Observation of Antiferromagnetic Phase Transitions by Ultrasonic Techniques

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Abstract
The usefulness of ultrasonic techniques in determining the phase boundaries of antiferromagnets (in the H‐T plane) is discussed. The various magnetic phase transitions which occur in antiferromagnets are described with particular emphasis on transitions in uniaxial antiferromagnets of the easy‐axis type. The phase boundaries between the antiferromagnetic, spin‐flop, and paramagnetic phases are discussed in the molecular field approximation. Some results of more sophisticated treatments are also included. The main ultrasonic phenomena which occur near the various phase transitions of uniaxial antiferromagnets are described. They are (a) λ anomalies in the attenuation and velocity near the second‐order transitions, (b) an attenuation spike (velocity dip) at the spin‐flop transition, and (c) an abrupt attenuation increase (velocity decrease) at the spin‐flop transition. Examples of phase boundaries determined ultrasonically in MnF2, FeF2, CoF2, Cr2O3, α‐Fe2O3, Cr, and EuTe are summarized.