A step forward in the electrical control of spin states

The solution to an important problem in spintronics research, found by an international collaboration including researchers at the LCN, has been published in the journal Nature Communications.

Spintronics is a multi-disciplinary research area that uses the quantum mechanical property of ‘spin’ that electrons and nuclei possess to create a new generation of computing technologies. Because spins behave like microscopic magnets, with a north and south pole, researchers aim to use spins to store and process information encoded in the spin state.

To create commercially viable spintronic memory devices, it is important to develop means to efficiently control spin states of magnets. Currently there is much interest in effects arising from the relativistic spin-orbit interaction which may meet this challenge. However, the two spin-orbit effects that have been observed so far and which have raised great interest, the spin-Hall effect and the interfacial inverse spin-galvanic effect, have been shown to be indistinguishable in their control of magnetic states.

In this paper, for the first time, these two effects have been observed and separated in the same material structure. The two effects allow electron spins to be manipulated by passing a conventional electric current though the material. The interaction of these spins with an adjacent magnetic layer can cause the magnetic north-south direction of this adjacent layer to flip, showing promise as a future magnetic random-access memory technology.

Conventionally, the two effects are measured in a bilayer consisting of a ferromagnetic metal and a paramagnetic metal. The spin-Hall effect, which creates a flow of spins into the ferromagnet, occurs throughout the paramagnet. In contrast, the inverse spin-galvanic effect is thought to create an accumulation of spins near to the interface of the two metals. However, the resulting forces on the ferromagnet due to the two effects are indistinguishable, as both forces can act in the same direction. This has frustrated efforts to understand the physics underlying the magnetic switching in these structures.

In this study, the researchers replaced the paramagnetic metal with a paramagnetic single-crystal of semiconductor GaAs. Unlike the metal paramagnets, the inverse spin-galvanic effect in the semiconductor paramagnet is created in a different way, by a property of the crystal structure instead of interfacially. The researchers showed that when current is passed in certain crystal directions, the force due to the spin-Hall effect is perpendicular to the force due to the inverse spin-galvanic effect. This allowed the researchers to separate the contributions of the two effects.

Lead author Tim Skinner, currently at the LCN, commented “Bilayer structures are an important prototype device for new spintronic memories. By understanding clearly the physical mechanisms behind the magnetic switching in these structures, we hope to design and optimise new materials for efficient, fast-access, non-volatile memories. This work demonstrates a material system in which we can study the separate contributions of these two related effects.”

Figure: Current passing through the paramagnetic semiconductor creates an accumulation of spins in one direction (inverse spin-galvanic effect). For a [100] or [010] current direction this results in a magnetic field acting parallel to the current (a) The current also generates a flow of spins into the ferromagnet, resulting in a magnetic field perpendicular to the current (b)

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