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We grew Mn4−xNixN epitaxial thin films on MgO(0 0 1) by molecular beam epitaxy, as well as studied their crystalline qualities and magnetic properties. The films were decomposed into Ni8N or Mn-Ni alloys when x ≥ 2, as confirmed by X-ray diffraction and reflection high-energy electron diffraction, but this decomposition was mitigated by reducing the substrate growth temperature. The lattice constants decreased with increased Ni substitution except when the Mn ratio was high, while the crystal orientation tended to degrade. The magnetic properties were measured via vibrating sample magnetometer, and it was found that the saturation magnetization (MS) and perpendicular magnetic anisotropy (PMA) diminished with a small amount of Ni substitution. Specifically, the MS value was remarkably decreased from 86.3 ± 1.1 emu/cm3 (Mn4N) to 19.0 ± 0.5 emu/cm3 (Mn3.85Ni0.25N), and the magnetic anisotropy constant was decreased from approximately 0.94–0.027 Merg/cm3, respectively. The PMA vanished with further Ni substitution. Ultimately, a small MS and a PMA were simultaneously achieved with a small amount of Ni substitution. These properties support spin transfer torque, which can be applied to the emerging non-volatile memory devices using domain wall motion.

Spin-transfer torque (STT) and spin-orbit torque (SOT) are spintronic phenomena allowing magnetization manipulation using electrical currents. Beyond their fundamental interest, they allow developing new classes of magnetic memories and logic devices, in particular based on domain wall (DW) motion. In this work, we report the study of STT driven DW motion in ferrimagnetic manganese nickel nitride (Mn4-xNixN) films, in which a fine adjustment of the Ni content allows setting the magnetic compensation at room temperature. The reduced magnetization, combined with the large spin polarization of conduction electrons, strongly enhances the STT so that domain wall velocities approaching 3000 m/s can be obtained for Ni compositions close to the compensation point. In addition, a reversal of the domain wall motion direction is observed when the magnetic compensation composition is crossed. This striking feature, related to the change of direction of the spin polarization with respect to that of the net magnetization, is clarified by ab initio band structure calculations., Comment: 24 pages, 4 figures

Spin-transfer torque (STT) and spin-orbit torque (SOT) are spintronic phenomena allowing magnetization manipulation using electrical currents. Beyond their fundamental interest, they allow developing new classes of magnetic memories and logic devices, in particular based on domain wall (DW) motion. In this work, we report the study of STT-driven DW motion in ferrimagnetic manganese nickel nitride (Mn

Epitaxial Fe 4− x Mn x N ( x = 0, 1, 2, 3, and 4) thin films were successfully grown on MgO(0 0 1) single-crystal substrates by molecular beam epitaxy, and their crystalline qualities and magnetic properties were investigated. It was found that the lattice constants of Fe 4− x Mn x N obtained from X-ray diffraction measurement increased with the Mn content. The ratio of the perpendicular lattice constant c to the in-plane lattice constant a of Fe 4− x Mn x N was found to be about 0.99 at x ⩾ 2. The magnetic properties evaluated using a vibrating sample magnetometer at room temperature revealed that all of the Fe 4− x Mn x N films exhibited ferromagnetic behavior regardless of the value of x . In addition, the saturation magnetization decreased non-linearly as the Mn content increased. Finally, FeMn 3 N and Mn 4 N exhibited perpendicular anisotropy and their uniaxial magnetic anisotropy energies were 2.2 × 10 5 and 7.5 × 10 5 erg/cm 3 , respectively.

We grow MnAl films on different underlayers by molecular beam epitaxy (MBE), and investigate their structural and magnetic properties. L 1 0 -ordered MnAl films were successfully grown both on an MgO(0 0 1) single-crystalline substrate and on an Mn 4 N(0 0 1) buffer layer formed on MgO(0 0 1) and SrTiO 3 (0 0 1) substrates. For the MgO substrate, post rapid thermal annealing (RTA) drastically improved the crystalline quality and the degree of L 1 0 -ordering, whereas no improvement in the crystallinity was achieved by altering the substrate temperature ( T S ) during MBE growth. However, high-quality L 1 0 -MnAl films were formed on the Mn 4 N buffer layer by simply varying T S . Structural analysis using X-ray diffraction showed MnAl on an MgO substrate had a cubic structure whereas MnAl on the Mn 4 N buffer had a tetragonal structure. This difference in crystal structure affected the magnetic properties of the MnAl films. The uniaxial magnetic anisotropy constant ( K u ) was drastically improved by inserting an Mn 4 N buffer layer. We achieved a perpendicular magnetic anisotropy of K u = 5.0 ± 0.7 Merg/cm 3 for MnAl/Mn 4 N film on MgO and 6.0 ± 0.2 Merg/cm 3 on STO. These results suggest that Mn 4 N has potential as an underlayer for L 1 0 -MnAl.

We grew epitaxial films of α′-Fe 8 N by molecular beam epitaxy on α-Fe buffered MgO(001) and MgAl 2 O 4 (MAO)(001) substrates. The lattice mismatches of these substrates with α′-Fe 8 N are −4.0% and 0.08%, respectively. We discuss effects on the crystal orientation of the grown layers in terms of the substrate material and the thickness of an α-Fe buffer layer. The 3-nm-thick α-Fe buffer layer on MAO was compressively strained and provided a more suitable in-plane lattice constant for growth of α′-Fe 8 N. The full width at half maximum of the x-ray ω -scan rocking curve peak intensity of the α′-Fe 8 N(002) plane was 0.09° when it was formed on α-Fe(3 nm)/MAO. This value was much smaller than that obtained for α-Fe(3 nm)/MgO samples (1.85°). (α′′+α′)-Fe 16 N 2 films, which had partly ordered N-sites, were formed by post-annealing of the α′-Fe 8 N films on α-Fe/MgO(001) and α-Fe/MAO(001) at 165 °C. The degree of N-site ordering reached 0.08 after 20 h for (α′′+α′)-Fe 16 N 2 grown on α-Fe(3 nm)/MAO(001). The saturation magnetizations were approximately 1650 emu/cm 3 , regardless of the annealing time and substrate used. The saturation field of the (α′′+α′)-Fe 16 N 2 films increased by the post-annealing.

We grew ferromagnetic Fe 4 N films by molecular beam epitaxy on MgO(001), MgAl 2 O 4 (MAO)(001), SrTiO 3 (STO)(001), and CaF 2 (001) substrates, possessing the lattice spacing close to Si(001) plane. Highly oriented epitaxial growth was confirmed for the Fe 4 N films on the MgO, MAO, and STO by reflection high-energy electron diffraction and x-ray diffractions. The degree of orientation of the Fe 4 N film on the STO was the best among these samples. This was attributed to the smallest lattice mismatch of −2.8% between Fe 4 N(001) and STO(001). On the other hand, crystallinity of the Fe 4 N film on the CaF 2 (001) substrate was poor due to a very large lattice mismatch of −30% between Fe 4 N(001) and CaF 2 (001) arising from the unexpected epitaxial relationship as Fe 4 N(001)[100] || CaF 2 (001)[100]. The saturation magnetization of the Fe 4 N films was approximately 1200 emu/cm 3 at room temperature for all the samples, and the magnetization easy axis was in-plane Fe 4 N[100]. We consider that STO is the suitable buffer layer for the growth of Fe 4 N on Si(001), hence to realize the Si-based spintronics devices using highly spin-polarized Fe 4 N.

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 Mn

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.

13 pages, 4 figures, and supplementary information; International audience; 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 magnetisation, 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 polarised 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 magnetisation and a large spin polarisation. Finally, we show that the application of gate voltages through the SrTiO3 substrates allows modulating the Mn4N coercive field with a large efficiency.

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.

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.

Ferrimagnets close to the magnetic compensation are excellent candidates to spin-torque-based spintronic applications, as their small magnetizations allow lowering switching currents. Here, we studied the magnetic compensation of Mn4 − xNixN epitaxial films by performing x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the L2,3 absorption edges of Mn and Ni atoms and compared them with those of Ni3FeN films. The XAS spectrum of the Ni3FeN films exhibits shoulders at approximately 2 eV above the Ni L2,3 main peaks, originating from orbitals hybridization between Ni 3d at face-centered (II) sites and N 2p at body-centered sites. However, such shoulders are not observed at the Ni L2,3 edges of the Mn4 − xNixN films (x = 0.1 and 0.25). These results indicate that the orbitals of Ni atoms do not hybridize with those of N atoms. Hence, Ni atoms preferentially occupy corner (I) sites, where hybridization is weak because of the relatively long distance between Ni at I sites and N atoms. The XMCD signals of Mn and Ni atoms reverse sign between x = 0.1 and 0.25. This shows that the directions of the magnetic moments carried by Mn and Ni atoms are reversed, indicating that the magnetic compensation occurs in the range 0.1

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.

The anomalous Hall effect in Fe4− x Mn x N (x = 0, 1, 2, 3, and 4) with antiperovskite structure were investigated. Anomalous Hall conductivity (σAH) strongly depended on the measurement temperature for x = 0 and 4, while constant for x = 1, 2, and 3. σAH decreased with x and even reversed sign for x ≥ 3 at T = 300 K, revealing extremely wide modulation range of σAH. Quantitative analysis based on the existing scaling law suggested that a competition between two components, skew scattering and side-jump, with different polarities is responsible for the x-mediated sign reversal.

Herein, 30 nm-thick Fe4−xMnxN (x = 0, 1, 2, 3, and 4) epitaxial films were grown on MgO(001) substrates by molecular beam epitaxy, and the anisotropic magnetoresistance (AMR) properties were measured at temperatures (T) between 10 and 300 K. A negative AMR effect was observed in the Fe4N and FeMn3N films at T ≤ 300 K and in the Mn4N film at T ≤ 100 K. In contrast, a positive AMR effect was observed in the Fe3MnN and Fe2Mn2N films at T ≤ 300 K. Using the relationship between the AMR ratio, the spin polarization of the density of states at the Fermi level, and the spin polarization of the electrical conductivity (Pσ), we derived the sign of Pσ to be negative in Fe4N, Fe3MnN, and Fe2Mn2N and to be positive in FeMn3N and Mn4N. These results show that the minority spin transport is dominant in Fe4−xMnxN for x = 0, 1, and 2 at lower temperatures, whereas the majority spin transport is dominant with increasing x in Fe4−xMnxN for x = 3 and 4.

We report the observation of the Hanle effect in the giant magnetoresistance measurement of metallic lateral nanostructures. By applying magnetic fields perpendicularly to the magnetization of the electrodes, we obtain resistance values lower than that of the parallel magnetization state, due to the extra decoherence provided by the random propagation time of the carriers. An analytical model is developed, enabling us to extract the characteristic transport parameters of CoFe, Al, and Cu from the fitting of the experimental results. We compare the obtained parameters to that of standard non-local Hanle measurements in lateral spin-valves, with the interest that this measurement configuration gives much larger signal variation.

Current-driven magnetic domain wall (DW) motion has been extensively studied not only theoretically, but also experimentally. The DW motion is induced by spin-transfer torque, that is, the transfer of spin angular momentum from conduction electrons to localized electrons. The velocity of DW motion is proportional to the spin polarization [P a = (σ ↑ − σ ↓ )/(σ ↑ + σ ↓ )] of electrical conductivity (σ) and its direction is the same as electron current when P σ > 0. The reverse DW motion is thus expected in ferromagnetic materials with negative spin polarization (P σ σ material and a negative P σ material, are of fundamental interest. We have paid a lot of attention to ferromagnetic Fe 4 N epitaxial films for application to spintronics devices because it is theoretically expected to have a large negative spin polarization (P σ = −1.0).2 Very recently, we confirmed its negative spin polarization by experiment.3,4

The magnetic structure of the domain wall (DW) of a 30-nm-thick Fe4N epitaxial film with a negative spin polarization of the electrical conductivity is observed by magnetic force microscopy and is well explained by micromagnetic simulation. The Fe4N film is grown by molecular beam epitaxy on a SrTiO3(001) substrate and processed into arc-shaped ferromagnetic nanostrips 0.3 μm wide by electron beam lithography and reactive ion etching with Cl2 and BCl3 plasma. Two electrodes mounted approximately 12 μm apart on the nanostrip register an electrical resistance at 8 K. By changing the direction of an external magnetic field (0.2 T), the presence or absence of a DW positioned in the nanostrip between the two electrodes can be controlled. The resistance is increased by approximately 0.5 Ω when the DW is located between the electrodes, which signifies the negative anisotropic magnetoresistance effect of Fe4N. The electrical detection of the resistance change is an important step toward the electrical detection of current-induced DW motion in Fe4N.

The 20–60 nm-thick epitaxial NixFe4-xN (x = 0, 1, 3, and 4) films were successfully fabricated on SrTiO3(001) single-crystal substrates by alternating the substrate temperature (Tsub), and their crystalline qualities and magnetic properties were investigated. It was found that the crystal orientation and the degree of order of N site were improved with the increase of Tsub for x = 1 and 3. The lattice constant and saturation magnetization decreased as the Ni content increased. This tendency was in good agreement with first-principle calculation. Curie temperature of the Ni3FeN film was estimated to be 266 K from the temperature dependence of magnetization. The Ni4N film was not ferromagnetic but paramagnetic due to its low degree of order of N site.

Current-induced magnetic domain wall (DW) motion in ferromagnetic ribbons is utilized in spintronic devices. The direction of the motion changes in response to the sign of the spin-polarizability of the current through the ribbon. The DW motion is expected to measure the sign. In this study, we investigate the magnetic structures of chamfered L-shaped nano-ribbons using micro-magnetic simulations, and show that the position at which the DW is produced can be controlled by applying an external magnetic field with a low spin-polarized current (SPC). In particular, we use the material parameters of Fe4N and permalloy to simulate the magnetic structure of the ribbon. The DW can be produced at either of two locations in a chamfered corner of the ribbon, and disappears upon applying an external magnetic field. From this point, after the field is removed, a new DW is produced at either of two locations, and its position can be controlled by adjusting the low SPC.

We grow 25-nm-thick Mn4N and Co0.2Mn3.8N epitaxial films on SrTiO3(001) by molecular beam epitaxy. These films show the tetragonal structure with a tetragonal axial ratio c/a of approximately 0.99. Their magnetic properties are measured at 300 K, and perpendicular magnetic anisotropy is confirmed in both films. There is a tendency that as the Co composition increases, an anisotropy field increases, whereas saturation magnetization and uniaxial magnetic anisotropy energy decrease. First-principles calculation predicts the existence of tetragonal Mn4N phase. This explains the c/a ∼ 0.99 in the Mn4N films regardless of their film thickness and lattice mismatch with substrates used.

We prepared CoxFe4−xN (x = 0, 1, 3) films on SrTiO3(STO)(001) substrates by molecular beam epitaxy. The epitaxial relationship with CoxFe4−xN[100](001) || STO[100](001) was confirmed by ω-2θ (out-of-plane) and ϕ-2θχ (in-plane) x-ray diffraction (XRD) measurements. The degree of order of atoms (S) in the CoxFe4−xN films was estimated to be ∼0.5 by the peak intensity ratio of CoxFe4−xN(100) (superlattice diffraction line) to (400) (fundamental diffraction line) in the ϕ-2θχ XRD patterns. Conversion electron Mossbauer spectroscopy studies for the CoxFe4−xN films revealed that some N atoms are located at interstitial sites between the two nearest corner sites in the CoxFe4−xN films, and/or Fe atoms are located at both the corner and face-centered sites in the CoFe3N and Co3FeN films. In order to realize high spin-polarized CoxFe4−xN films having large S, further optimization of growth condition is required to prevent the site-disorders.

We grow a 15-nm-thick ferromagnetic Fe4N epitaxial film on a SrTiO3(001) substrate by molecular beam epitaxy, and process it into approximately 0.5-µm-wide and 24-µm-long L-shaped ferromagnetic narrow wires by electron-beam lithography and Cl2 reactive ion etching. Their longitudinal directions are set in parallel to the magnetic easy axes, Fe4N[100] and [010]. With applying external magnetic field in the direction parallel to Fe4N[100] or [010], the position of domain wall is controlled either on the upper side or lower side of the corner. This experiment is the preliminary step toward current-driven domain wall motion in Fe4N having a negative spin polarization.