Valence state of chromium ions in the half-metallic ferromagnet CrO2 probed by Cr53 NMR

We have performed $^{53}\mathrm{Cr}$ NMR measurements on a high-purity polycrystalline sample to investigate the static and dynamic properties of a half-metallic ferromagnet ${\mathrm{CrO}}_{2}$. Two $^{53}\mathrm{Cr}$ NMR lines, corresponding to magnetically nonequivalent Cr nuclei, were observed in the ferromagnetic phase of ${\mathrm{CrO}}_{2}$ despite all of the Cr ions being situated on the crystallographic equivalent sites. We measured the temperature dependences of the $^{53}\mathrm{Cr}$ spin-lattice relaxation rate ${({T}_{1})}^{\ensuremath{-}1}$ in the ferromagnetic phase for the temperature range $T=4.2--360\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. It was found that in the range of low temperatures ($T\ensuremath{\le}60\phantom{\rule{0.28em}{0ex}}\mathrm{K}$) the relaxation of nuclear magnetic moments is determined mainly by the orbital contribution proportional to the temperature, conditioned by the fluctuation of the orbital currents of $d$-band electrons. At temperatures $T>60\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, the main mechanism leading to the nuclear spin-lattice relaxation is a three-magnon process of scattering at which the relaxation of the nuclear spin is accompanied by the absorption of a magnon and the creation of two magnons. Based on the analysis of temperature dependences of ${({T}_{1})}^{\ensuremath{-}1}$ for two nonequivalent Cr ions, we found that their valence state is the same and corresponds to valence ${\mathrm{Cr}}^{4+}$, whereas the difference of resonance frequencies for these ion nuclei is conditioned by the different magnetic local fields in their location.