Brillouin scattering from single-crystal dysprosium and gadolinium films

Abstract

Brillouin light scattering studies have been made on the temperature dependence and dispersion of long-wavelength acoustic phonons in single-crystal Dy films and long-wavelength acoustic magnons in single-crystal Gd films. The spectra were analyzed by using elastic and magnetic continuum models for the Dy and Gd systems, respectively.

The room temperature dispersion and the temperature dependence of the velocities of surface and guided acoustic waves have been studied in c-axis single-crystal Dy films grown on sapphire substrates with Nb (or Ta) and Y buffer layers. The room temperature data compared very well with the dispersion of acoustic phonon velocities calculated using the bulk elastic constants in the elastic model. We observed modest anomalies in the temperature dependences for the velocities of acoustic waves in Dy films and bulk Dy. However, we observed a very large anomaly in the temperature dependence of the relative peak intensities in the Brillouin spectra for the 450A Dy film. It was concluded that changes in the elastic constants of the Dy layer by itself are unlikely the cause of this large anomaly, and it may be due to interactions with the magnetic system.

Two bulk acoustic magnons and one shallow surface magnon were observed in a 300A c-axis Gd film. The temperature dependence of the frequencies of these magnons was shown to depend on the bulk values of the c-axis magnetic stiffness constant $D\sb{\rm c}$ defined by $\omega({\bf q}) = \Sigma D\sb{i}q\sb{i}\sp2$ where $\vert qa\vert \ll 1,$ and the axial-anisotropy constant $P\sb2$, defined by $H\sb{\rm aniso.} = P\sb2(S\sb{\rm z})\sp2$+ .... From the Brillouin data and the magnetization of the Gd film determined by a SQUID magnetometer, the values of $D\sb{\rm c}$(T) and $P\sb2$(T) were extracted over limited temperature ranges. These values are shown to be consistent with bulk values from the literature.