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

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-III,L-II-, and Co L-III,L-II-edge x-ray-absorption spectroscopy and Mn L-III,L-II-, and Co L-III,L-II-edge x-ray magnetic circular dichroism (XMCD). Without 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) O-2 exposure, the spin and orbital moments of Co do not change noticeably, while the Mn L-III,L-II-edge XMCD almost completely vanishes. After 5.5-L 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 < 0.005(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.