How Spin and Current Cross Paths
University of Michigan Physics graduate student Benjamin Norman, Professor Vanessa Sih, and their research team have discovered a surprising feature of the recently demonstrated phenomenon that an electric current can align electron spins. This effect was unexpected because large magnetic fields are usually required to polarize electron spins. However, this phenomenon can be understood in the context of special relativity, where a moving electron experiences an electric field as an effective magnetic field. Therefore, it is generally accepted that this effect should scale with the effective magnetic field, yet all research to date has found no clear trend. In a paper recently published in Physical Review Letters, the Michigan team present data that clearly contradicts this prediction, showing that, counter-intuitively, spins align more efficiently in weaker effective magnetic fields.
While today’s electronics mainly rely on manipulating electric charge, the binary nature of the electron spin makes it a promising candidate for representing information. This field of study is known as spintronics. Similar to a compass needle, electron spins are expected to point along the direction of any nearby magnetic fields. In non-magnetic materials, spins are not typically aligned, and one way to align spins is to apply a large magnetic field, but this is not practical for electronics. Therefore, an electrical method for aligning spins is desirable.
The research team investigated the nature of electrical spin alignment using a sample designed so that the magnitude and direction of the in-plane current could be varied and compared the measured electrical spin alignment efficiency with the effective magnetic field. The results contradict theoretical models for how current-induced spin alignment should depend on the effective magnetic field and therefore new models to explain the phenomenon are required. This work has high relevance for the recent effort to learn more about the control of electron spin.
The team’s research is described in the February 5, 2014 issue of Physical Review Letters.