Barkhausen transitions in single layer and bilayer thin permalloy films

Abstract

The low coercivity of uniaxial thin nickel-iron multilayer films, prepared according to E. W. Hill, J. P. Li, and J. K. Birtwistle [J. Appl. Phys. 69, 4526 (1991)], makes them attractive for magnetic sensor applications. Barkhausen jumps, causing noise, limit the performance of such sensors. It has been reported [R. F. Soohoo, J. Appl. Phys. 69, 5871 (1991)] that, for any given sample under identical initial conditions, these Barkhausen jumps are deterministic rather than noisy in nature. This is consistent with the fact that the defect structure of a given sample is deterministic. This study compares power spectra and coercivity distributions from Barkhausen pulses obtained by applying varying alternating fields to thin permalloy films. The effect of an orthogonal bias field is also examined. The samples under examination consist of both single layer and bilayer films, produced as continuous layers by vacuum evaporation and ion milled into 40 mm diameter discs. The permalloy is of 80/20 composition and has a total thickness of 100 nm. The bilayer films have a nonmagnetic tantalum interlayer of thickness 5 nm. The rate of change of the applied field was varied between 2918.4 and 11.4 Oe/s, at an initial amplitude of 90 Oe. The results show that the Barkhausen pulses have a strong deterministic trend, but also contain a smaller nondeterministic component. As the frequency and amplitude are decreased, this random effect becomes larger. As might be expected when an orthogonal bias field is applied to the samples, the amplitudes of the pulses are decreased. The films also exhibit a corresponding reduction in coercivity.