering rate was approximately 0.8 Å s−1. Magneto-Ionic Characterization. In situ magnetoelectric surface. When comparing Co and CoMn-path 1 structures, the measurements were carried out at room temperature using a minimum energy required by N to enter the CoMn (111) commercial vibrating sample magnetometer (VSM) from Micro orientation is slightly larger than for Co (111). However, Sense (LOT, Quantum Design), with a maximum applied magnetic further energy barrier calculation for CoMnN-path 2 structure field of 2 T. Electrolyte gating was conducted between the counter gives a value of 21.8 meV for (111) interfaces, which is lower electrode (a Pt wire) and the working electrode (the investigated than that obtained in case of path 1. Therefore, overall, the CoN/Cu/Ti/Si or CoMnN/Cu/Ti/Si thin films) in a home-made differences in energy barriers between Co and CoMn along electrolytic cell using an external Agilent B2902A power supply, as this lattice orientation are less evident than along the (001) indicated in earlier works. The electrolyte consisted of anhydrous propylene carbonate with Na+- and OH−-solvated species (10−25 orientation, and they are path dependent. In fact, the ppm), in which metallic sodium was immersed to react with any considered N paths in the calculations are particular, while possible trace of water. Negative voltages in this work signify the in the experiment not all N atoms have essentially the same accumulation of negative charges at the working electrode (and vice path into the surface and thus a collective effect of perhaps versa for positive voltages). The magnetization (M) is obtained by several paths is observed. Also, it is instructive to comment on normalizing the magnetic moment to the sample volume exposed to the difference in the positions of the minima/maxima in Figure the electrolyte. Note that the linear slopes in the hysteresis loops at 4c,d. We believe that it is caused by the different crystallo- high fields (arising from diamagnetic or paramagnetic contributions) graphic orientations. For the (111)-oriented surface, the were eliminated by correcting the background signal (i.e., at fields always significantly larger than the saturation fields). maximum energy is reached when the N atom is almost Structural and Compositional Measurements. The θ/2θ X- located in the first atomic surface layer. However, in the (001) ray diffraction (XRD) patterns were collected on a materials research surface, the maximum energy is found when the N atom is diffractometer (MRD) from Malvern PANalytical company, equipped integrated more into the surface. In this case, it feels less with a PIXcel1D detector, using Cu Kα radiation. High-resolution comfortable due to smaller interlayer spacing to accommodate transmission electron microscopy (HRTEM), high-angle annular it, and thus the barrier is larger. Remarkably, these calculations dark-field scanning transmission electron microscopy (HAADF- show that the lattice orientation plays a key role in the STEM), and electron energy loss spectroscopy (EELS) were carried observed variations in energy barriers upon Mn-substitution, out on a TECNAI F20 HRTEM/STEM microscope operated at 200 kV. Cross-sectional lamellae were prepared by a focused ion beam, which provides a valuable guide for precisely engineering placed onto a copper transmission electron microscopy grid, and magneto-ionics from a structural perspective. topped with a protective platinum layer. Transport Measurements. To determine the electrical proper- ■ CONCLUSIONS In summary, here we demonstrate that magneto-ionic effects in ties of CoN and CoMnN thin films, both films were deposited directly onto high-resistivity Si substrates. Resistivity values were recorded from 30 to 300 K by utilizing the 4-contact van der Pauw CoN films are drastically enhanced by the introduction of Mn configuration in a closed He refrigeration system. (in low percentages) to the binary composition of the Ab initio Calculations. First-principles calculations were based on transition metal nitride. Such enhancement is ascribed to the the projector-augmented wave (PAW)51 method as implemented in key role of Mn in modifying the microstructure and electric the VASP package52 using the generalized gradient approximation.53 transport properties of the CoN films, as well as the change in To model a Mn-doped Co and compare it to bare Co, we used (4 × the formation energy of the metal-ion bonds. The incorpo- 2) supercells with a four-monolayer thickness for both Co(90%)- Mn(10%) and Co. To evaluate the energy barrier a N atom needs to ration of Mn was proven effective to bring a more amorphous overcome in order to be inserted in the surface, the nudged elastic microstructure and semiconducting properties to CoN. As a band method (NEB)49,50 was used on the nitrogen pathway. At each result, a 6.7-fold enhancement of the saturation magnetization step, the atomic coordinates were relaxed until the forces became as and higher magneto-ionic cyclability are achieved. From ab small as 1 meV Å−1. A kinetic energy cutoff of 500 eV was applied for initio calculations, the energy barrier in CoMn−N is overall the plane-wave basis set and 25 × 25 × 1 k-point meshes were used. smaller than for Co−N, providing hints to understand the more efficient N-ion motion. The reported enhanced ion motion effects in CoMnN films by moderate Mn introduction ■ * ASSOCIATED CONTENT sı Supporting Information (and the extension to other transition-metal systems) are The Supporting Information is available free of charge at appealing for diverse technological areas (beyond magneto- https://pubs.acs.org/doi/10.1021/acsami.2c12847. 44587 https://doi.org/10.1021/acsami.2c12847 ACS Appl. Mater. Interfaces 2022, 14, 44581−44590 ACS Applied Materials & Interfaces www.acsami.org Research Article Dependence of the carrier densities (donor/acceptor) of no 875018; REMINDS 2021-ERC-Advanced Grant, agree- as-prepared CoMnN films as a function of temperature; ment no 101054687), the European Union’s Horizon 2020 magnetoelectric characterizations for CoN and CoMnN research and innovation program (European Training Net- films; structural characterization by HRTEM; and work, ETN/ITN Marie Skłodowska-Curie grant no 861145; compositional characterization by TEM and energy- and Integrated Infrastructure, RADIATE, grant no 824096), dispersive X-ray analysis (EDX) line scans (PDF) the Spanish Government (PID2020-116844RB-C21 and PDC2021-121276-C31), the Generalitat de Catalunya (2017- ■ AUTHOR INFORMATION Corresponding Authors SGR-292), and the KU Leuven (BOF program) is acknowl- edged. We acknowledge the technical support from the MiNa Laboratory at IMN in Madrid, who received funding from the Enric Menéndez − Departament de Física, Universitat CM (project S2018/NMT-4291 TEC2SPACE), MINECO Autònoma de Barcelona, E-08193 Cerdanyola del Vallès, (project CSIC13-4E-1794), and EU (FEDER, FSE). A.Q. Spain; orcid.org/0000-0003-3809-2863; acknowledges financial support from the Spanish Ministry of Email: Enric.Menendez@uab.cat Science, Innovation and Universities through the “Severo Jordi Sort − Departament de Física, Universitat Autònoma de Ochoa” Programme for Centers of Excellence in R&D Barcelona, E-08193 Cerdanyola del Vallès, Spain; Institució (FUNFUTURE CEX2019-000917-S) and the “Juan de la Catalana de Recerca i Estudis Avançats (ICREA), E-08010 Barcelona, Spain; orcid.org/0000-0003-1213-3639; Cierva�Formación” contract (FJC2019-039780-I). J.S. thanks Email: Jordi.Sort@uab.cat the Spanish “Fábrica Nacional de Moneda y Timbre” (FNMT) for fruitful discussions. E.M. is a Serra Húnter Fellow. Authors Zhengwei Tan − Departament de Física, Universitat Autònoma de Barcelona, E-08193 Cerdanyola del Vallès, ■ REFERENCES (1) Park, B. H.; Kang, B. S.; Bu, S. D.; Noh, T. W.; Lee, J.; Jo, W. Spain Lanthanum-substituted Bismuth Titanate for Use in Non-volatile Sofia Martins − Departament de Física, Universitat Autònoma Memories. Nature 1999, 401, 682−684. de Barcelona, E-08193 Cerdanyola del Vallès, Spain (2) Choi, W. S.; Chisholm, M. F.; Singh, D. J.; Choi, T.; Jellison, G. Michael Escobar − Departament de Física, Universitat E.; Lee, H. N. Wide Bandgap Tunability in Complex Transition Metal Autònoma de Barcelona, E-08193 Cerdanyola del Vallès, Oxides by Site-specific Substitution. Nat. Commun. 2012, 3, 689. Spain (3) Manyala, N.; DiTusa, J. F.; Aeppli, G.; Ramirez, A. P. Doping a Julius de Rojas − Departament de Física, Universitat Semiconductor to Create an Unconventional Metal. 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