Magnetic Properties of Anti-Perovskite Type Transition Metal Ferromagnetic Nitrides

Ferromagnetic anti-perovskite type nitrides (Fig.1) have special features suitable for application to spintronics devices. We have succeeded in growing Co_xFe_4-xN and Co_xMn_4-xN 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_xMn_4-xN 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_xMn_4-xN(x≤0.8) epitaxial films from the magnetization curves measured by a superconductive quantum interface device magnetometer. The M_S of the Mn₄N and Co_0.8Mn_3.2N 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 Co_0.8Mn_3.2N 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 L₃ edges, under the applied magnetic field perpendicular to the films, showed the clear open hysteresis, indicating that PMA surely appeared in the Mn₄N and Co_0.8Mn_3.2N 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.