Tatsunori Sanai

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 on MgO(001) and SrTiO3(001) substrates

We grew Mn4N epitaxial thin films capped with Au layers on MgO(001) and SrTiO3(001) substrates by molecular beam epitaxy. Perpendicular magnetic anisotropy (PMA) was confirmed in all the samples at room temperature from the magnetization versus magnetic field curves using superconducting quantum interference device magnetometer. From the ω-2θ x-ray diffraction (XRD) and ϕ-2θχ XRD patterns, the ratios of perpendicular lattice constant c to in-plane lattice constant a, c/a, were found to be about 0.99 for all the samples. These results imply that PMA is attributed to the in-plane tensile strain in the Mn4N films.

Negative spin polarization at the Fermi level in Fe4N epitaxial films by spin-resolved photoelectron spectroscopy

We evaluated the valence band structures of 10-nm-thick ferromagnetic Fe4N epitaxial films on SrTiO3(001) substrates at room temperature using spin-resolved photoelectron spectroscopy. Negative spin polarization is confirmed at the Fermi level. The experimentally obtained photoelectron spectra are well explained by first-principles calculations based on the psuedopotential method.

Hard x-ray photoelectron spectroscopy study on valence band structure of semiconducting BaSi2

The valence band structures of a 35-nm-thick BaSi2 epitaxial film on Si(111) have been explored at room temperature by hard x-ray photoelectron spectroscopy (HAXPES). The experimentally obtained photoelectron spectrum is well reproduced by first-principles calculations based on the pseudopotential method. The top of the valence band consists mainly of Si 3s and 3p states in BaSi2, suggesting that the effective mass of holes is small in BaSi2. This is favorable from the viewpoint of solar cell applications. The observed spectrum shifted slightly to the lower energy side due to n-type conductivity of BaSi2. The valence band top was observed at about 0.8 eV below the Fermi level in the HAXPES spectrum.

X-ray magnetic circular dichroism for CoxFe4−xN (x = 0, 3, 4) films grown by molecular beam epitaxy

We evaluated orbital (morb) and spin magnetic moments (mspin) of CoxFe4−xN (x = 0, 3, 4) epitaxial thin films grown by molecular beam epitaxy using x-ray magnetic circular dichroism, and discussed the dependence of these values on x. Site-averaged mspin value of Fe atoms was deduced to be 1.91 μB per atom, and that of Co atoms to be 1.47 μB per atom in Co3FeN at 300 K. These values are close to 1.87 μB per Fe atom in Fe4N and 1.43 μB per Co atom in Co4N, respectively. This result implies that the Fe and Co atoms in the Co3FeN films were located both at corner and face-centered sites in the anti-perovskite lattice. Spin magnetic moments per unit cell were decreased linearly with increasing x in CoxFe4−xN. This tendency is in good agreement with theory predicted by the first-principle calculation.

Electronic structures and magnetic moments of Co3FeN thin films grown by molecular beam epitaxy

We evaluated electronic structures and magnetic moments in Co3FeN epitaxial films on SrTiO3(001). The experimentally obtained hard x-ray photoemission spectra of the Co3FeN film have a good agreement with those calculated. Site averaged spin magnetic moments deduced by x-ray magnetic circular dichroism were 1.52 μB per Co atom and 2.08 μB per Fe atom at 100 K. They are close to those of Co4N and Fe4N, respectively, implying that the Co and Fe atoms randomly occupy the corner and face-centered sites in the Co3FeN unit cell.

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.