Spintronics is a new concept that could increase the speed and storage capacity of computers by transferring information in the intrinsic angular momentum—or spin—of electrons. One of the main challenges in spintronics is devising methods to electrically generate and control spin-polarized electrons.

Now Iwao Matsuda and colleagues1 at the University of Tokyo have produce spin-polarized electrons throughout a metal film based on a phenomenon called the Rashba effect, which separates electrons of different spin.

Fig. 1: A thin silver film deposited on a silicon substrate confines electrons in a quantum well (top). This state is spin-degenerate, meaning that all the electrons have different spins. When a single-atom layer of bismuth-silver alloy is added (bottom), there is a ‘spin split’ at the surface caused by the Rashba effect. The spin split spreads downwards, leading to spin-polarization in the silver film – i.e. electrons transversing (conducting) along the same direction have the same spin orientation.Copyright © 2008 Iwao Matsuda

The Rashba effect arises when electrons are trapped in a quantum well—a dip in potential energy that confines the electrons in one dimension (the z-axis in Fig. 1, top). If the well is not symmetrical, the electrons experience an electromagnetic field that splits them into different energy bands depending on their spin.

Most importantly, the Rashba spin-splitting can be modified by an electrical voltage, so it can benefit spintronic versions of common devices such as transistors. It was recently discovered that heavy metal crystals such as gold, bismuth or antimony show particularly large Rashba spin-splitting, but the splitting is usually only seen at the metal surface.

Matsuda and co-workers studied dual thin films comprising a layer of silver, which generates quantum well states, covered with a layer of bismuth-silver alloy that shows a large Rashba spin-splitting at the surface (Fig. 1, bottom). The researchers produced images of electrons in the films with photoemission spectroscopy.

The images showed evidence that the quantum well state in the silver mixed with the spin-split surface state in the alloy, causing the quantum well state to become spin-polarized itself. Therefore the Rashba spin-splitting was spread from electrons at the surface to electrons throughout the whole film.

This work is the first experimental indication that spin polarization can spread to regions other than the surface of non-magnetic metals – promising higher performance of spintronic devices.

“Our idea is to regulate electronic and spin properties of quantum films just by adding interface atomic layers,” says Matsuda. “Moreover, the films are grown on silicon substrates, so established concepts from silicon technology can be applied.”