Static And Dynamic Measurement Methods Of The Anisottopy In Exchange-bias Thin Films

Aiming at the significance of material anisotropy in high-frequency magnetic properties, and centering on the quantitative measurements of the uniaxial anisotropy of ferromagnetic layers and unidirectional anisotropy caused by anti-ferromagnetic pinning in exchange-bias bilayers, this dissertation is focused on the theorical and experimental studies on the anisotropies of the exchange-bias bilayer from the static and dynamic aspects. The main research findings are summarized as follows:1. This dissertation puts forward that the rotational process of magnetization deviating from the easy axis is consistent with the coherent rotation in exchange bias system. Combined with Stoner-Wohlfarth model, the static rotational magnetization curves of different film systems are calculated theoretically. The research results are: For all in-plane uniaxial anisotropic ferromagnetic films and exchange bias bilayers whose uniaxial and unidirectional anisotropic fields are collinear and non-col linear, three magnetization rotation processes, continuous non-reversal, discontinuous reversal and continuous reversal, can occur under different external fields. Due to the anti-ferromagnetic pinning, the period of rotational magnetization curve changes from π of the uniaxial anisotropic ferromagnetic film to2π of the exchange bias bilayer. For different exchange bias bilayers, there exist two critical reversal fields, and with the increase of the ratio between unidirectional and uniaxial anisotropic fields, the values of them get closer, which leads to the range of magnetization rotational process of discontinuous reversal getting smaller and smaller.2. This dissertation also puts forward that the rotating magnetization curve approach can be employed to accurately obtain unidirectional and uniaxial anisotropic fields of exchange bias samples, and with VSM the measurement and research of static rotating magnetization curve of FeNi/FeMn samples are realized. The research results are:The accuracy direction of easy axis can be obtained from the rotating magnetization curve. By fitting the experimental curves, unidirectional and uniaxial anisotropic fields as well as the critical reversal fields and angles can be obtained. Measuring the rotating magnetization curves of FeNi/FeMn samples with different thickness, under the same external fields the same or different magnetization rotational processes can be found. Such atmosphere is due to the differences of the critical reversal fields and angles of different samples, which essentially reflects the differences among the unidirectional anisotropic field, uniaxial anisotropic field and the ratio of them.3. This dissertation finally puts forward that the ferromagnetic pinning can improve the resonance frequency and the anisotropic fields of exchange-bias bilayers can be obtained through the relationship between the resonant frequency and the external fields. The research results are:The resonant frequencies of FeNi/FeMn bilayers are higher than the values of FeNi films. Three different fr2～H curves correspond to three different magnetization states of FeNi/FeMn bilayers under different external fields. The ferromagnetic pinning makes the uniaxial anisotropic field of FeNi film increase and the step phenomenon of hysteresis loops of (FeNi/FeMn)n multilayer correspond to the two resonance peaks on the magnetic spectrum. By fitting the fr2～H curves of FeNi/FeMn/FeNi sample, it is found that the uniaxial and unidirectional anisotropic fields of firstly deposited FeNi layer are larger than those of secondly deposited FeNi layer, which is caused by the difference of exchange-coupling interaction and leads to the occurance of the step phenomenon of hysteresis loops.