LOW-TEMPERATURE PROPERTIES OF SOFT MAGNETIC-MATERIALS - MAGNETIC VISCOSITY AND 1 F THERMAL NOISE/

We study the magnetic properties of soft ferromagnetic cores in the lo w field linear regime at low temperature (T less-than-or-equal-to 4.2 K) in view of their use in connection with superconducting quantum int erference device magnetometers. We have tested several amorphous and p olycrystalline alloys in the form of strip-wound toroidal cores. Both high and near zero magnetostrictive alloys are studied. All samples ar e found out to be the source of thermal magnetic noise of 1/f spectral density in agreement with the fluctuation dissipation theorem. This t ype of thermal noise comes from a frequency independent imaginary perm eability. This feature is equivalently described in terms of the magne tic viscosity effect or by considering the constant (residual) term in the classical Legg's equation of magnetic losses at low fields. Measu rements on a few Co-based amorphous alloys show that the magnetic visc osity is independent of temperature. Explanations in terms of thermal activation processes and quantum tunneling are discussed.