Permeability resonances in permalloy stripes (abstract)

Abstract
This work presents the experimental low field (Hac≊0.01 Oe) complex permeability spectra μ(f)=μ′(f)−jμ″(f) (between 1 and 200 MHz) of L=1-cm-long NiFe stripes as a function of stripe width W (10 μm to 1 cm), film thickness T (0.1 to 4 μm), and the history-dependent initial domain configuration. It will be demonstrated that occurrence of strong resonances in μ(f) is closely linked to the domain structure of the stripes, and in particular the linear density of the 180° transverse domain walls (observed by Bitter techniques) characteristic of a periodic flux-closed domain pattern. Square films (L=W=1 cm) of all measured thicknesses that have a low linear density (<30 walls/cm) of 180° domain walls show no observable resonances, and their spectra μ(f) are instead well described by classical eddy current predictions. In contrast, highly pronounced resonance peaks (μ″≳μdc≊2000) are observed in 0.1 μm≤T≤2 μm stripes that have been anhysteretically demagnetized with a longitudinal field, where the static domain structures have linear wall densities ≊1000 walls/cm. When demagnetized with a transverse field, alternative static domain configurations with roughly 1/5 the 180° wall density show substantial reduction in the amplitude of the resonance peaks. In contrast, the frequency location of the resonances is found to essentially be independent of both stripe width and wall density, but roughly scales inversely with film thickness: fres≊(20 MHz)(T/μm)−1 for the first, primary resonance peak. This phenomenon is believed to be linked to resonances in the internal motion of the 180° walls during low amplitude flux conduction, and has potential negative implications for the frequency limitations on the reproduce performance of NiFe thin-film inductive heads. Theoretical/phenomenological explanations of the measurement data will also be discussed.