Study On Vertical Anisotropy And Giant Magnetoresistance Effect Of Magnetic Multilayers

Magnetic thin films with out-of-plane easy axis has great potentials for the future applications of high density magnetic random access memories (MRAM) due to its good magnetic and thermal stability as the device scales down to submicron size, as well as the been considered as the predicted low critical current for spin torque driven magnetization switching. In this thesis, we focus on the study of performance optimization for the perpendicularly magnetized multilayers and spin valves, which mainly include:the perpendicular anisotropy of [CoFeB/Pt] multilayer, and the perpendicularly exchange-coupled composite structure consisting of a [Co/Ni]N multilayer and a ferrimagnetic TbFe layer which has been utilized as the spin valve reference layer.The main results in this thesis are summarized as follows:Firstly, we have obtained the perpendicularly magnetized [CoFeB/Pt]5multilayer structure. The perpendicular anisotropy and thermal stability of [CoFeB/Pt]5multilayers are investigated with various CoFeB thicknesses (tCoFeB) and a fixed Pt thickness of lnm. Magnetic hysteresis loops exhibit a square shape for0.22-0.40nm tCoFeB and suggest multilayers with appropriate CoFeB and Pt thicknesses display perpendicular magnetic anisotropy (PMA). After post-deposition annealing at temperatures Ta <300℃, no obvious change occurs in the loop shape for tCoFeB=0.22nm, while the perpendicular coercivity increases with increasing Ta for the sample with tCoFeB=0.45nm due to the enhanced (111) texture. At Ta-300℃, the original square loops for both samples starts to tilt, showing that the occurrence of PMA degradation is independent of the CoFeB thickness. XRD results indicate the observed decay of PMA in CoFeB/Pt multilayers upon postannealing is associated with the interdiffusion and alloying effects at the CoFeB/Pt interfaces.Secondly, we have investigated the relationship between magnetism and composition of TbFe, and the exchange coupling interactions between the perpendicularly magnetized [Co/Ni]N and TbFe layers for various TbFe compositons. By slightly raising the TbFe thickness, the coercivity of the coupled structure greatly increases, thereby giving an effective way to widen the difference in switching fields for the free and reference layers. In the perpendicular spin valve with [Co/Ni]5/TbFe reference layer and [Co/Ni]3free layer, such composite reference layer can not only provide a wide switching plateau, but also retain a high perpendicular GMR ratio. However, if the deposition power of Tb is40W the Tb-rich TbFe would decrease the perpendicular anisotropy of the free layer, leading to the magnetization away from the out-of-plane direction. Nevertheless, increasing the thickness of the Cu buffer layer could effectively overcome this problem and increase the GMR ratio up to6.4%.