Relationship between morphology and soft-magnetic properties of Co-Sr-F nano-granular films

Abstract

With the rapid improvement of electromagnetic (EM) equipment, the high-frequency (HF) soft magnetic films have been studied intensively as HF components such as a film type inductor, an electromagnetic noise suppressor sheet, and so on.[1, 2] In particular, the nano-granular films are well-known as one of the best choices as HF components, which can prevent the ensuing plunge in electrical resistivity as ferromagnetic resonance frequency (fFMR) increases. Herein, we select Co-Sr-F nano-granular films as the material of HF components and investigate their structure and magnetic properties in detail.The structure is nanocomposite, that is, Co nano-granules are evenly dispersed in a ceramic matrix. TEM observations indicate Co nano-granules change from a spherical to a columnar shape as the Co content increases. The electrical resistivity decreases from 109 to 104 μΩ*cm with a steep around 50~55 at.% of Co content as the Co content increases because of the change in the morphology of the film.In the range of Co content above 50 at.%, their coercivities are obtained and their saturation magnetizations (Ms) increase linearly(Fig.1). These results indicate that the films are ferromagnetic in the range of Co content above 50 at.%. To clarify fFMR of these films, the change in permeability of these films with frequency is measured, and the observed fFMR is summarized in Fig. 2 as a function of Co content. For Co content between 51~55 at.% (stage I), fFMR remains constant around 4.8 GHz, while, for Co content above 55 at.% (stage II), fFMR increases to 12 GHz monotonically. These behaviors can be explained by the change in the morphology of Co nano-granules. Namely, in stage I, as Co nano-granules are spherical, Kittel formula[3] is expressed as fFMR=γHz (γ is the gyromagnetic ratio). In stage II, as Co nano-granules are columnar, Kittel formula is described as fFMR=γ(Hz+1/2MS) and fFMR increases linearly with the increase of Ms. Therefore, these results demonstrate fFMR and film morphology correlates each other apparently, and suggest that Co-Sr-F nano-granular films are one of the candidates for HF components.  Fig. 1 Magnetization curves of Co-Sr-F nano-granular thin film.  Fig. 2 The dependence of fFMR on Co content.