A ${\mathrm{Cr}}_2$${\mathrm{O}}_3$ single crystal was grown by the flame fusion method. It has rhombohedral symmetry and showed an antiferromagnetic Curie temperature ($T_c$) of 307°K. Below $T_c$ the magnetic susceptibility $\chi$ perpendicular to the [111] direction is almost independent of temperature while the value of $\chi$ parallel to [111] approaches zero at very low temperatures. According to the Van Vleck theory, this behavior indicates that the spins are aligned along [111]. Above 77°K the parallel susceptibility was found to be proportional to the $\frac{3}{2}$ power of the temperature. Resonance measurements (23 500 Mc/sec) at the Curie temperature showed a shift in the peak of the absorption curve as a function of temperature. This shift was towards lower magnetic fields, and exhibited an anisotropy effect depending on the direction of the magnetic field which is in agreement with theory. From the temperature dependence of the resonance absorption and the Kittel theory of antiferromagnetic resonance, an estimate of the anisotropy constant and its temperature dependence can be made.

• Received 15 February 1956

DOI:https://doi.org/10.1103/PhysRev.102.1000