Abstract:
This paper reports on the results of an experimental investigation into the influence of linear magnetic birefringence of transverse elastic waves due to magnetoelastic interaction on the conical refraction of sound in antiferromagnetic $\alpha$-Fe$_2$O$_3$. It has been found that, in a weakly ferromagnetic state, the incoming flux of the sound energy is separated into two fluxes corresponding to two normal modes of transverse vibrations in the sample. They propagate along the $\mathbf{C}_3$ trigonal axis of this crystal with different phase velocities due to the linear magnetic birefringence of sound. It has been established that the angle of deviation $(\theta_\eta)$ from the $\mathbf{C}_3$ axis for the group velocity of the normal mode of elastic vibrations, which does not interact with the magnetic subsystem, is determined by the internal conical refraction and does not depend on the magnitude of the magnetic field in the basal plane $(\mathbf{H}\perp\mathbf{C}_3)$. It has also been shown that the angle of deviation $(\theta_\xi)$ of the group velocity of the interacting mode of normal vibrations depends on the magnitude of the magnetic field and tends to $\theta_\eta$ at large values of $H$. The group velocity vectors lie in the same azimuth plane passing through the $\mathbf{C}_3$ axis and deviate in opposite directions. The obtained results confirm the conclusions drawn from the theoretical description of the conical refraction of sound in antiferromagnets.
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