Layered Film And Multilayer Magnetism And Magnetic Relaxation Study

Since the discovery of giant magnetoresistance (GMR), it has received much attention because of its importance in applications of various GMR devices in the fields of machine, car, avigation, and ultrahigh-density magnetic recording. For example, the spin valve GMR reading head has allowed a dramatic increase of areal recording density in hard disk drives. As the heart of the GMR devices, exchange biased ferromagnet (FM) /antiferromagnet (AFM) has played an important role. Therefore, the FM/AFM exchange bias has attracted much attention in both basic research and applications.Recently, the study of ferromagnetic resonance (FMR) linewidth of magnetic films has received much interest. On the one hand, from the FMR linewidth, one can obtain the damping parameter for magnetization precession and information regarding magnetic inhomogeneity involved in films. On the other hand, as the working frequency of the GMR devices and magnetic recording media shifts towards microwave region, the study of the mechanism of magnetic relaxation in metallic ferromagnetic thin films becomes increasingly important.The dissertation consists of two main parts: First, training effect of the exchange biasing and magnetic relaxation will be discussed in FM/FeMn bilayers; Secondly, magnetic properties and magnetic relaxation will be addressed in Co/Pt multilayers. The major results are summarized as follows.I. Exchange coupling in FM/FeMn bilayers1. The exchange coupling and its training effects are studied as a function of the ferromagnetic layer magnetization by using various ferromagnet/FeMn bilayers with ferromagnet materials Ni, Fe19Ni81, Fe50Ni50, Co, and Fe. The exchangecoupling energy Jex increases with increasing MFM as Jex . Thetraining effect of the exchange field is related to both the ferromagnetic magnetization and the magnetization reversal mechanism. For ferromagnet/FeMn bilayers with similar magnetization reversal mechanism, the relative change of theexchange field decreases with increasing magnetization in an exponential manner. The dependence of training effect on FM magnetization can be explained by the Fulcomer and Charap's model. These results are believed to facilitate a better design of giant magneto-resistance devices.2. Exchanged-biasing in wedged-permalloy/uniform-FeMn bilayers is studied by ferromagnetic resonance and magnetometry measurements with applied field parallel to the film plane. For all samples, the exchange field measured by the ferromagnetic resonance is close to the magnetometry measured value. Since the coercivity equals to the uniaxial anisotropy field, the FM magnetization reversal can be explained by the coherent rotation model. An additional weak resonance peak was observed besides an intense resonance, indicating the existence of interfacial diffusion. For the wedged-Py/FeMn bilayers, the in-plane isotropic resonance field shift is positive and inversely proportional to the ferromagnetic layer thickness. It is originated from either specific geometry of Py layer thickness or interfacial diffusion. For the wedged-Py/FeMn bilayers, the exchange field, the coercivity, the anisotropy field and the positive isotropic resonance field shift are all inversely proportional to the FM layer thicknes, demonstrating an interface nature of the FM/AFM exchange coupling.3. We have prepared a wedged-CoNi/uniform-FeMn bilayer by dc magnetron sputtering and measured the out-of-plane FMR spectra. The recorded angular dependence of the resonance field was analyzed by the Landau-Lifshitz-Gilbertequation. The effective demagnetizing field increases and Lande g factordecreases with increasing FM layer thickness. The dependence of the linewidth on the out-of-plane angle and the FM layer thickness can be explained very well in terms of the intrinsic Gilbert damping effect and the broadening induced by the magnetic inhomogeneity. Calculations show that two contributions increase with decreasing the FM layer thickness and that the intrinsic Gilbert damping effect play...