Physical origin of bimodal complex permeability spectrum of a noise-suppression sheet

Abstract

A noise-suppression sheet (NSS), which is composed of very thin soft magnetic flake particles with organic binder, is indispensable to suppress the electromagnetic compatibility problem for recent mobile communication devices. NSS is mounted on the circuit board and absorbs extra-electromagnetic waves inside the mobile device. This absorption property is characterized as a peak of imaginary complex permeability μ’’ owing to the ferromagnetic resonance [1]. Although a commercial NSS with flake particles of Sendust (Fe-Si-Al alloy) generally exhibits a bimodal μ’’ spectrum, the physical origin of this behavior has not been understood so far. This situation would be a bottleneck for the next generation mobile communication. In this study, we investigated the μ’’ spectra of NSSs with various magnetic materials (Fe74.8Al25.2, Fe50.2Co47.6V2.2, and Fe) and compared with those of constituent single flake particles. μ’’ spectrum measurements of single flake particles are realized by means of TC-PERM which is the newly developed complex permeability measurement method with very high sensitivity [2]. Figure 1 shows the complex permeability of NSS of Fe and the constituent single flake grain of 180 μm in diameter and 1 μm in thickness. As well as the commercial NSS of Sendust, the bimodal μ’’ spectrum is clearly observed for NSS of Fe. Moreover, quite similar μ’’ spectrum is also observed for the single flake grain. This fact strongly indicates that the bimodal μ’’ spectrum of NSS is attributed to the nature of a constituent single flake. We found that the magnetic vortex model can reasonably explain this bimodal μ’’ spectrum, that is, the low- and high-frequency peaks are owing to the resonances of vortex core gyration motion and flux closure magnetization state, respectively. The calculated low- and high-frequency peaks quantitatively agree with the experimental results as marked in Fig. 1(b). This good agreement can be also confirmed for other materials. This work is supported by MIC SCOPE JP195003002.  Fig. 1 Complex permeability spectra of (a) NSS and (b) a single constituent flake grain of Fe.