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 low field linear regime at low temperature (T≤4.2 K) in view of their use in connection with superconducting quantum interference device magnetometers. We have tested several amorphous and polycrystalline alloys in the form of strip‐wound toroidal cores. Both high and near zero magnetostrictive alloys are studied. All samples are found out to be the source of thermal magnetic noise of 1/f spectral density in agreement with the fluctuation dissipation theorem. This type of thermal noise comes from a frequency independent imaginary permeability. This feature is equivalently described in terms of the magnetic viscosity effect or by considering the constant (residual) term in the classical Legg’s equation of magnetic losses at low fields. Measurements on a few Co‐based amorphous alloys show that the magnetic viscosity is independent of temperature. Explanations in terms of thermal activation processes and quantum tunneling are discussed.