Soft X-ray absorption spectroscopy and magnetic circular dichroism study of valence and spin states of half-metallic CrO$_2$ nanorods

Half-metallic ferromagnetic CrO$_2$, with the Curie temperature $T_C \sim390~K$, is very interesting because most of the transition metal oxides are antiferromagnetic insulators. It has been proposed \footnote{M. A. Korontin et al., Phys. Rev. Lett. {\bf 80}, 4305 (1998).} that the metallic ferromagnetism in CrO$_2$ originates from the oxygen-mediated double exchange interaction between mixed-valent Cr ions, caused by self-doping. But this issue is controversial.\footnote{J. H. Shim et al., Phys. Rev. Lett. {\bf 99}, 057209 (2007).} We have investigated the valence and spin states in CrO$_2$ nanorods by employing soft X-ray absorption spectroscopy (XAS) and soft X-ray magnetic circular dichroism (XMCD). The valence states of Cr ions are found to be Cr$^{3+}$-Cr$^{4+}$ mixed-valent at the surfaces, but nearly Cr$^{4+}$ in the bulk. The temperature-dependent XMCD intensity is observed, in agreement with the bulk $T_C$. We will discuss the electronic structure and its half-metallic ferromagnetism in CrO$_2$.