Fabrication of magnetic thin film structures for control of electromagnetic interference

Abstract

We are concerned with methods for the absorption of unwanted electromagnetic energy, which is commonly referred to as electromagnetic interference or EMI. High permeability magnetic metals used for the control of EMI have high conductivities and hence support eddy currents, which act to reflect the incident wave before it can be absorbed. Eddy currents can be reduced by fabricating the film in a multilayer design, with the magnetic layers separated by electrically insulating dielectric layers, and also by sectioning the film into electrically isolated regions. However magnetic properties are significantly affected by sample shape. In order to find high performance EMI absorbers, we examine the low frequency BH loop, magnetic flux density B versus applied magnetic field H, and complex permeability spectra of multilayer permalloy films laser processed to define stripes geometries upon the samples. The stripes are defined parallel to the as-deposited magnetic hard axis (perpendicular to the direction of magnetization). We begin by examining different laser energy densities for groove definition in films of different thicknesses. We find the sample geometry can compensate for the inherent anisotropy field of the as-deposited film, resulting in fabrication of films with precisely controlled permeabilities. We find that for multilayer thin films the demagnetizing field, which is a function of groove depth, is able to reorient the easy and hard axes from the initial orientation. At the point of re-orientation, the samples become isotropic with large permeabilities. As the groove becomes more clearly defined the hard axis permeability increases, while the easy axis permeability decreases. The role of stripe width and ambient processing atmosphere have been investigated and are reported.