Magnetization processes and low frequency fluctuations in micromagnetic structures

Abstract

The study presented in this thesis is concerned with low frequency fluctuations in magnetic systems. Three different systems were investigated. The first system is (CoFe/Pt)n multilayers patterned into micrometer scale Hall bars. Low frequency magnetic fluctuations in the active volume were probed by anomalous Hall effect. Magnetic noise was only observed during magnetization switching. It was shown that 1/fα noise spectrum, with α~2, dominates when at the macroscopic nucleation field, and close to negative saturation when domains annihilate. This is attributed to the irreversible magnetization switching and Barkhausen jumps. While in the slanted region of the hysteresis curve, a stationary 1/f noise spectrum was observed. This fluctuation is identified as the result of superposition a large number of reversible excursions of domain wall segments, which are roughly a hundred nanometers in length. First-order reversal curves have also been measured for the same samples. They show two dominant sets of 'hysterons', which coincide with the nucleation field and the negative saturation field, while the magnetization switching in between has high reversibility. These observations largely agree with the anomalous Hall voltage noise data.