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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 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.

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 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.

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.