Radial-Anisotropy Thin-Film Magnetic Material for High-Power-Density Toroidal Inductors

Thin-film nanogranular magnetic materials are attractive for micro-fabricated 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-power-density and high-quality-factor toroidal inductors is discussed in this paper. The radial anisotropy is induced during sputter deposition of magnetic thin films by an applied radial magnetic field. A magnetic field orientation fixture including a permanent magnet and carefully shaped iron pieces for fabricating smaller toroidal cores with down to 1.6-mm inner diameter and up to 5.7-mm outer diameter has been designed and fabricated. Radial-anisotropy toroidal cores with 1.7-mm inner diameter, 5.6-mm outer diameter, 10-micrometer thickness have been fabricated. Hysteresis measurements of fabricated samples showed desired radial anisotropy. Magnetic field orientation fixtures with shared magnets enabling fabricating even smaller toroidal cores are proposed. High-power-density and high-quality-factor toroidal inductors using the radial-anisotropy thin-film magnetic material are optimized for a high-frequency power converter.