Mn4N Thin Films Research Articles

Fabrication of highly oriented Mn4N/Pt epitaxial bilayer structure on MgO(001) for spintronics applications

Magnetization control by electric current or spin current is an attractive technology with potential applications in memory and logic devices, and Mn4N has attracted much attention in recent years as a candidate material for current-induced magnetization switching and magnetic domain wall motion, as well as a skyrmion material. For these applications, a bilayer structure of Mn4N and heavy metals must be formed with high crystalline quality. In this study, we have achieved the epitaxial growth of Mn4N thin films on MgO(001) with a Pt underlayer. The reflection high-energy electron diffraction images and x-ray diffraction ω-rocking curves show that the Mn4N films fabricated on the Pt underlayers exhibit higher crystalline quality than the Mn4N films directly formed on the MgO. Furthermore, anomalous Hall effect loops with bump-like structures were observed. This is considered to be due to the topological spin texture. The Mn4N/Pt epitaxial bilayer structure has potential for use in spin–orbit torque applications and is a candidate configuration for topological spin textures.

Ferrimagnetic-ferromagnetic phase transition in Au-doped Mn4N epitaxial films confirmed by x-ray magnetic circular dichroism

The antiperovskite ferrimagnet Mn4N has perpendicular magnetic anisotropy and small spontaneous magnetization, both of which are favorable properties for current induced domain wall motion. Previously we have investigated the magnetic structure of 3d-element-doped Mn4N thin films and demonstrated ultrafast domain wall velocities reaching 3000 m s−1 in the vicinity of the magnetic compensation composition of Ni-doped Mn4N at the current density of j = 1.2 × 1012 A m−2 at room temperature (RT). In this study, we investigate the effect of Au doping on the magnetic structure of Mn4N films, and present a composition ratio-dependent sign reversal of the anomalous Hall effect at RT. X-ray magnetic circular dichroism measurement revealed that the magnetic moment of the face-centered Mn atoms of Mn4−xAuxN reversed between x = 0.1 and 0.2, and became parallel to that of the corner-site Mn atoms for x = 0.2 and 0.3. This result suggests that the ferrimagnetic-ferromagnetic phase transition occurred in Au-doped Mn4N epitaxial films as in the In-doped Mn4N epitaxial films.

Introduction of VN underlayer and caplayer for preparation of Mn4N(001) single-crystal thin film with perpendicular magnetic anisotropy

A Mn4N thin film is prepared on MgO(001) single-crystal substrate by introducing VN underlayer and caplayer which are respectively expected to inhibit the oxidation from substrate and surface. The thin film is prepared by ultra-high vacuum radio-frequency magnetron sputtering. The growth, the structure, and the magnetic properties are investigated. A fully epitaxial VN/Mn4N/VN film with sharp interfaces is formed on the substrate. The Mn4N film has low out-of-plane and in-plane orientation dispersions of about 1° and high N site order degree of 0.88. The lattice of Mn4N film is slightly deformed along the perpendicular direction (c/a = 0.9872) possibly due to accommodation of the lattice mismatch at Mn4N/VN interfaces. The film shows a low saturation magnetization of 85 kA/m (85 emu/cm3) and a strong perpendicular magnetic anisotropy. The present study has shown that introduction of VN underlayer and caplayer is useful for preparation of well-defined Mn4N thin films with perpendicular magnetic anisotropy.

Interfacial mixing effect in a promising skyrmionic material: Ferrimagnetic Mn4N

Interfacial mixing of elements is a well-known phenomenon found in thin film deposition. For thin-film magnetic heterostructures, interfacial compositional inhomogeneities can have drastic effects on the resulting functionalities. As such, care must be taken to characterize the compositional and magnetic properties of thin films intended for device use. Recently, ferrimagnetic Mn4N thin films have drawn considerable interest due to exhibiting perpendicular magnetic anisotropy, high domain-wall mobility, and good thermal stability. In this study, we employed x-ray photoelectron spectroscopy (XPS) and polarized neutron reflectometry (PNR) measurements to investigate the interfaces of an epitaxially grown MgO/Mn4N/Pt trilayer deposited at 450 °C. XPS revealed the thickness of elemental mixing regions of near 5 nm at both interfaces. Using PNR, we found that these interfaces exhibit essentially zero net magnetization at room temperature. Despite the high-temperature deposition at 450 °C, the thickness of mixing regions is comparable to those observed in magnetic films deposited at room temperature. Micromagnetic simulations show that this interfacial mixing should not deter the robust formation of small skyrmions, consistent with a recent experiment. The results obtained are encouraging in terms of the potential of integrating thermally stable Mn4N into future spintronic devices.

Wide modulation of coercive fields in Mn4N ferrimagnetic thin films caused dominantly by dislocation microstructures

Perpendicular magnetic anisotropy and coercive fields (Hc) are governed by the degree of order of N (S) in ferrimagnetic Mn4N thin films. However, in this study, we observed non-negligible modulation of Hc extending from 10 to 7.5 kOe for three Mn4N films grown at a substrate temperature (Tsub) in the range of 400–500 °C, even with identical S, which has not been discussed to date. The possible mechanisms for this phenomenon were explored by transport properties and fine structural analysis. The results indicated that longitudinal resistivity and anomalous Hall resistivity remain unchanged for three Mn4N films. Conversely, the number of dislocations and in-plane grain size (D) increased, whereas the residual strain (ε) decreased with increasing Tsub. This indicates that Hc of the Mn4N film with the same S is sensitive to the variation in dislocations, D and ε. Tunable Hc of this type can be effectively applied in magnetic and spintronic devices using ferrimagnetic Mn4N films, because Hc is an important determinant of the thermal stability of magnetization and working power consumption.

Tunable magnetic skyrmions in ferrimagnetic Mn4N

Thin films of ferrimagnetic Mn4N are candidate materials to host magnetic skyrmions that have demonstrated thermal stability up to 450 °C. However, there are no experimental reports observing skyrmions in this system. Here, we discuss the results of sputter grown 15–17 nm Mn4N thin films on the MgO substrate capped with Pt1−xCux layers. Vibrating sample magnetometry measurement of out-of-plane hysteresis loops confirmed that magnetic properties are insensitive to the cap layer composition. Imaging based on magnetic force microscopy measurements observed 300–50 nm sized skyrmions, as the Cu concentration was increased from x = 0–0.9. We performed density functional theory calculations and found that the interfacial Dzyaloshinskii–Moriya interactions (iDMI) follow a trend: Mn4N/MgO(001) < Cu/Mn4N(001) < Pt/Mn4N(001). We infer from these calculations that x in the Pt1−xCux capping layer can serve as a robust tuning knob to tailor the iDMI and control the skyrmion size. This work provides guidance to achieve smaller Néel-type skyrmions in Mn4N thin films, which is an important step forward for building thermally stable skyrmionic devices.

Rare-earth-free ferrimagnetic Mn4N sub-20 nm thin films as potential high-temperature spintronic material

Ferrimagnetic alloy thin films that exhibit perpendicular (out-of-plane) magnetic anisotropy (PMA) with low saturation magnetization, such as GdCo and Mn4N, were predicted to be favorable for hosting small Néel skyrmions for room temperature applications. Due to the exponential decay of interfacial Dzyaloshinskii–Moriya interaction and the limited range of spin–orbit torques, which can be used to drive skyrmion motion, the thickness of the ferrimagnetic layer has to be small, preferably under 20 nm. While there are examples of sub-20 nm, rare earth-transition metal (RE-TM), ferrimagnetic thin films fabricated by sputter deposition, to date, rare-earth-free sub-20 nm Mn4N films with PMA have only been reported to be achieved by molecular beam epitaxy, which is not suitable for massive production. Here, we report the epitaxial growth of sub-20 nm Mn4N films with PMA at 400 °C–450 °C substrate temperatures on MgO substrates by reactive sputtering. The Mn4N films were achieved by reducing the surface roughness of MgO substrate through a high-temperature vacuum annealing process. The optimal films showed low saturation magnetization (Ms = 43 emu/cc), low magnetic anisotropy energy (0.7 Merg/cc), and a remanent magnetization to saturation magnetization ratio (Mr/Ms) near 1 at room temperature. Preliminary ab initio density functional theory calculations have confirmed the ferrimagnetic ground state of Mn4N grown on MgO. The magnetic properties, along with the high thermal stability of Mn4N thin films in comparison with RE-TM thin films, provide the platform for future studies of practical skyrmion-based spintronic materials.

Perpendicular magnetic anisotropy in ferrimagnetic Mn4N films grown on (LaAlO3)0.3(Sr2TaAlO6)0.7(0 0 1) substrates by molecular beam epitaxy

Many studies have shown that Mn4N thin films possess perpendicular magnetic anisotropy (PMA) caused by in-plane tensile strain. Since cubic (LaAlO3)0.3(Sr2TaAlO6)0.7 [LSAT] (0 0 1) has a smaller lattice constant than Mn4N with a mismatch of 0.8%, a compressive strain is expected to be induced in Mn4N epitaxial films grown on an LSAT substrate. We grew Mn4N thin films on an LSAT(0 0 1) substrate by molecular beam epitaxy, and investigated the optimum growth temperature (TS) from the aspects of crystallinity, magnetic properties, and magneto-transport properties. Mn4N films were grown epitaxially at temperatures between TS = 700 and 800 °C with maintaining PMA, and the optimum TS was determined to be approximately 750 °C. In-plane and out-of-plane X-ray diffraction measurements showed that the Mn4N films were under tensile stress in contrast to our prediction. From the thickness dependence of magnetization, a dead layer of approximately 10 nm existed at the Mn4N/LSAT interface. Transmission electron microscopy observation revealed that Mn-N composites other than Mn4N existed in the early stage of MBE growth.

Strong correlation between uniaxial magnetic anisotropic constant and in-plane tensile strain in Mn4N epitaxial films

Ferrimagnetic Mn4N is a promising candidate for current-induced domain wall motion assisted by spin-transfer and spin–orbit torques. Mn4N can be doped to have perpendicular magnetic anisotropy (PMA) and a small spontaneous magnetization. However, the origin of the PMA of Mn4N has yet to be fully understood. Here, we investigated the relationship between the ratios of the perpendicular lattice constant c to the in-plane lattice constant a of Mn4N epitaxial thin films (c/a) and the uniaxial magnetic anisotropic constant (Ku) in Mn4N thin films grown on MgO(001), SrTiO3(001), and LaAlO3(001) substrates. The lattice mismatches between Mn4N and these substrates are approximately −6%, −0.1%, and +2%, respectively. All the Mn4N thin films had PMA and in-plane tensile distortion (c/a < 1) regardless of the Mn4N thickness and substrate. Although the magnitude of c/a depended on several factors, such as the Mn4N layer thickness and substrate, we found a strong correlation between c/a and Ku; Ku increased markedly when c/a deviated from 1. This result indicates that the origin of PMA is tensile distortion in Mn4N films; hence, it might be possible to control the magnitude of Ku by tuning c/a through the Mn4N layer thickness and the substrate.

Magnetic properties of MBE grown Mn4N on MgO, SiC, GaN and Al2O3 substrates

Mn4N is a compound magnetic material that can be grown using MBE while exhibiting several desirable magnetic properties such as strong perpendicular magnetic anisotropy, low saturation magnetization, large domain size, and record high domain wall velocities. In addition to its potential for spintronic applications exploiting spin orbit torque with epitaxial topological insulator/ferromagnet bilayers, the possibility of integrating Mn4N seamlessly with the wide bandgap semiconductors GaN and SiC provides a pathway to merge logic, memory and communication components. We report a comparative study of MBE grown Mn4N thin films on four crystalline substrates: cubic MgO, and hexagonal GaN, SiC and sapphire. Under similar growth conditions, the Mn4N film is found to grow single crystalline on MgO and SiC, polycrystalline on GaN, and amorphous on sapphire. The magnetic properties vary on the substrates and correlate to the structural properties. Interestingly, the field dependent anomalous Hall resistance of Mn4N on GaN shows different behavior from other substrates such as a flipped sign of the anomalous Hall resistance.

Large Current Driven Domain Wall Mobility and Gate Tuning of Coercivity in Ferrimagnetic Mn4N Thin Films.

Spintronics, which is the basis of a low-power, beyond-CMOS technology for computational and memory devices, remains up to now entirely based on critical materials such as Co, heavy metals and rare-earths. Here, we show that Mn4N, a rare-earth free ferrimagnet made of abundant elements, is an exciting candidate for the development of sustainable spintronics devices. Mn4N thin films grown epitaxially on SrTiO3 substrates possess remarkable properties, such as a perpendicular magnetization, a very high extraordinary Hall angle (2%) and smooth domain walls at the millimeter scale. Moreover, domain walls can be moved at record speeds by spin-polarized currents, in absence of spin-orbit torques. This can be explained by the large efficiency of the adiabatic spin transfer torque, due to the conjunction of a reduced magnetization and a large spin polarization. Finally, we show that the application of gate voltages through the SrTiO3 substrates allows modulating the Mn4N coercive field with a large efficiency.

Magnetic and magneto-transport properties of Mn4N thin films by Ni substitution and their possibility of magnetic compensation

Ferrimagnets are of interest in spintronics owing to the enhancement of spin transfer and spin–orbit torque in the vicinity of the magnetic compensation point. Here, we study the possibility of achieving compensation at room temperature in Mn4−xNixN films grown on SrTiO3(001) substrates by molecular beam epitaxy. The magnetic and magneto-transport properties of 30-nm-thick epitaxial Mn4−xNixN films (x = 0, 0.1, 0.25, and 0.5) were investigated. Ni introduction in Mn4N causes the spontaneous magnetization (MS) to decrease by 45% at x = 0.1 and by 75% at x = 0.25. All samples exhibited the perpendicular magnetic anisotropy required for spintronic applications as well as displaying a sharp magnetization reversal. Remarkably, the sign of the anomalous Hall resistivity changed in the samples between x = 0.1 and 0.25. Further, the magnetization increased for x = 0 and 0.1 and decreased for x = 0.25 when the temperature was decreased. Based on these results, we propose that there is a magnetic compensation composition between x = 0.1 and 0.25, where the magnetization of Mn4−xNixN becomes zero at room temperature. The small MS and perpendicular magnetic anisotropy of the Mn4−xNixN thin films, thus, make them suitable candidates for use in spin transfer torque-based devices.

Millimeter-sized magnetic domains in perpendicularly magnetized ferrimagnetic Mn4N thin films grown on SrTiO3

The use of epitaxial layers for domain wall-based spintronic applications is often hampered by the presence of pinning sites. Here, we show that when depositing Mn4N (10 nm) epitaxial films, the replacement of MgO(001) by SrTiO3(001) substrates allows minimizing the misfit, and to obtain an improved crystalline quality, a sharper switching, a full remanence, a high anisotropy and remarkable millimeter-sized magnetic domains, with straight and smooth domain walls. In a context of rising interest for current-induced domain wall motion in rare-earth ferrimagnets, we show that Mn4N/SrTiO3, which is rare-earth free, constitutes a very promising ferrimagnetic system for current-induced domain wall motion.

Contribution from Ising domains overlapping out-of-plane to perpendicular magnetic anisotropy in Mn4N thin films on MgO(001)

Single phase ε-Mn4N thin and ultrathin films are grown on MgO(001) using molecular beam epitaxy. Reflection high-energy electron diffraction and out-of-plane X-ray diffraction measurements are taken for each sample in order to determine the in- and out-of-plane strain for each sample. Vibrating sample magnetometry and superconducting quantum interference device measurements, which are performed on the thin and ultrathin films respectively, are used to plot the magnetization of each sample versus both in- and out-of-planeH→-fields and to determine the magnitude of perpendicular magnetic anisotropy in these films. Three significant components of perpendicular magnetic anisotropy are observed in these films and are attributed to sample strain (1 component) and shape (2 components). Among these components, the most significant component 0.8–4.9Mergcm3 is identified as a second term of shape anisotropy, which possesses a negative linear relationship with sample thickness over the range from 9nm to 310nm. Atomic (magnetic) force microscopy measurements show the presence of a surface localized magnetic polarization (22–82%), which increases with decreasing thickness, when the net magnetizations of the films are zero. The second term of shape anisotropy as well as the surface localized polarization, which each depend on sample thickness, are each regarded as a consequence of Ising domains overlapping out-of-plane in these films.

Metallic transport and large anomalous Hall effect at room temperature in ferrimagnetic Mn4N epitaxial thin film

We report the electrical transport properties of ferrimagnetic Mn4N (001) epitaxial thin films grown by pulsed laser deposition on MgO (001) substrates. The Mn4N thin films were tetragonally distorted with a ratio of out-of-plane to in-plane lattice constants of 0.987 and showed perpendicular magnetic anisotropy with an effective magnetic anisotropy constant of 0.16 MJ/m3, which is comparable with that of a recently reported molecular-beam-epitaxy-grown film. The thin films exhibited metallic transport with a room temperature resistivity of 125 μΩ cm in addition to a large anomalous Hall effect with a Hall angle tangent of 0.023.

Effect of substrate-film interface on magnetic properties of Mn 4N films

Ferrimagnetic Mn 4N thin films were deposited on blank Si(100), SiN x -coated Si(100) and polyimide-coated Si(100) substrates by dc reactive magnetron sputtering. Ordered Mn 4N structure with (002) texture was identified in these films at substrate temperatures from 150 to 250°C without further annealing. The films on blank Si(100) and SiN x -coated Si(100) exhibit much higher coercivity than those on polyimide-coated Si(100). However all films show a perpendicular coercivity of around two times higher than parallel coercivity. The saturation flux density is 240 to 610 G, increasing with elevated substrate temperature. The interface between substrate and film is found to dominate residual stress in films and to affect the absolute value of coercivity. The abnormal perpendicular magneto-anisotropy of these films is attributed to the columnar grains with ordered (002) structure and stress effect.