Magnetic susceptibility and magnetization fluctuation measurements of mixed gadolinium-yttrium iron garnets

Abstract

We study the magnetic properties of gadolinium-yttrium iron garnet $({\text{Gd}}_{x}{\text{Y}}_{3\ensuremath{-}x}{\text{Fe}}_{5}{\text{O}}_{12},x=3,1.8)$ ferrite ceramics. The complex initial permeability is measured in the temperature range of 2--295 K at frequency of 1 kHz and in the frequency range of 100 Hz--200 MHz at temperatures of 4, 77, and 295 K. The magnetic viscosity-induced imaginary part of the permeability is observed at low frequencies. Measurements of the magnetization noise are made at 4 K. Using the fluctuation-dissipation theorem, we find that the observed magnetization fluctuations are consistent with our measurements of the low-frequency imaginary part of the permeability. We discuss some implications for proposed precision measurements as well as other possible applications.