Ultrasonic propagation-time measurements in Zn1-xMnxSe down to low temperatures: Shear-wave anomalies and the spin-glass transition


Ultrasonic transit-time measurements have been made down to 1.5 K on hexagonal, wurtzite-structured Zn1-xMnxSe, with x=0.37 and 0.53. The velocities of longitudinal waves increased monotonically with decreasing temperature, as expected from the anharmonicity of crystal bonding forces. However, the shear-wave velocities fell below that expected from ordinary anharmonic effects starting at about 150 K and went through a broad, shallow minimum with its lowest point near the spin-glass transition temperature, Tsg. The depression was smaller at x=0.37 than at x=0.53 and smaller at 145 MHz than at 30 MHz. We attribute it to ultrasonic strain-induced modulation of the Mn-Mn superexchange interaction and the concomitant changes in Mn spin-spin correlations involving some kind of anisotropy. Comparison with results on Cd1-xMnxTe indicates anion dependence. An analysis is given of our results which employs an empirical expression involving the magnetic susceptibility. The expression was constructed by combining theoretical relations developed by others for ordinary antiferromagnetic materials and for spin glasses. It is inferred that some type of ordering begins near 160 K (associated with isolated spin clusters) in addition to the ordering which begins at the much lower spin-glass temperature.

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Physical Review B