Ultra-high resistive and anisotropic CoPd-CaF₂ nanogranular soft magnetic films prepared by tandem-sputtering deposition

Ultra-high resistive and anisotropic soft magnetic films for gigahertz applications are desirable to demonstrate the really practical films. Here we present a study of novel nanogranular films fabricated by tandem-sputtering deposition. Their electromagnetic properties and nanostructure have also been discussed. These films consisted of nanocrystallized CoPd alloy-granules and CaF₂ matrix, and a specimen having a composition of (Co_0.69Pd_0.31)₅₂-(Ca_0.31F_0.69)₄₈ exhibited distinct in-plane uniaxial anisotropy after uniaxial field annealing with granule growth. Its complex permeability spectra have a ferromagnetic resonance frequency extending to the Super-High-Frequency band due to its higher anisotropy field, and its frequency response was quite well reproduced by a numerical calculation based on the Landau-Lifshitz-Gilbert equation. Furthermore, it was clarified that the CaF₂-based nanogranular film exhibits a hundredfold higher electrical resistivity than conventional oxide or nitride-based films. Higher resistivity enables the film thickness to achieve a margin exceeding threefold against eddy current loss. The greater resistivity of nanogranular films is attributed to the wide energy bandgap and superior crystallinity of CaF₂ matrix.