Exchange Bias And Giant Magnetoresistance Effect In Magnetic Multilayers

During the past two decades, extensive experimental and theoretical investigations have been performed on giant magnetoresistance (GMR) because in spin-valve it has very important applications in commercial magnetic storage and sensor devices and a new discipline - spintronics was born. This dissertation is focused on the exchange bias which is a very important part in spin-valve and spin-dependent scattering phenomenon. The main contents are summarized as follows:In the first part, exchange biasing in GdFe/NiCoO and GdFe/FeMn bilayers has been investigated and the positive exchange bias phenomenon has been observed firstly in ferrimagnet/antiferromagnet system. The exchange field changes from negative values to positive values with increasing cooling field and coercivity acquires a maximum near the crossover of exchange field. These results can be ascribed to a competition between the FM-AFM exchange interaction and an external field-AFM surface magnetic coupling interaction. The angular dependence of exchange field and coercivity has been investigated for both negative and positive exchange bias in GdFe/FeMn system. It is found the negative exchange bias and the positive one have similar angular dependence that can be described by a magnetization coherent rotation model.In the second part, Si/Ta/Co/Cu/Co/FeMn (NiO) spin valves were prepared by a delayed sputtering procedure. The spin valves are found to have stronger (111) preferred orientation in Co and Cu layers, compared with the traditional continuous method. At the same time, the GMR ratio and spin-dependent scattering are enhanced by a factor of 20%-30%. These results indicate that the coherent growth and the (111) preferred orientation can enhance the spin-dependent scattering of spin valves. To optimize the spacer Cu thickness, the GMR ratio is 7.7% and 12.1% at room temperature.In the third part, for Fe/Cu/CoGd spin-valves, GMR has been measured as a function of different thickness and composition of CoGd layers and temperature. The GMR has been attributed to contributions of spin-dependent scattering at interfaces and in bulk. The bulk contribution is dependent on the composition of GdCo alloy. The bulk contribution is positive (negative) with the Co content is high (low). The variation of the bulk asymmetric factor in the GdCo layer with the alloy composition can be explained in terms of the variation of either the spin alignment of Co and Gd atoms or the spin polarization of GdCo layer. However, the interfacial contribution produces positive giant magnetoresistance ratio for various compositions of GdCo alloys. The temperature dependence of GMR ratio could also be considered the competition between the interface scattering and bulk scattering.