Kazuki Kabara

Sign of the spin-polarization in cobalt-iron nitride films determined by the anisotropic magnetoresistance effect

We present the anisotropic magnetoresistance (AMR) properties in Fe4N, Co3FeN and Co4N epitaxial thin films grown on SrTiO3(001) substrates using molecular beam epitaxy. A negative AMR effect was observed in the Fe4N and Co3FeN films below 300 K. This behavior was attributed to the negative spin-polarizations of both the electrical conductivity and density of states at the Fermi level. The sign of the AMR ratio changed at ∼140 K in the Co4N film and was negative below 140 K. Diffraction lines corresponding to the (100) plane were clearly observed in the ϕ-2θχ (in-plane) x-ray diffraction patterns for the Fe4N and Co3FeN films but not for the Co4N film. This indicated that nitrogen atoms were present at the body center of each unit cell in the Fe4N and Co3FeN films, whereas they were lacking in the Co4N film. This assumption is discussed with energetic calculations using a spin density-functional theory.

Perpendicular magnetic anisotropy of Mn4N films fabricated by reactive sputtering method

Manganese nitride films were fabricated on MgO substrates by changing N2 flow ratio into Ar gas ( PN2) during reactive sputtering deposition of the films, and their crystal structures and magnetic properties were investigated. Single phased e-Mn4N films were obtained when PN2 was 5%–9%, and the tetragonal lattice distortion was identified in all the Mn4N films (the lattice constant ratio, c/a = 0.99). Perpendicular magnetic anisotropy was observed in all the specimens. The Mn4N film, fabricated with PN2 = 8%, has a low saturation magnetization (Ms = 110 emu/cc) and relatively high magnetic anisotropic energy (Ku = 8.8 × 105 erg/cc). Both Ms and Ku of the films drastically changed with mixing other phases (α-Mn, β-Mn, η-Mn3N2, and possibly γ-Mn) by varying PN2.

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.

Fabrication of MgAl2O4 tunnel barrier by radio frequency-sputtering method and magnetoresistance effect through it with Fe or Fe4N ferromagnetic electrode

Spinel MgAl2O4 thin films were deposited on MgO single-crystal substrates and epitaxial Fe (or Fe4N) thin films by RF-sputtering from a ceramic target. Epitaxial relationship was confirmed by X-ray diffraction analysis between the crystalline spinel MgAl2O4 films and the respective substrate and underlayers, while no diffraction peak was observed from the films deposited on amorphous substrates. Spin-valve type magnetic tunnel junctions (MTJs) with a stacking structure of Fe [Fe4N]/MgAl2O4/CoFeB/Ru/Fe/MnIr exhibited normal [inverse] tunnel magnetoresistance (TMR) effect, reflecting the sign of spin polarization of Fe [Fe4N]. The maximum magnitude of the TMR ratio obtained for the Fe-based and Fe4N-based MTJs was 67% and 18%, respectively. The resistance area product values of the MTJs were significantly larger than the reported values for the MTJs with a post-oxidized spinel MgAl2O4 barrier.

Negative anisotropic magnetoresistance resulting from minority spin transport in NixFe4−xN (x = 1 and 3) epitaxial films

We grew 50 nm-thick NixFe4−xN (x = 1 and 3) epitaxial films on a SrTiO3(001) single-crystal substrate by molecular beam epitaxy and measured their anisotropic magnetoresistance (AMR) ratios rAMR in the temperature range of 5–300 K with current directions set along either NixFe4−xN [100] or [110]. A negative rAMR was obtained up to 200 K or higher. Their magnitude | rAMR | increased with decreasing temperature. From the negative AMR effect and the negative spin-polarization of density of states for NixFe4−xN at the Fermi level, it can be stated that the minority spin transport is dominant in NixFe4−xN, similar to Fe4N and Co3FeN. The rAMR depends on the current direction that arises from the current direction dependence of s-d scattering. In the case of Ni3FeN, the rAMR decreased to nearly zero at 260 K. This temperature agreed well with the Curie temperature determined from the temperature dependence of magnetization. The AMR curves were reproduced well by using both cos2ϕ and cos4ϕ components below 100 K...

Transverse anisotropic magnetoresistance effects in pseudo-single-crystal γ′-Fe4N thin films

Transverse anisotropic magnetoresistance (AMR) effects, for which magnetization is rotated in an orthogonal plane to the current direction, were investigated at various temperatures, in order to clarify the structural transformation from a cubic to a tetragonal symmetry in a pseudo-single-crystal Fe4N film, which is predicted from the usual in-plane AMR measurements by the theory taking into account the spin-orbit interaction and crystal field splitting of 3d bands. According to a phenomenological theory of AMR, which derives only from the crystal symmetry, a cos 2θ component (C2tr) exists in transverse AMR curves for a tetragonal system but does not for a cubic system. In the Fe4N film, the C2tr shows a positive small value (0.12%) from 300 K to 50 K. However, the C2tr increases to negative value below 50 K and reaches to -2% at 5 K. The drastic increasing of the C2tr demonstrates the structural transformation from a cubic to a tetragonal symmetry below 50 K in the Fe4N film. In addition, the out-of-plane and in-plane lattice constants (c and a) were precisely determined with X-ray diffraction at room temperature using the Nelson-Riely function. As a result, the positive small C2tr above 50 K is attributed to a slightly distorted Fe4N lattice (c/a = 1.002).

Magneto-transport properties of pseudo-single-crystal Mn4N thin films

The anisotropic magnetoresistance (AMR) effect and the anomalous Hall effect (AHE) were investigated in the temperature range of 5–300 K for a pseudo-single-crystal Mn4N thin film. The sign of the AMR ratio changed from positive to negative when the temperature was lowered. Below 100 K, the cos 2θ component of the AMR curves significantly increased in magnitude, and a cos 4θ component appeared. Based on the electron scattering theory, which takes into account the tetragonal crystal field effect, it is suggested that the dominant scattering process in the Mn4N film is up-spin conduction electrons into up-spin d orbitals. The magnitude of the anomalous Hall conductivity (σAH) slightly increased with decreasing temperature, from 300 K to 150 K, and then it drastically dropped when the temperature was below 100 K. A sign change for σAH, from negative to positive, was observed at 30 K. The starting temperature at 100 K for the drastic change in the AHE corresponds well with that of the AMR, suggesting that the...

Anomalous Hall effects in pseudo-single-crystal γ′-Fe4N thin films

The anomalous Hall effects (AHE) were investigated at various temperatures for the pseudo-single-crystal Fe4N films, deposited on MgO substrates with changing the degree of order (S) of the nitrogen site. Both the anomalous Hall resistivity and the longitudinal resistivity simply decrease with lowering temperature for all the specimens. The AHE of the Fe4N films is presumed to arise from an intrinsic mechanism because of the relationship between the anomalous Hall resistivity and longitudinal resistivity. The anomalous Hall conductivity, σAH, exhibits a specific behavior at low temperature. In the case of the film with S = 0.93, the σAH drastically drops below 50 K, while it simply increases with lowering temperature in the range of 50–300 K. This low-temperature anomaly decays with decreasing S of the film and nearly vanishes in the films with low S. The threshold temperature and the dependence on S of the low-temperature anomaly of the σAH well correspond to those of the anisotropic magnetoresistance ef...