A new connection between electricity and magnetism

Electricity arises from the flow of electron charge, ferromagnetism from the ordering of electron spins. The interactions between electricity and ferromagnetism have been crucial to our technological development over the last centuries: the magnetic fields generated by electrical currents and the voltages generated by moving magnets are two cornerstones of our modern lifestyle. Important applications range from the largest power plants, to electric motors, down to the nanoscale write heads in our laptop hard drives. In contrast, the physics of ordered electron spin transport (a spinpolarized current) has become important only relatively recently. The new field of spintronics [1] is based on the ongoing prediction, discovery, and interpretation of additional interactions between electricity and magnetism that follow from the flow of spin-polarized electrons. Writing in Physical Review Letters, Shengyuan Yang and colleagues from the University of Texas, Austin, present a theoretical description and measurements of a new spintronic interaction: the voltage induced by a magnetic domain wall moving along a ferromagnetic wire [2]. So far, spintronic interactions have mainly been studied in devices made from layered ferromagnetic metal films, where the layers are thin enough that flowing electron spins carry magnetic information from layer to layer without drastic realignment. The most dramatic example is the finding that the electrical resistance between two magnetic layers depends on their relative magnetic orientation. The discovery of this effect, called giant magnetoresistance [3, 4], led rapidly to the development of novel sensor and data storage technologies and was recognized with the 2007 Nobel Prize in Physics, awarded to Albert Fert and Peter Grünberg. A related effect, dubbed “spin transfer torque,” occurs when spins flowing from one layer to another can reorient the magnetization in the layers [5]. The effects of spin transfer torque, such as precession and switching of magnetism in small junction devices, have been measured under a wide variety of conditions. The complementary effect, spin pumping [6], occurs when the dynamics of the magnetization in the layers drives a spin current in the device. The consequences of spin pumping are more difficult to observe than those of spin transfer torques but are still well established experimentally [7, 8]. Spintronic effects are not only found in multilayer structures. For example, in single-layer magnetic nanowires, the analog to giant magnetoresistance is domain-wall resistance: the additional resistance due to a domain wall [Fig. 1] between two regions of opposite magnetization. A current in a nanowire can also move a domain wall via the same spin transfer torques that causes precession and switching in multilayers [9– 11]. As electrons flow through a ferromagnet, their spins tend to align with the magnetization. When they pass into a region of nonuniform magnetization, such as a domain wall, the electron spins rotate to stay aligned with the local magnetization direction. A reaction torque on the changing magnetization in the domain wall can cause the pattern of magnetization at the domain wall to move in the direction of the electron flow. Several theoretical groups [12–15] have predicted the existence of a new effect that is both complementary to current-induced domain-wall motion and also analogous to spin pumping in layered structures: a voltage or current induced by domain-wall motion. The measurements of Yang et al. are the first observations of these effects [2]. Conceptually, it is straightforward to measure this domain-wall motion-induced voltage: simply propagate a domain wall down a ferromagnetic wire while measuring the voltage difference between the ends of the wire, as illustrated in Fig. 1. In practice, however, the voltage induced by the moving domain wall is only one of several voltages arising from different effects, and the challenge to the experimenter is to sort out this one small signal from other signals that compete with it. In particular, voltages induced by changing magnetic fluxes in the detection circuit, as described by Faraday’s