Acoustic properties of hematite near the orientation phase transitions in magnetic fields and under pressure

This paper reports on the results of the experimental investigation of the quasi-phonon (quasi-sound) branch of magnetoelastic waves in hematite ($\alpha$-$\mathrm{Fe}_2\mathrm{O}_3$) in the easy-plane state near the orientation phase transitions in magnetic fields $H$ and under uniaxial pressure $P$. It has been found that, in an applied external magnetic field $H>1.0$ kOe and at a uniaxial pressure $P>40$ MPa, the amplitude of the transmitted quasi-sound abruptly increases. The minima of quasi-sound velocities observed at the orientation phase transition points in the magnetic field $H$ are consistent with the predictions of the existing theory of magnetoelastic waves, but their width significantly exceeds the theoretical values. The minima of quasi-sound velocities at a uniaxial pressure $P$ are observed only in magnetic fields $H<3$ kOe and, at $H>3$ kOe, turn into extended plateaus.