Magnetic-Field Effect on the Sound Velocity at Low Temperature in a Dilute Kramers-Ion Glass

Measurements of sound velocity in aluminosilicate glasses with various dysprosium contents are reported. They are performed at a temperature down to 10 mK and in a magnetic field up to 50 kOe. An effect of the magnetic field on the logarithmic temperature dependence of the sound velocity is revealed. It exists for Dy doping (Kramers ions) and not for Ho doping (non-Kramers ions). This effect is ascribed to magnetic tunnelling states which are spins whose local anisotropy axis is almost perpendicular to the local magnetic field.