Radial-anisotropy nanogranular thin-film magnetic material for toroidal inductors

Thin-film nanogranular magnetic materials are attractive for micro-fabricated very-high-frequency power magnetics, but their anisotropy makes them difficult to use in toroidal geometries. A method to deposit magnetic materials with the radial anisotropy required for high-performance toroidal inductors is introduced in this paper. The radial anisotropy is induced during sputter deposition of Co-Zr-O thin films by an applied radial magnetic field. A magnetic field orientation fixture which consists of two magnets and iron components is designed to create a radial field predominantly parallel to the substrate plane. Initial prototype toroidal cores with 8-mm inside diameter, 16-mm outside diameter, and thickness of 6 μm have been fabricated. Hysteresis measurements along the radial direction (easy axis) and the circumferential direction (hard axis) of deposited samples show the desired anisotropy orientation. Small-signal measurements show that the fabricated toroidal core has flat relative permeability near 60 to several hundred megahertz, and reaches a quality factor higher than 100 in the range of tens of megahertz.