Low density artificial microwave magnetic composites

Abstract

The high frequency permeability of composites made of hollow conducting planes or tubes is derived. It is shown that such composites may exhibit permeability levels significantly different from unity, even for nonmagnetic metal volume fractions lower than 1%. This behavior is related to the skin effect, but it can occur for metal shell thicknesses much lower than the skin depth. The influence of the shape of the hollow inclusion on the permeability is reported. The maximum of the imaginary permeability can be tuned over a wide frequency range by adjusting the conductivity of the metal and the characteristic dimensions of the inclusions. The properties of composites made of hollow ferromagnetic planes are also investigated using experimental values obtained from materials made of ferromagnetic laminations.

High-frequency behavior of magnetic composites

“…Therefore, randomization factor of 2 3 appears in the right part of Eq. (2). In a single-domain film-shaped particle, one permeability component may be high, and two other components are close to unity.…”

“…For other domain structures, magnetic frequency dispersion may be more intricated, with either multiple resonance peaks [13] or a single broad peak of non-Lorentzian form; analogues of Eqs. (2) and (3) that are valid for these cases are discussed in Section 6.…”

“…A generalization of Eqs. (2) and (3) to complex magnetic dispersion curves and composite materials is given in Section 6.…”

“…The scope of the paper is limited to composites comprising tiny ferroor ferri-magnetic powders at wavelengths large compared to the powder particle size. Artificial magnets, i.e., the materials, where no static magnetic moment is induced, though the induced moment at high frequencies may be rather high [1][2][3], are beyond the scope. Everywhere in the paper, relative values of permittivity and permeability values and volume concentrations in 3d isotropic composites are implied.…”

GHz permeability of soft CoZr films: The role of exchange–conductivity coupling

“…Since the supplementary high frequency peak is observed on samples with thickness comparable to the skin depth, it is important to assess whether it may be due to skin effect. A common approach to account for skin effects is to solve the Maxwell equations in the sample describe as slab with a scalar permeability m. Taking the appropriate definition of the measured permeability m eff [3], one finds…”