Valence state of chromium ions in the half-metallic ferromagnet ${\mathrm{CrO}}_2$ probed by ${}^{53}\mathrm{Cr}$ 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 ${\left(T_1\right)}^{-1}$ in the ferromagnetic phase for the temperature range $T=4.2–360\mathrm{K}$. It was found that in the range of low temperatures ($T\le 60\mathrm{K}$) 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 $T>60\mathrm{K}$, 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 ${\left(T_1\right)}^{-1}$ 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.

Figure 1

${\mathrm{CrO}}_2$ crystal structure. Solid gray lines show the elementary crystal cell. Colored planes denote the equatorial planes. Dashed lines show the oxygen octahedra surrounding each Cr atom, consisting of four O atoms in the equatorial plane and two apical atoms. Magnetic moments of chromium ions lie in the equatorial planes of ${\mathrm{CrO}}_6$ octahedra and directed along the crystal $c$ axis. $x, y, z$ are the local axes of ${\mathrm{CrO}}_6$ octahedra; $X, Y, Z$ are the principal axes of the ${}^{53}\mathrm{Cr}$ hyperfine coupling tensor.

Figure 2

(a) ${}^{53}\mathrm{Cr}$ NMR spectra at temperatures $T=4.2$ and 293 K. Two lines in the NMR ${}^{53}\mathrm{Cr}$ spectrum correspond to the chromium nuclei in positions Cr(A) and Cr(B). (b) ${}^{53}\mathrm{Cr}$ NMR spectra, measured at the different power of spectrometer radiofrequency transmitter.

Figure 3

The dependencies of resonance frequencies of chromium nuclei in positions Cr(A) and Cr(B) on the temperature. Solid lines show the result of data approximation by functions ${\nu }_n\left(T\right)={\nu }_n\left(T=0\right)\left(1-a{T}^{3/2}\right)$ (at $T\le 300\mathrm{K}$) and ${\nu }_n\left(T\right)={\nu }_n\left(T=0\right)k{\left(T_C-T\right)}^{\beta }$ (at $T\ge 300\mathrm{K}$). The measurement error did not exceed the symbol size.

Figure 4

(a) Recovery of ${}^{53}\mathrm{Cr}$ nuclear magnetizations ${}^{53}M_Z\left(t\right)$ shown in a semilog scale of the quantity ${}^{53}m\left(t\right)=[M_Z\left(\infty \right)-M_Z\left(t\right)]/M_Z\left(\infty \right)$ vs the time interval $t_{\mathrm{inv}}$ after applying an inversion rf pulse at $T=240\mathrm{K}$ (open circles) and $T=32\mathrm{K}$ (closed circles). (b) Temperature dependencies of spin-lattice relaxation rates of chromium nuclei in positions Cr(A) and Cr(B). The data were approximated by a function $1/T_1=AT+B{T}^{\mathrm{n}}$. Inset: temperature dependence of the relationship of times of relaxation. The measurement error did not exceed the symbol size.

Figure 5

The dependences of the spin echo decay $E\left(2t_{\mathrm{del}}\right)$ of ${}^{53}\mathrm{Cr}$ [Cr(A) site] vs double delay between the pulses $2t_{\mathrm{del}}$ in the ${\mathrm{CrO}}_2$ at $T=77\mathrm{K}$. Inset: the initial part $E\left(2t_{\mathrm{del}}\right)$, measured with the step $\mathrm{\Delta }{t}_{\mathrm{del}}=1\mu \mathrm{s}$. The data for chromium in the Cr(B) position are not shown because of their identity with the data for chromium in the Cr(A) position.