High-Field Antiferromagnetic Resonance inCr2O3

Abstract

Antiferromagnetic resonance (AFMR) experiments in single-crystal ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$ are summarized with specific emphasis on the high-field resonance mode. Most of the experiments involve pulsed magnetic fields and millimeter wavelength radiation. A brief description of the experimental techniques is included. The experimental results include AFMR as a function of temperature (from 4.2°K to the Néel temperature), frequency (36 to 135 kMc/sec), magnetic field, and angle between the magnetic field and the $c$ axis. Results of the spinflop resonance mode are also presented as a function of temperature, field, frequency, and angle. Magnetic measurements at the spin-flop field and low-field static susceptibility measurement both parallel and perpendicular to the $c$ axis are also given as a function of temperature. Normalized plots of the AFMR based on the molecular field approximation are presented as a function of angle and field and compared with experiment. The AFMR experiments agreed with the molecular field results when the static susceptibility data were included in the calculations. The characteristic quantity ${\left(2\mathrm{}{H}_E{H}_A\right)}^{\frac{1}{2}}$, where $H_E$ is the exchange field and $H_A$ is the anisotropy field, is 60±3 kG from 4.2 to 235°K. The unusual temperature independence of ${\left(2\mathrm{}{H}_E{H}_A\right)}^{\frac{1}{2}}$ is partly accounted for by the crystalline field contribution to $H_A$ which is only 700 G at 4.2°K. Assuming only dipolar and crystalline field contributions to $H_A$ the crystalline field portion at low temperatures is 1000 G. This corresponds to an axial $D$ contribution which is 1/9 that of ${\mathrm{Cr}}^{3+}$ in ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ and of the opposite sign. More recent optical and related data which confirm this result are discussed and some possible mechanisms of the unusual temperature dependence are indicated. A nonzero value of the parallel static susceptibility at low fields is also observed and briefly discussed.