Thermal magnetic noise in a strip wound crystalline ferromagnetic core at 4.2 K

A dc SQUID magnetometer‐based system has been developed and used to measure, in the frequency range 50–2300 Hz, the complex magnetic permeability μ_r(ν) and the magnetization noise at 4.2 K in a strip wound toroid. This toroidal core has been made of the 3‐μm‐thick ribbon fabricated from a crystalline magnetically soft alloy, Ultraperm. Below 1 kHz a constant value of −arg[μ_r(ν)]≊2×10⁻³ and 1/ν shaped noise spectral density have been measured. For frequencies higher than 1 kHz a linear growth of the imaginary part μ_r and a white noise have been found. The noise due to the sample is found in quantitative agreement with the standard fluctuation‐dissipation formula for the thermal noise, while a comparison of the permeability imaginary part magnitude with the theoretical value has indicated a partially shorted windings in the toroid, which have decreased the toroid roll‐off frequency down to 1 MHz.