Reserch In The Dynamic Properties Of Exchange-biased NiFe/FeMn Multilayers

With the rapid development of information technolog, magnetic magnetic electron devices will become more and more integrated, small and high-speed. Traditional magnetic materials can no longer meet the requirements of the growing magnetic electronic devices,which we need to develop magnetic materials with high cut-off frequency, permeability and bandwidth. The anisotropic magnetic metal thin film which compared to traditional magnetic material has a higher saturation magnetization, and can break the Snoek limit. It also have a large permeability and resonance frequency which can meet the needs of magnetic electronic devices. So far, the metal magnetic thin film has been widely used in microwave devices, but the operating frequency need to be improved. By making use of the exchange-bias effect between ferromagnetic(FM) and anti-ferromagnetic(AFM), we can push the cut-off frequency to a higher degree.In this paper, a series of NiFe/FeMn exchange bias multilayer films were fabricated by DC magnetron sputtering. The structure of the films were analyzed by X-ray diffraction. The static magnetic properties are characterized using a vibrating sample magnetometer(VSM), while the dynamic properties are measured by a vector network analyzer(VNA). The results showed that the crystallinity of the films were good, exchange bias can greatly improve the high-frequency properties of the film. In Ta/NiFe(15nm)/ FeMn(t)/Ta sample, despite the very small exchange bias field(HE = 14 Oe), the resonance frequency reaches 2.84 GHz. This can be explained by the contribution of rotational anisotropy which can be obtained by finding the difference between dynamic anisotropy and static anisotropy. We found that rotational anisotropy increase firstly and then decrease with increase of AF thickness which was larger at thinner AF layer.By fabricating Ta/[NiFe(15nm)/FeMn(t)]n/Ta exchanged multi-cycle films, we can further improved the high frequency performance. At the thickness of tAF = 4nm, we can push the resonance frequency to 4.02 GHz. Due to different FM were pinned by the different FM layer, two separate loops were observed in Hysteresis loops. And double resonance peaks in multi-cycle films greatly improved microwave absorption properties. Adjustment of the FM layer thickness and the number of multilayer cycles can cause the double resonance peaks to join together to produce broadband permeability spectra.In addition, the magnetization reversal process of exchange-biaed bilayers were studied by using anisotropic magnetoresistance(AMR). It was found that the reversal magnetization process were influenced by exchange-biased effect.