Magneto-Elastic Dependence of the Propagation of Sound in Gadolinium at the Critical Point

The change in the attenuation of sound and in the elastic constant $c_{33}$ of gadolinium has been measured at 5 MHz as a function of applied magnetic fields to 13 kOe in the temperature range 270-320 °K. The minimum in the elastic constant at the Curie point moved upwards in temperature and tended to diminish and broaden with increasing field. Studies of the change in attenuation due to magnetic field in the same region revealed sharp $\lambda$-shaped increases up to a field of 1. 8 kOe. At higher fields this peak split into two diminishing maxima, one moving upwards and the other downwards with increasing field. The zero-field-attenuation data decreased as $\Delta \alpha \sim {\epsilon }^y$, where $\epsilon =\left|\frac{(T-T_c)}{T_c}\right|$ and $y=-1.8\mathrm{}\pm 0.2$ in the paramagnetic region. In the presence of a magnetic field, the changes in the velocity are attributed to changes in the spin-correlation function.