Random Telegraph Noise in Ferromagnetic Garnets

Magnetic noise in ten garnet samples is reported. The magnetic noise, fluctuations in the transmitted light intensity using the Faraday effect to observe the magnetization, was measured when the samples were driven by an applied AC magnetic field. The data were taken as a function of the area sampled, the magnitude of the AC magnetic field, and the AC frequency (40-250 Hz). In zero applied magnetic field, the magnetization of the samples consisted of either stripe or serpentine domains. The measured noise is white at fields somewhat below and somewhat above the coercive field for all length scales. Intermediate fields, close to the coercive field of a sample, exhibit a Lorentzian power spectral density (PSD) as would be expected for random telegraph noise (RTN). This PSD was seen at all length scales. Surprisingly, the Lorentzian noise exhibits the same characteristic frequency at all positions measured in a given sample. With increasing area the magnitude of the Lorentzian noise decreases following a 1/area relation consistent with the Lorentzian noise arising from many uncorrelated RTN oscillators. The results will be discussed in terms of random pinning of domains to create complex domain structures.