MAGNETIC VISCOSITY, THERMAL RELAXATION, AND THERMAL-EQUILIBRIUM NOISE IN CO-BASED AMORPHOUS-ALLOYS AT MILLIKELVIN TEMPERATURES

We report measurements of the magnetic reversible complex permeability and of the related thermal noise for a few Co-based soft ferromagnetic amorphous alloys. The measurements were performed in the 16 mK-4.2 K temperature range and in the 0.1 Hz-1 kHz frequency range with a superconducting quantum interference device magnetometer. The data show the existence of a temperature independent magnetic viscosity together with a thermal relaxation of the spin subsystem to the heat bath. This last feature shows up as a partial rolloff of the real permeability from an isothermal low-frequency value to an adiabatic high-frequency one. The noise due to the sample is found in good agreement with the standard fluctuation-dissipation prediction for the thermal noise.