Elastic properties of LiTbF4 ferromagnetics near the Curie point

The propagation of a transverse ultrasound at 13.4 MHz frequency in the [001] direction of a LiTbF4 monocrystal is stUdied both experimentally and theoretically. Relative changes in the ultrasound velocity, ΔV/V, versus temperature (in the 1.57 to 4.2 K range) and magnetic field, H(up to 30 kOe) are measured by the echo-pulse method. An anomalous increase in the ultrasound velocity with a temperature decrease is observed below the ferromagnetic phase transition, Tsub(C)=2.87 K. In a zero magnetic field a relative increase in the velocity, ΔV/V, at 1.57K with regard to 4.2 K made up +7X10-5. A considerable suppression of the effect has been observed with a greater magnetic field. The experimental curves plotted account for the sample magnetostriction. Using the perturbation theory method in a molecular field approximation a microscopic calculation is made of the crystal elastic constants, with the Tb3+ ions bount to the lattice accounted for. A goort agreement with the experiment is obtained