Study Of Anomalous Hall Effect In Co-based Thin Film Materials

In recent years, the anomalous Hall effect (AHE) in Co-based thin film materials (such as Co/Pt multilayers and CoFeB films) has attracted great interests because of potential applications in magnetic sensors, memories, and logic devices. However, there still exist some problems for them in the real application. For example, Co/Pt multilayers possess weak AHE and poor thermal stability due to Co-Pt interdiffusion during annealing. And high temperature annealing can deteriorate magnetic and transport properties of CoFeB thin films. These existing problems could limit their applications. Focusing on addressing the problems mentioned above, we systematacially study the AHE in Co-based thin film materials in this thesis. The main research results are as follows:The influence of Mg intercalation on the AHE has been studied in perpendicular MgO/[Co/Pt]2/Co/MgO multilayers. The AHE in Co/Pt multilayers with Mg metal layer at the Co/MgO interface is 125% larger than that in the multilayers without Mg insertion. XPS analysis shows that the improved AHE is mainly attributed to the modification of the chemical states at the interface region of Co/MgO via Mg intercalation, which results in the enhancement of the saturation magnetization in perpendicular Co/Pt multilayers.Thermal stability of the AHE in MgO/[Co/Pt]2/Co/MgO multilayers has been investigated by introducing a Hf metal layer at the Co/MgO interface. The AHE in perpendicular MgO/[Co/Pt]2/Co/Hf/MgO multilayered structure after 350℃ annealing is 14% larger than that in the as-grown films, which is mainly due to the enhancement of the side jump and intrinsic contributions to the AHE via interface modification.We have investigated the effect of annealing on the AHE in perpendicular X/[Co/Pt]2/Co/X (X=Pt, MgO and HfO2) multilayers. Annealing can weaken the AHE in Co/Pt multilayers sandwiched by Pt layers or MgO layers, mainly due to Co-Pt interdiffusion during annealing. Surprisingly, the AHE in HfO2/[Co/Pt]2/Co/ HfO2 multilayers after 300℃ annealing is 18% larger than that in the as-deposited films despite Co-Pt interdiffusion, which is attributed to the improvement of the skew scattering contribution to the AHE. The influence of the HfO2/Co interface and the Co/HfO2 interface on thermal stability of the AHE in perpendicular Co/Pt multilayers has been studied. Thermally stable AHE behavior is obtained in perpendicular [Pt/Co]3/HfO2/Pt multilayers despite Co-Pt interdiffusion, due to the improvement of the side jump and intrinsic contributions to the AHE by the modification of the Co/HfO2 interface after annealing. However, annealing can weaken the AHE in perpendicular Pt/HfO2/[Co/Pt]3 multilayers owing to Co-Pt interdiffusion.The influence of HfO2 buffer layer on the AHE in Hf/CoFeB/MgO films has been investigated. The enhancement of 158% of the AHE in the 350℃ annealed CoFeB films is obtained by introducing HfO2 buffer layer, which is mainly ascribed to the enhanced saturation magnetization due to the suppression of Hf atoms diffusion during annealing, and the improvement of the skew scattering, side jump and intrinsic contributions to the AHE.