Study Of The Anomalous Hall Effect And Magnetic Properties Of Iron And Iron-based Alloys

In the past two decades, the study on magnetism is prominent in the area of condensed matter physics. It is not limited to static and macro magnetism but expanded to spin dynamics in transport, even in the single electron limit. Due to the promising future of using the freedom of spin in data storage and calculation, this area draws intensive attention and investment. Experimentally, the tunneling magneto-resistance (TMR) and Colossal magnetoresistance (CMR), spin Hall effect, anomalous Hall effect, spin torque and quantum spin Hall effect are typical new phenomena found in this area. It has been developed into a comprehensive discipline which contains device fabrication, mechanism study and materials development, and has a specific name as spintronics. Among various topics, anomalous Hall effect is one of the most fascinating problem because it is discovered by Edwin Hall himself in 1889. But the debate about its origin has been lasting to date. The existing mechanisms includes an intrinsic mechanism proposed by Kaplus and Luttinger, an extrinsic mechanism called skew scattering which is found by Smit and another extrinsic one-side jump found by Berger. The complicated theoretical model and experimental results make it difficult to settle this debate. Today people found that the mechanism of anomalous Hall effect also works in spin Hall effect, which highlights the importance of understanding its mechanism and behavior with temperature and dimension.In this thesis, the author investigates the classical system of AHE-single crystal of Fe. Fe films are grown epitaxially on GaAs(001) and MgO(001). Its resistivity is tuned by film thickness. This way of tuning resistivity has the benefit of keeping the intrinsic mechanism unchanged within certain extend, while the extrinsic mechanisms vary with resistivity. In contrast, the old way of tuning resistivity-adding impurities-has the disadvantage that while the extrinsic mechanisms are changing, the intrinsic one also varies because of the alloying. It is this difference that brings some undiscovered results:1. We prove that the widely used function describing the anomalous Hall resistivity and the strength of scattering (the longitudinal resistivity) is not suitable describing our experimental data. Then we established a new function based on experiments. It is capable describing AHE in various materials at different temperature. The terms in the function are interpreted to have connection to the skew scattering, side jump and intrinsic mechanisms. Moreover, it summaries the experimental results at finite temperature, which is not fully understand in theory. This progress should stimulate further discussions in theory.2. Through tuning of the interface and impurity level of our samples, the AHE at different interface, substrates and impurities is measured and discussed. We show that the previously established new function also works in Fe-based alloys. The intrinsic and extrinsic mechanisms are separated and discussed consequently. In some special cases the experimental data deviates from the new function, which indicates in the case of magnetic impurities, some mechanisms of AHE remains unclear.3. Based on our progress, the AHE in ultra-thin Fe films are investigated. The intrinsic contribution is observed to decrease in thinner films. The situation for extrinsic contributions depends on whether localization effect happens. This part of results provides experimental facts for theory to explore. Furthermore, the localization correction to anomalous Hall conductivity is studied within the established framework of extrinsic mechanisms. Side jump mechanism is found to dominant in ultra-thin Fe films on GaAs, which support the interpretation that one term in our new function is connected to side jump mechanism. Meanwhile the side jump contribution in Fe on MgO is found to be minor.The last chapter of this thesis is some calculation about magneto-optical Kerr effect in multi-layer system. Combining with experiment, we show that the magneto-optical constant of nano-scale Fe may be larger than that of the bulk Fe. Other optical constants like reflective index of Fe and Au remains the same. The calculation results demonstrate that in optically anisotropic materials, for instance some materials prepared by molecular beam epitaxy, the magneto-optical Kerr effect can still be used to measure the magneto-crystalline anisotropy.