Development of magneto-dielectric materials based on Li-ferrite ceramics: III. Complex relative permeability and magneto-dielectric properties

Abstract

This paper reports on the effects of Bi 2O 3 and Co on the magnetic properties of Li 0.50Fe 2.50O 4 ceramics. The magnetic properties (static permeability) of the lithium ferrite ceramics with low concentration of Bi 2O 3 (0.2% and 0.5%), as a function of sintering temperature, can be explained by the magnetic circuit model, while those of the samples with high concentration (1–5%) are in a good agreement with the Globus model. Both models are not applicable to the results of pure Li 0.50Fe 2.50O 4 samples, because their magnetic properties are determined by both the density and grain size. Further modification of Li 0.50Fe 2.50O 4 with Co makes it possible to produce ferrite ceramics with promising magneto-dielectric properties. Maximum static permeability is observed in Li 0.50–0.50 x Co x Fe 2.50–0.50 x O 4 at x = 0.01, which can be qualitatively explained by magnetocrystalline anisotropy and ion ordering induced local uniaxial anisotropy. It is found that the Li 0.50–0.50 x Co x Fe 2.50–0.50 x O 4 (with x = 0.030–0.035) ceramics sintered at ≤950 °C have matching permeability and permittivity of 13–15, as well as sufficiently low magnetic and dielectric loss tangents of ≤10 −2, over 3–30 MHz (HF band). From the laws of physics, HF antennas can be physically reduced by a factor of 13–15, with unchanged electrical dimension, if loaded with this class of magneto-dielectric materials.