Ultrasonic attenuation in ferromagnet under an applied magnetic field

•Effect of an external magnetic field on the sound attenuation coefficient is investigated.•Temperature and magnetic field variations of sound attenuation coefficient are examined.•A quantitative comparison between obtained results and available ultrasonic measurements in MnP is presented. Effect of an external magnetic field on the sound attenuation coefficient is investigated using the Landau-Ginzburg model incorporating a tricritical to mean-field crossover. The dynamics is treated within the framework of the Onsager theory. Temperature variation of the attenuation coefficient has been obtained for arbitrary values of the magnetic field. For non-zero sixth order term in the Landau-Ginzburg expansion and for sufficiently high magnetic field the maximum in attenuation occurs above the Curie point and is displaced towards the higher temperatures as the magnetic field is increased. In the high temperature region a very rapid increase of attenuation with reduced temperature is demonstrated before the maximum is attained. This increase is much steeper than in the analogous hydrodynamic regime below the Curie temperature. At constant temperature the quadratic field dependence of attenuation on magnetic field is found in the low-field region until a maximum at a certain magnetic field is attained. This maximum is followed by a power-law decay to zero in the high-field region. A quantitative comparison between obtained results and available ultrasonic measurements in MnP is presented. Good agreement is achieved for moderate and large magnetic fields and temperatures not very close to the Curie temperature.