High frequency dynamic magnetic properties of epitaxial FeSi thin films on (001) MgO

Developing high frequency soft magnetic thin films is still a hot issue due to the requirement of the technology in high frequency field. Traditionally, tuning the high frequency performance of magnetic thin films is realized by tuning the in-plane uniaxial anisotropy field of magnetic thin films with amorphous or nanocrystalline structure. There are several methods to obtain the in-plane uniaxial magnetic anisotropy such as applying an in-plane magnetic field during thin film deposition, magnetic field annealing, addition of elements or compounds, and patterned thin films to create shape anisotropy method [1]. Moreover, with the increase of the grain size, the high frequency performance becomes worse due to the decrease of the in-plane uniaxial anisotropy field and associated the cut-off frequency according to the random exchange anisotropy model of Herzer [2]. However, there are few works are concerned the high frequency properties of epitaxial single crystal film. In this work, Fe0.84Si0.16 (FeSi)/Ta (5 nm) thin films with thickness of 70 nm have been deposited on (001) MgO substrates by rf magnetron sputtering at 400 °C with a base pressure below 3'10 −5 Pa and a processing Ar pressure of 0.2 Pa, and then heat-treated for 1 h at 800 °C with the heating rate of 50 °C/min. MgO/FeSi/Ta shows (002) and (004) peaks, which indicated that FeSi(001)[110]//MgO(001) [100] film is epitaxial growth. The mainly chemical order phase is B2 with a significant A2 disorder phase according to the X-rays diffraction peak intensity ratio [3]. Angular dependence of remanence ratio (ARR) and hysteresis loops with the magnetic field applied parallel to in-plane film were measured by vibrating sample magnetometer (VSM) at room temperature. A four-fold symmetry of remanence ratio from the ARR graph was observed, indicating the cubic magnetic crystalline anisotropy of FeSi films. However two adjacent easy axis direction remanence ratios are not completely equivalent, which may be caused by a superstition of an extra in-plane uniaxial anisotropy with easy axis along the [100] direction as shown in Fig. 1. Thus, we may conclude the presence of a superstition of a strong four-fold magneto-crystalline anisotropy and a small in-plane uniaxial magnetic anisotropy with easy magnetization axis along the [100] and [010] crystallographic directions of FeSi and hard magnetization axis along the [110] and [110] crystallographic directions. The saturation field along [110] direction reaches 550 Oe and the saturation magnetization is about 13 kGs. The dynamic magnetic properties are investigated by the one-terminal micro-strip transmission-line perturbation method in the frequency range 0.1–9 GHz. The permeability spectra are shown in Fig. 2 with an extra magnetic field applied along the [100] easy axis direction. One can see the natural ferromagnetic resonance frequency is around 8.0 GHz for epitaxial FeSi thin films on (001) MgO substrate. The quite large resonance frequency is ascribed to the large magneto crystalline anisotropy field of FeSi films with cubic anisotropy. The obtained Gibert damping constant (a) through fitting the complex permeability (μ") based on Lan-dau-Lifshitz-Gibert equation with the variation of applied magnetic field is around 0.016, which is larger than that of pure Fe film. It maybe understood by the chemical disorder of Fe and Si atoms with A2 structure.