Experimental evidence for mixed-valent Cr ions in half-metallic CrO₂: Temperature-dependent XMCD study

By employing soft X-ray magnetic circular dichroism (XMCD) and soft X-ray absorption spectroscopy (XAS), we have investigated the temperature (T)-dependent valence and spin states of Cr ions in half-metallic ferromagnet CrO₂ nanorod powder. The lineshape-analysis of both the Cr 2p XAS and XMCD spectra of CrO₂ reveals that the Cr ions in bulk CrO₂ have both Cr⁴⁺ t_2g²↑ and Cr³⁺ t_2g³↑ valence configurations. This work provides the direct experimental evidence for the Cr³⁺-Cr⁴⁺ mixed-valent states in bulk CrO₂ (Cr^3.8+), demonstrating that the half-metallic ferromagnetism in CrO₂ originates from the double-exchange interaction. The T-dependent intensity variation of the Cr 2p XMCD spectrum agrees well with the bulk magnetization M(T) and the Curie temperature, indicating that the measured Cr 2p XMCD data represent the intrinsic bulk ferromagnetism in CrO₂, and support the above finding of the XMCD lineshape analysis.