Complex permeability measurements of radial-anisotropy thin-film magnetic toroidal cores

This paper presents complex permeability measurements of a series of multi-layer thin-film radially anisotropic toroidal cores, which can be effectively used in toroidal inductors for high-frequency power applications. Good performance of radially anisotropic cores enables the miniaturization of magnetics while maintaining high efficiency for achieving power-supply-in-package or power-supply-on-chip. The cores were created through sputter deposition of nano-composite Co-Zr-O in the presence of a radial magnetic field to induce radial anisotropy. Measurements were performed using an impedance analyzer, a commercial magnetic material test fixture, and a custom-built test fixture, which allows measuring cores that are smaller than the minimum size supported by the commercial fixture. Tested samples have an outer diameter of 5.5 mm, inner diameters of 1.7, 2.3, and 3.4 mm, with magnetic film thicknesses of 6, 10, and 40 μm per sample. Small-signal measurements show that the toroidal cores reach quality factors above 100 below 100 MHz. Furthermore, the measurements show that the real part of the relative permeability does not vary across the tested samples, but the self-resonant frequency and quality factor depend on the inner diameter and magnetic film thickness. Wire-wound toroidal inductors using the radially anisotropic cores were built and tested to verify the core characteristics.