Ultrasonic Propagation in EuO

Abstract

The attenuation of 10 to 90‐MHz longitudinal and shear ultrasonic waves propagating along the [100] and [110] directions of EuO single crystals was measured between 63° and 78°K. At zero applied magnetic field, the attenuation was constant above the Curie temperature, T_C=70°K, but increased as the temperature was lowered through T_C. The increase in attenuation was much greater for the shear modes than for the longitudinal modes. Preliminary experiments at 4.2°≤T≤78°K with 30‐MHz longitudinal and 10‐MHz shear waves propagating along the [100] direction showed that the excess attenuation at T<T_C passes through a maximum near 60°K and is small at 4 2°K. An external magnetic field $\text{H≳5\hspace{0.17em}}\mathrm{kG}$ removed the excess attenuation at T<T_C. No attenuation peak was found at T_C. The excess attenuation at T<T_C is tentatively interpreted as being due to ferromagnetic domains. The elastic constants at 78°K are (in 10¹¹ cgs units) c₁₁ = 19.2±0.6, c₄₄ = 5.42±0.13, c₁₂ = 4.25±0.85. These lead to an adiabatic bulk modulus B = (9.2±0.6) ×10¹¹ cgs units, and a Debye temperature θ = 350°K.