Acoustical study of tunneling states in magnetic rare-earth glasses : wave propagation and phonon echoes

An acoustical study of tunneling states in aluminosilicate glasses with various holmium contents is reported. The measurements are performed down to 10 mK, in the frequency range 10-500 MHz and in magnetic fields up to 60 kOe. The coupling parameters of the tunneling states are obtained from the sound velocity measurements and from the self induced transparency. The linewidth is obtained from the free precession of the tunneling states and from hole burning experiments. Phonon echoes are observed in samples with various holmium contents, with and without magnetic field. The echo amplitude increases by a factor of 3 as the magnetic field increases from 0 to 60 kOe. The transverse relaxation time T2 is obtained from the spontaneous phonon echoes and the stimulated phonon echoes allow us to reach the longitudinal relaxation time T1. The latter is also obtained from saturation recovery experiments. Lastly, an experiment of spectral diffusion is reported. Our experimental results show that the static parameters of the tunneling states (deformation potential and density of states) change with the magnetic-ion content (between 1.5 % and 10.1 %) but are not changed when a magnetic field (up to 60 kOe) is set up. On the contrary, the relaxation times T1 and T2 are roughly the same whatever the magnetic-ion content and T1 increases by a factor of 6 as the magnetic field increases from 0 to 60 kOe. The relaxation of the tunneling states by the rare-earth ions is considered.