Spin state and orbital ordering in CuCr2O4 investigated by NMR

Abstract

Cu-63,Cu-65 and Cr-53 nuclear magnetic resonance spectra for CuCr2O4 were measured at various magnetic fields and temperatures. The microscopic evidence of orbital ordering in CuCr2O4 was obtained from a dipolar hyperfine field, NQR, and magnetic anisotropy analysis of the linewidth broadening of the Cu and CrNMR spectra measured in the external magnetic field. The energy gap in the dispersion relation of the spin wave excitation was measured from the temperature dependence of the resonance frequency of Cu and Cr ions in CuCr2O4. The energy gap of the Cu ions is about 10 K (+/- 5 K), and that of the Cr ions is about 40 K (+/- 5 K). These values imply that the spin-orbit coupling of Cr ions is stronger than that of Cu ions related to the orbital ordering in CuCr2O4. The magnetic field dependence of the Cr NMR frequency shows that the angle between the Cr3+ magnetic moment and the Cu2+ magnetic moment is about 98 degrees (+/- 2 degrees).