The rf permeability of dc planar magnetron sputtered FeNi multilayer films (abstract)

Abstract

Recording heads are commonly built in a multilayer stack to eliminate or reduce domain walls by allowing flux closure, or magnetostatic coupling, between the magnetic layers across nonmagnetic spacers. Use of an electrically insulating spacer material will reduce eddy current shielding increasing the high frequency performance of the head. As magnetic recording systems continue their trend to higher operating frequencies knowledge of the rf permeability of head-like multilayer materials becomes quite important. We examine the complex, rf permeability spectra of dc planar magnetron sputtered FeNi multilayer films as a function of spacer material, number of layers, and thickness of the spacer and magnetic layer. The permeability spectra are measured using a permeameter over the frequency range 3–300 MHz. The spacer materials examined include several metals and metal alloys, the corresponding oxides, silicon, and silicon nitride. The magnetic layer is, by weight, 81.8% Ni and 18.2% Fe. We have found that for a constant magnetic layer thickness, as the nonmagnetic spacer thickness decreases from 30 to 10 nm the permeability magnitude of the multilayer stack increases and the resonance frequency shifts to lower values. As the spacer thickness increases above 30 nm the permeability response remains constant with a negligible value of μ″. The permeability spectra are approximately the same for spacer layers made of metal or their corresponding oxides. Spacer layers made of Si3N4 result in permeability spectra with the highest permeability magnitudes. For example, the permeability magnitude of a multilayer stack composed of four 120 nm FeNi layers separated by 10 nm spacers is approximately 2000 for metal spacers, 2300 for Si spacers, and 4200 for Si3N4 spacers. For a fixed number of multilayer pairs and constant spacer layer thickness making the FeNi layer thicker tends to increase the permeability magnitude, reduce the resonance frequency, and lower the low frequency coercive force.