Image courtesy of John Xiao
Schematic of mechanism for generating a magnetic field. The system consists of a thin metal layer [Platinum (Pt)], a metal spacer layer [Copper (Cu)], and a ferromagnetic layer (composed of a material called “Permalloy”, a 20% iron-80% nickel alloy, labeled Py in the figure). A current of electrons (black arrows, Je) in Pt metal layer separates electrons according to their spin direction (red and blue arrows). The electrons with spin direction marked with red arrow move downward and can penetrate neighboring metal layers (middle and bottom layers). The effective field that is produced is confined to the bottom layer and may be used to cause switching of magnetization direction in the ferromagnetic layer.
Scientists have achieved the first experimental confirmation that a current of electrons with the same spin alignment (a pure spin current) in a thin metal film can penetrate neighboring metal layers (see figure) and cause switching of the magnetization direction in the bottom layer.
This demonstration reveals a new method to control the magnetization direction in the nanomagnets used in electronics, sensors, computer hard disks, and computer memory.
Spintronics – short for spin electronics- is an emerging field of nanoscale science and technology involving the detection and manipulation of electron spin. The spin of the electron provides an extra lever for scientists to tweak and offers the possibility of totally new functionalities in electronics, including faster and more efficient computers and other devices. Electron spins can either be “spin up” or “spin down” with the direction of the electron spin aligning with an applied magnetic field. Precise control of this phenomena or magnetization is a major challenge in the spintronics field. In this study, a flow of electrons with same spin direction, referred to as a pure spin current, is injected into a material to switch the magnetization direction. This effect is called a “spin transfer torque” (STT). Researchers at the University of Delaware have focused on the understanding of STT effect in thin films comprised of two layers, one a ferromagnet (FM) and the second a heavy metal (HM). In this bilayer material, a spin current passing from the HM through the HM/FM interface reverses the direction of the magnetization of FM layer. The experiments have demonstrated, for the first time, that spin current can produce a rotation of the magnetization within the film plane (represented by a field-like term in models) in addition to the commonly observed rotation of the magnetization out of the film plane (represented by a torque like-term in models). This observation presents a possible approach for control and switching of the magnetization direction of the FM as well as an important tool to understand the mechanism of STT in FM/HM bilayers.
John Q. Xiao
Department of Physics and Astronomy, University of Delaware, Newark, DE 19716
DOE Experimental Program to Stimulate Competitive Research (EPSCoR: DE-FG02-07ER46734) for measurements and analysis and NSF (ECCS-1001715) for sample fabrication and instrument development.
Xin Fan, Jun Wu, Yunpeng Chen, Matthew J. Jerry, Huaiwu Zhang and John Q. Xiao, Observation of the nonlocal spin-orbital effective field. Nat. Commun. 4:1799 doi:10.1038/ncomms2709 (2013).