Magnetic states of Mn and Co atoms atCo2MnGe/MgOinterfaces seen via soft x-ray magnetic circular dichroism

Abstract

The magnetic states of Mn and Co atoms in Co-rich ${\text{Co}}_{2}\text{MnGe}$ Heusler alloy thin films facing an MgO barrier were studied by means of soft x-ray magnetic circular dichroism (XMCD). In particular, the ${\text{Co}}_{2}\text{MnGe}$ film-thickness dependence of the Mn and Co magnetic moments was investigated. With a decrease in the ${\text{Co}}_{2}\text{MnGe}$ film thickness to 1--2 monolayers (MLs), the spin magnetic moment of Mn decreased and the $\text{Mn}\text{ }{L}_{2,3}$-edge x-ray absorption spectra (XAS) showed a ${\text{Mn}}^{2+}$-like multiplet structure in MnO, in contrast to samples thicker than 4 ML, indicating that the Mn atoms of the 1 and 2 ML samples were oxidized. The Co spin magnetic moment increased slightly with decreasing thickness. A ${\text{Co}}^{2+}$-like multiplet structure in CoO was not observed in all the $\text{Co}\text{ }{L}_{2,3}$-edge XAS and XMCD, indicating that, even in the ultrathin samples, the Co atoms were not oxidized, and were more strongly spin polarized than those in the thicker samples. Co spin magnetic moments of $1.40--1.77\text{ }{\ensuremath{\mu}}_{\text{B}}$ larger than the theoretical value for ideal stoichiometric ${\text{Co}}_{2}\text{MnGe}$ $(\ensuremath{\sim}1\text{ }{\ensuremath{\mu}}_{\text{B}})$ and the Co-rich film composition imply the presence of Co antisites that would lower the spin polarization.