A New Twist for Magnets

• Main Author: Dan Ralph
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: North Pole; Tilting
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1062.1715; http://ralphgroup.lassp.cornell.edu/papers/Science-291-999-2001.pdf

The manipulation of magnets with electrical currents is an integral part of everyday technology. It is the operating principle behind electric motors and determines how information is written onto magnetic-memory devices such as computer hard drives. The underlying physical mechanism has been understood since the early 1800s: Moving electric charges generate a magnetic field, which exerts a force on a magnet. A surprising realization has recently emerged in this seemingly mature field. There is a second, fundamentally distinct mechanism by which an electric current can reorient a magnet, and for very small devices, this mechanism can be much more powerful than current-induced magnetic fields. The new mechanism, known as spin transfer, is based on the interaction of a magnet with the intrinsic spin of an electron, rather than with the electron's moving charge. On page 1015 of this issue, Weber et al. (1) report direct measurements of this spin-dependent interaction between an electron and the elemental ferromagnets iron, cobalt, and nickel. Berger (2) and Slonczewski (3) first proposed such a spin-transfer effect. If an electron travels through a thin film of magnetic material, the magnet exerts a torque on the electron, tilting its spin. According to Newton's Third Law, the electron must exert an equal and opposite torque on the magnet, which causes the magnet's moment vector (the direction from its south to north pole) to tilt as well. The effect is called spin transfer because spin angular momentum is delivered from the electron to the magnetic material. The torque produced by a single electron is very small, but if all the electrons in a current are spin-polarized such that their spins all point in the same direction, then the sum of their contributions can produce a substantial torque on the magnet. The existence of this effect was demonstrated recently in layered metallic devices (4-8). Electrons were first passed through a magnetic layer that acted as a spin filter to produce a partially polarized current. ...