Making ferrimagnetic Mn4N ferromagnetic by non-magnetic element doping

Abstract

We show that antiperovskite Mn4N transforms from ferri- to ferromagnetism at room temperature (RT) when the Mn content is decreased by the addition of non-magnetic In atoms. Mn4N films have attracted increasing attention as candidates for the current induced domain wall motion (CIDWM) devices. They have a small magnetization (Ms ~ 100 kA/m [1]) and perpendicular magnetic anisotropy, contributing to high CIDWM mobility with low threshold current. In addition, Mn4-xNixN [2] and Mn4-xCoxN [3] epitaxial films have x-dependent magnetic compensation (MC) points at RT, probed by X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements. In Mn4-xNixN nanowires at the vicinity of the MC, we demonstrated ultrafast and efficient CIDWM (3,000 m/s with j=1.2×1012 A/m2) by pure spin transfer torque at RT [4]. In this work, we focused on Mn4-xInxN epitaxial films as new candidates for such materials [5].We grew 25nm-thick Mn4-xInxN films (x=0, 0.15 and 0.27) on MgO(001) by MBE. XAS and XMCD measurements were performed at the twin APPLE II undulator beam line BL16A of KEK-PF in Japan.Figure 1 shows XAS and XMCD spectra of Mn-L2,3 absorption edges of the samples. In the XAS spectra of all the samples, distinct shoulders marked by arrows appear at about 2 eV above the main L3 edges, which are caused by fcc-Mn(II) atoms [3]. In the XMCD spectrum of Mn4N(x=0), corner-site Mn(I) and fcc-Mn(II) atoms are attributable to peaks α and β, respectively [3]. Their different signs show that the magnetic moments of Mn(I) and Mn(II) are aligned antiparallel. When the In composition x increases up to 0.27, the sign of peak β reverses while that of peak α remains unchanged. It means that the magnetic moment of Mn(II) reverses and now aligns parallel to that of Mn(I), showing a ferri-to-ferromagnetic transition at RT. Figure 2 shows the anomalous Hall effect (AHE) curves at RT. The sign reversal of ρAHE reconfirms the reversal of the magnetic moment of Mn(II) between x = 0.15 and 0.27 in Fig. 1.  Fig. 1 XAS and XMCD spectra at Mn-L2,3edges in (a) Mn4N, (b) Mn3.85In0.15N and (c) Mn3.85In0.27N.  Fig. 2 Anomalous Hall effect curves of Mn4−xInxN (x=0.15 and 0.27) films on MgO(001).