Anomalous Hall effect in ferromagnetic metallic thin films.

摘要

Anomalous Hall effect (AHE) has gained renewed interest regarding its origin (or mechanism) in ferromagnetic materials and its importance in the characterization of the magnetic state in diluted magnetic semiconductor materials. Although this effect was discovered almost a century ago, its microscopic origin is still under debate. In this thesis, we study the origin of AHE in ferromagnetic metallic films, by which we investigate the rationality of the existing theoretical models and point out the errors generated in practice when these models are utilized to interpret experiments.;Both extrinsic and intrinsic mechanisms were proposed decades ago for the origin of AHE. The former considers the spin-orbit coupling dependent scattering as the source of the AHE, while the latter states that the intrinsic Berry phase gives rise to AHE. We fabricated ferromagnetic single-layered granular films with magnetron sputtering. We changed the scattering intensity by introducing various impurities and studied the impact of these impurities on AHE. With these experiments, we study the disspationless features of the anomalous Hall current. Apart from verifying this feature, we found that due to the lack of an enough span of both the AHE and Ohmic resistivity data points, the judgement of a responsible mechanism for AHE based on the scaling law fitting with these data is not reliable.;We further study the AHE in ferromagnetic/non-magnetic bilayers and ferromagnetic/ferromagnetic bilayers as well. In these bilayer films, the AHE is generated only in ferromagnetic layers and the non-magnetic layers act as discharging channels affecting the anomalous Hall electric field through shunting effect in longitudinal direction and shortcircuit effect in transverse direction. For this reason, the scaling law between the longitudinal and transverse resistivity in these samples is no longer applicable.;Extrinsic mechanisms suggest a straightforward way to testify the origin of the AHE. However, due to the difficulty of exact simulation of the imperfectness in solids, the SS and SJ models just provide a qualitative method. On the contrary, the intrinsic mechanism, i.e., Berry-phase-inducing AHE states that AHE is due to the intrinsic equivalent internal magnetic field. This magnetic field gives rise to AHE in magnetic domains as it does to OHE. Furthermore, this model can calculate AHE quantitatively. We discuss the intrinsic aspects of AHE mechanism with Fe/Gd bilayer films. We found that, the considered intrinsic AHE in some materials can possibly be explained in other manners.