Ultrasonic study of terbium in a magnetic field

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Abstract
A model is presented which describes longitudinal ultrasonic propagation characteristics in magnetic materials. The model specifically incorporates magnetoelastic and exchange interactions when an external magnetic field is applied. Spin fluctuations and spin polarization are seen to play a dominant role, and particular attention is focused on the effect of critical fluctuations near the magnetic-phase-transition temperatures. The model is applied to single-crystal terbium. Longitudinal ultrasonic attenuation data are presented under conditions of constant temperature and constant magnetic field. The constant-temperature data reveal a very large attenuation enhancement by a magnetic field in the paramagnetic region. The constant-field data show anomalous attenuation maxima even when the applied field is sufficient to quench the spin-spiral state; a magnetic phase diagram for terbium is deduced from the constant-field data. A quantitative comparison between the experimental results and theory is presented for the constant-temperature data, and a qualitative comparison between experimental results and theory is presented for the constant-field data. Good agreement is achieved.