Thermal magnetization noise as a function of frequency in amorphous ferromagnets

We report the observation of 1/frequency magnetization noise in toroidal strip‐wound cores of amorphous ferromagnetic alloys at liquid‐helium temperatures and in the frequency range 0.1 Hz–1 kHz. The materials tested are Co‐based alloys, and exhibit very low magnetostriction and good soft magnetic properties. The experimental apparatus consists of a radio frequency SQUID magnetometer coupled to the sample by a superconducting flux transformer which is devised to allow measurements of its complex inductance. We show that the magnetization noise obeys in detail the fluctuation‐dissipation theorem. A temperature‐dependent partial roll‐off of the complex permeability is found at ∼100 Hz, possibly linked to pinning wells for domain‐wall motion. The link between 1/frequency noise and magnetic viscosity is briefly discussed.