Magnetic Properties of Anti-Perovskite Type Transition Metal Ferromagnetic Nitrides

Abstract

Ferromagnetic anti-perovskite type nitrides (Fig.1) have special features suitable for application to spintronics devices. We have succeeded in growing Co x Fe4 - x N and Co x Mn4 - x N films epitaxially on SrTiO3(001) substrates using molecular beam epitaxy for the first time, and characterized their magnetic properties.1-3 Co3FeN is theoretically predicted to have the large negative spin-polarization of density of states at the Fermi level (P D),4 when Fe atoms are located at the corner (I) sites and Co atoms at the face center (II) sites of the cubic unit cell. We performed x-ray magnetic circular dichroism (XMCD) measurements for the Co3FeN epitaxial films, and evaluated the orbital and spin magnetic moments (m spin) of Co and Fe atoms. Clear XMCD spectra were observed at Fe and Co L 2,3 absorption edges for the Co3FeN films. The m spin per Fe and Co atoms in the Co3FeN film was evaluated to be 2.08 and 1.52 μ B, respectively.3 In the ordered large spin-polarized Co3FeN phase, the m spin per Fe and Co atoms is theoretically predicted to have 3.09 and 1.41 μ B.3 The experimentally obtained m spin of Fe atoms, 2.08 μ B, is close to 2.29 μ B, the site averaged value in Fe4N.5 This result shows that the Fe atoms in the Co3FeN film are located at both I and II sites. Mössbauer measurements were also performed in the Co3FeN film and indicated the consistent results. Growth conditions should be optimized so that Fe and Co atoms are located only at I and II sites, respectively, in order to achieve large spin-polarized Co3FeN. Perpendicular magnetic anisotropy (PMA) was reported in the Mn4N films.6 In addition, large positive P D is predicted by the first principle calculation.7 Co x Mn4 - x N is predicted to have PMA and small saturation magnetization (M S), which is good for application to current-induced magnetic domain wall motion devices. PMA was observed in the Co x Mn4 - x N(x≤0.8) epitaxial films from the magnetization curves measured by a superconductive quantum interface device magnetometer. The M S of the Mn4N and Co0.8Mn3.2N films was 145 and 15 emu/cc at 300 K, respectively. The effective PMA energy of the Mn4N films was about 2.2 Merg/cc.2 The c/a ratio, where c and a are lattice constants along perpendicular and in-plane direction of Mn4N films, respectively, was approximately 0.99.2 We think that PMA in the Mn4N films is induced by the in-plane tensile strain. Distinct XMCD spectra at Mn and Co L 2,3 absorption edges were observed for the Co0.8Mn3.2N film. Shape of the XMCD signal at Mn L 2,3 edges was broader than that of Mn4N. In contrast, XMCD signal at Co L 2,3 edges was sharp. This indicates that the Mn atoms are more likely to locate at II sites, and the Co atoms at I sites instead. It is predicted that the Mn atoms at II sites have the itinerant feature due to band hybridization with neighboring N atoms, while the Co atoms at I sites the localized feature. The element specific magnetization curves measured at the Mn and Co L 3 edges, under the applied magnetic field perpendicular to the films, showed the clear open hysteresis, indicating that PMA surely appeared in the Mn4N and Co0.8Mn3.2N films. 1 T. Sanai, K. Ito, T. Suemasu et al., J. Cryst. Growth 357, 53 (2011). 2 Y. Yasutomi, K. Ito, T. Suemasu et al., J. Appl. Phys. 115, 17A935 (2014). 3 K. Ito, T. Suemasu et al., Appl. Phys. Lett. 103, 232403 (2013). 4 Y. Takahashi et al., J. Magn. Magn. Mater. 323, 2941 (2011). 5 A. Sakuma, J. Magn. Magn. Mater. 102, 127 (1991). 6M. Tsunoda and K. Kabara, presented at ICAUMS2012, 2pPS-47. 7 M. S. Miao et al., Phys. Rev. B 72, 033204 (2005).Fig.1 Lattice structure of anti-perovskite type nitrides.