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Abstract:
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The concept of Berry phase , since its proposition in 1984 , has found numerous applications and appears in almost every branch of physics today . In this work , we study several physical effects in ferromagnetic metal materials
which are manifestations of the Berry phase . We first show that when a domain wall in a ferromagnetic nanowire is undergoing precessional motion , it pumps an electromotive force which follows a universal Josephson -type relation . We discover that the integral of the electromotive
force over one pumping cycle is a quantized topological invariant equal to integer multiples of h /e , which does not depend on the domain wall geometry nor its detailed dynamic evolution .
In particular , when a domain wall in a nanowire is driven by a constant magnetic field , we predict that the generated electromotive force is proportional to the applied field with a simple coefficient consisting of only fundamental constants . Our theoretical prediction has been successfully confirmed by experiments . Similar effect known as spin pumping occurs in magnetic multilayer heterostructures ,
where a precessing free magnetic layer pumps a spin current into its adjacent normal metal layers . Based on this effect , we propose two magnetic nanodevices that can be useful in future spintronics applications : the magnetic
Josephson junction and the magneto -dynamic battery . The magnetic
Josephson junction has a drastic increase in resistance when the applied current exceeds a critical value determined by the magnetic anisotropy . The magneto -dynamic battery acts as a conventional charge battery in a circuit with well -defined electromotive force and internal resistance . We investigate the condition under which the power output and efficiency of the battery can be optimized . Finally we study the side jump contribution in the
anomalous Hall effect of a uniformly magnetized ferromagnetic metal . The side jump contribution , although arises from disorder scattering , was believed to be independent of both the scattering strength and the disorder density .
Nevertheless , we find that it has a sensitive dependence on the spin structure of the disorder potential . We therefore propose a classification scheme of disorder scattering according to their spin structures . When two or more classes of disorders are present , the value of side jump is no longer fixed but depends on the relative disorder strength between classes . Due to this competition , the side jump contribution could flow from one class dominated limit to another class dominated limit
when certain system control parameter changes . Our result indicates that the magnon scattering plays a role distinct from the normal impurity
scattering and the phonon scattering in the anomalous Hall effect , because they belong to different scattering classes . |