Ultrasonic Behavior near the Spin-Flop Transitions of Hematite

Ultrasonic propagation in natural single crystals of hematite (α-Fe₂O₃) was studied in magnetic fields up to 200 kG at various temperatures between 4.2 and 300^∘{}K. Anomalies in the ultrasonic attenuation were observed near: (1) the spin-flop transition at H_c∥ (in a field parallel to the trigonal axis), (2) the spin-flop transition at H_c⊥ (in a field normal to the trigonal axis), and (3) the Morin transition. Anomalies in the velocity of some acoustical modes of propagation were also observed at H_c∥. The temperature variation of H_c∥ and H_c⊥ was studied in one of the hematite crystals which had a low impurity content. At 77^∘{}K, H_c∥=68.2±0.7 kG and H_c⊥=160.8±2 kG. The Morin transition in this crystal occurs at TM=(260.6±0.5)^∘{}K. The ultrasonic data can be accounted for by considering the motion of domain walls under the action of an oscillating elastic stress. Other explanations are also considered. The data for H_c∥ and H_c⊥ are compared with the results of other workers.