We have studied the electronic and magnetic states of Co and Mn atoms at the interface of the Co$_mathrm{2}$Mn$_{beta}$Si (CMS)/MgO ($beta$=0.69, 0.99, 1.15 and 1.29) magnetic tunnel junction (MTJ) by means of x-ray magnetic circular dichroism. In pa
rticular, the Mn composition ($beta$) dependences of the Mn and Co magnetic moments were investigated. The experimental spin magnetic moments of Mn, $m_mathrm{spin}$(Mn), derived from XMCD weakly decreased with increasing Mn composition $beta$ in going from Mn-deficient to Mn-rich CMS films. This behavior was explained by first-principles calculations based on the antisite-based site-specific formula unit (SSFU) composition model, which assumes the formation of only antisite defect, not vacancies, to accommodate off-stoichiometry. Furthermore, the experimental spin magnetic moments of Co, $m_mathrm{spin}$(Co), also weakly decreased with increasing Mn composition. This behavior was consistently explained by the antisite-based SSFU model, in particular, by the decrease in the concentration of Co$_mathrm{Mn}$ antisites detrimental to the half-metallicity of CMS with increasing $beta$. This finding is consistent with the higher TMR ratios which have been observed for CMS/MgO/CMS MTJs with Mn-rich CMS electrodes.
BiFeO$_3$ (BFO) shows both ferroelectricity and magnetic ordering at room temperature but its ferromagnetic component, which is due to spin canting, is negligible. Substitution of transition-metal atoms such as Co for Fe is known to enhance the ferro
magnetic component in BFO. In order to reveal the origin of such magnetization enhancement, we performed soft x-ray absorption spectroscopy (XAS) and soft x-ray magnetic circular dichroism (XMCD) studies of BiFe$_{1-x}$Co$_x$O$_3$ ({it x} = 0 to 0.30) (BFCO) thin films grown on LaAlO$_3$(001) substrates. The XAS results indicated that the Fe and Co ions are in the Fe$^{3+}$ and Co$^{3+}$ states. The XMCD results showed that the Fe ions show ferromagnetism while the Co ions are antiferromagnetic at room temperature. The XAS and XMCD measurements also revealed that part of the Fe$^{3+}$ ions are tetrahedrally co-ordinated by oxygen ions but that the XMCD signals of the octahedrally coordinated Fe$^{3+}$ ions increase with Co content. The results suggest that an impurity phase such as the ferrimagnetic $gamma$-Fe$_2$O$_3$ which exists at low Co concentration decreases with increasing Co concentration and that the ferromagnetic component of the Fe$^{3+}$ ion in the octrahedral crystal fields increases with Co concentration, probably reflecting the increased canting of the Fe$^{3+}$ ions.
We have studied magnetism in anatase Ti$_{1-x}$Co$_x$O$_{2-delta}$ ({it x} = 0.05) thin films with various electron carrier densities, by soft x-ray magnetic circular dichroism (XMCD) measurements at the Co $L_{2,3}$ absorption edges. For electricall
y conducting samples, the magnetic moment estimated by XMCD was $<$ 0.3 $mu_B$/Co using the surface-sensitive total electron yield (TEY) mode, while it was 0.3-2.4 $mu_B$/Co using the bulk-sensitive total fluorescence yield (TFY) mode. The latter value is in the same range as the saturation magnetization 0.6-2.1 $mu_B$/Co deduced by SQUID measurement. The magnetization and the XMCD intensity increased with carrier density, consistent with the carrier-induced origin of the ferromagnetism.
