Variation of Sound Velocity with Magnetic Field in Mn-Doped YIG

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Abstract
The variation of the sound velocity with magnetic field in Mn-doped YIG has been studied in both rods and spheres. The samples were single crystals containing 2.3 wt % Mn. With the rods, which were approximately 0.3×0.3×2.0 cm, the sound velocity was measured directly while with the spheres (about 0.3 cm in diameter) the resonant frequency was measured. For a rod with the sound propagation in the 〈111〉 direction, the change in ultrasonic shear velocity for a field change from 200 G (required for saturation) to 2 kG was 13% at 87°K, 13% at 106°K; 9.5% at 140°K, 8% at 213°K, and 3.4% at 290°K. For rods with the sound propagation in the 〈100〉 direction, the change in velocity with magnetic field at any temperature was greatly reduced compared to that observed for the 〈111〉 direction. Furthermore, the velocity of longitudinal waves did not show any appreciable field dependence. With spheres magnetized along 〈111〉, the resonant frequency of the lowest shear mode changed by 30% at 77°K for a field change from 0.8 kG (sufficient to saturate the sample) to 6.0 kG. Some of the experimental observations can be interpreted on the basis of the simple theory of magnetostrictively coupled phonon and spin waves. However, this picture fails in some important aspects, indicating that the interaction of the acoustic waves with the magnetization in Mn-doped YIG is more complex.