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 2 p XAS and XMCD spectra of CrO2 reveals that the Cr ions in bulk CrO2 have both Cr 4 + t 2 g 2 ↑ and Cr 3 + t 2 g 3 ↑ valence configurations. This work provides the direct experimental evidence for the Cr 3 + - Cr 4 + 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 2 p XMCD spectrum agrees well with the bulk magnetization M(T) and the Curie temperature, indicating that the measured Cr 2 p XMCD data represent the intrinsic bulk ferromagnetism in CrO2, and support the above finding of the XMCD lineshape analysis.