Spintronics turns a corner
May 24, 2001
Physicists in the US have overcome one of the biggest challenges in 'spintronics' - electronic devices that exploit the spin on electrons as well as the charge. Vincent LaBella and colleagues at the University of Arkansas have successfully injected a stream of electrons with identical spins into a semiconductor. They also discovered that the fine detail of the surface of the semiconductor greatly affects the ability of the electrons to retain their spin. These important insights into the behaviour of 'spin-polarized' currents bring ultracompact devices a step closer (V LaBella et al 2001 Science 292 1518).
Quantum mechanics states that the spin on an electron may either be 'up' or 'down'. The easiest way to align - or 'polarize' - these spins is to place them in a magnetic field. In previous experiments, an electric potential was applied across a ferromagnet connected to a semiconductor. This pulls the polarized electrons from the ferromagnet into the semiconductor, but the process is not very efficient and most of the electrons revert to a mixture of random spins.
LaBella and colleagues, however, managed to keep 92% of the electrons in their spin-polarized state. They used the tip of a scanning tunnelling microscope made of single-crystal ferromagnetic nickel wire as a source of fully polarized electrons. There were no spin-up electrons at the Fermi level in the nickel wire - which means that these electrons cannot contribute to conduction. But there were spin-down electrons at this energy - and this guaranteed that the current in the wire consisted solely of spin-down electrons, which were then injected into a slab of gallium arsenide semiconductor. "Other groups have achieved similar efficiencies, but only at around 10 kelvin," LaBella told PhysicsWeb. "We have reached this figure by using a tunnelling method to inject the electrons".
A technique known as spin-polarized tunnelling-induced luminescence spectroscopy is used to probe the spin of electrons in the semiconductor. This relies on the ability of polarized electrons to polarize the light that semiconductor devices - such as LEDs - emit when an electron recombines with a positive hole.
The team achieved their feat at 100 kelvin by injecting the electrons into a flat portion of the semiconductor along a certain crystal plane. But they were surprised to find that this figure fell by a factor of six when the electrons were added to a nanometre-sized region with a different crystal orientation. "This demonstrates that defects at the interfaces of materials are extremely detrimental to injection efficiency", explains LaBella. The observation should also shed light on the underlying processes that govern spin polarization in electrons.