July 24, 2014

New class of materials could power memory devices

A new phase of matter known as topological insulators, until recently known only for esoteric quantum-mechanical properties, might have a practical use in controlling magnetic memory and logic devices.

A team of Cornell and Penn State University physicists has demonstrated for the first time that electrical currents flowing along the surface of topological insulators can exert a torque on an adjacent magnetic layer that is 10 times more efficient than any other known mechanism. This breakthrough provides a new strategy for making next-generation memory technologies that use the least possible energy and current.

The research, led by Dan Ralph, the F.R. Newman Professor of Physics at Cornell, and Nitin Samarth of Penn State, is published online July 24 in the journal Nature. The team used the topological insulator bismuth selenide (a combination of bismuth and selenium) for their experiments.

Like conventional insulators, topological insulators do not allow current to flow through the material, but they are different because they are wrapped in a conducting surface. Electrons flowing on the surface also do something unique: The direction of an electron’s spin is always locked perpendicular to its direction of motion. This locking provides a means for the flow of an electrical current along the surface to produce a buildup of spin that can apply torque to an adjacent magnet.

Ralph and colleagues are trying to develop new magnetic nonvolatile memory and logic devices. One of the main challenges in doing so is to find a way to quickly flip the devices’ magnetization using the least possible current.

The new results show that electrical current flowing within a thin film of bismuth selenide – at room temperature no less – can be used for this purpose.

The researchers caution that actual memory devices are a long way off, but the paper, Ralph noted, can be viewed as an exciting first step for a new branch of science.

The first author of the paper, “Spin Transfer Torque Generated by a Topological Insulator,” is Cornell graduate student Alex Mellnik; co-authors include Eun-Ah Kim, associate professor of physics, and colleagues.

The collaborative Cornell and Penn State work is supported by a grant from DARPA (the Defense Advanced Research Projects Agency). The Cornell team is also supported by the National Science Foundation Materials Research Science and Engineering Centers program through the Cornell Center for Materials Research, by the Army Research Office, and by the Kavli Institute at Cornell for Nanoscale Science.