Magnetism of an ultrathin Mn film on Co(100) and the effect of oxidation studied by x-ray magnetic circular dichroism

Abstract

The electronic and magnetic structures of a Mn ultrathin film grown on a 3-ML (monolayer) Co film have been investigated during stepwise oxidation by means of O K-, Mn ${L}_{\mathrm{I}\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}}\ensuremath{-},$ and Co ${L}_{\mathrm{I}\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}}\ensuremath{-}\mathrm{e}\mathrm{d}\mathrm{g}\mathrm{e}$ x-ray-absorption spectroscopy and Mn ${L}_{\mathrm{I}\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}}\ensuremath{-},$ and Co ${L}_{\mathrm{I}\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}}\ensuremath{-}\mathrm{e}\mathrm{d}\mathrm{g}\mathrm{e}$ x-ray magnetic circular dichroism (XMCD). Without ${\mathrm{O}}_{2},$ strong interaction between the Mn and Co $3d$ orbitals was suggested and Mn-Co ferromagnetic coupling was confirmed. We observed significant suppression of the d hole number and the spin and orbital moments of Co after Mn deposition compared to those before Mn deposition. These findings imply that the Mn d electrons are transferred to the minority-spin levels of Co. At 0.5-L (Langmuir) ${\mathrm{O}}_{2}$ exposure, the spin and orbital moments of Co do not change noticeably, while the Mn ${L}_{\mathrm{I}\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}}\ensuremath{-}\mathrm{e}\mathrm{d}\mathrm{g}\mathrm{e}$ XMCD almost completely vanishes. After 5.5-L ${\mathrm{O}}_{2}$ exposure, an antiparallel spin alignment between Mn and Co was observed. The estimated orbital moments of Mn is reduced from 0.06 (before oxidation) to $l0.005{\ensuremath{\mu}}_{B}$ (after oxidation). It is concluded that unoxidized Mn is in the ${d}^{5}{+d}^{6}$ state while oxidized Mn is in the ${d}^{5}$ high-spin state. Such variance of the electron configuration of Mn can explain the unusual magnetic properties. Antiferromagnetic coupling between Co and oxidized Mn may originate from the ${d}^{5}$ high-spin configuration of Mn rather than from the superexchange interaction between Mn and Co via the O atom.