Towards electromagnetic metamaterial: Negative permeability spectra of YIG composite materials

Yttrium Iron Garnet (YIG) granular composite materials in the high particle content show negative permeability spectra under external magnetic field in the microwave range. In this study, the complex permeability spectra (μ_r = μ_r\´ - jμ_r") of 56.2 vol.% YIG composite under external dc field have been analyzed using a permeability dispersion formula having the domain wall (DW) vibration and the gyromagnetic spin rotation susceptibilities, χ_d and χ_s. Frequency dispersion parameters of the DW and spin resonances were evaluated. With increasing dc external field, static susceptibility of the two components decreases and the resonance frequencies shift toward higher frequency range. The DW resonance contributes to the negative permeability as well as the spin resonance up to about 600 Oe. Above 600 Oe, DW contribution becomes very small; spin resonance contribution becomes dominant. The variation of the maximum frequency in the imaginary part of susceptibilities with damping factors has fairly good agreement with the theoretical calculation.