Toshiki Gushi

Perpendicular magnetic anisotropy in CoxMn4−xN (x = 0 and 0.2) epitaxial films and possibility of tetragonal Mn4N phase

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.

Epitaxial growth and magnetic properties of NixFe4-xN (x = 0, 1, 3, and 4) films on SrTiO3(001) substrates

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.

Mössbauer study on epitaxial CoxFe4−xN films grown by molecular beam epitaxy

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.

Fabrication of L-shaped Fe4N ferromagnetic narrow wires and position control of magnetic domain wall with magnetic field

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.

Transition from minority to majority spin transport in iron-manganese nitride Fe4−xMnxN films with increasing x

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.

Controlling magnetic domain wall positions with an external magnetic field and a low spin-polarized current in chamfered L-shaped ferromagnetic thin ribbons

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.

Electrical detection of magnetic domain wall in Fe4N nanostrip by negative anisotropic magnetoresistance effect

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.

Control of domain wall position in L-shaped Fe 4 N negatively spin polarized ferromagnetic nanowire

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 <; 0) compared to those with positive spin polarization, because minority spin dominates the electrical conduction. Thereby, spintronics devices composed of both a positive P_σ material and a negative P_σ material, are of fundamental interest. We have paid a lot of attention to ferromagnetic Fe₄N epitaxial films for application to spintronics devices because it is theoretically expected to have a large negative spin polarization (P_σ = -1.0). Very recently, we confirmed its negative spin polarization by experiment.