Condensed-matter physics

Taking control of spin currents

Conventional wisdom dictates that an electron's magnetic moment and momentum are strongly coupled only in materials made of heavy elements. An experiment demonstrates a striking counterexample. See Letter p.492

Spintronic devices promise enhanced performance over conventional electronics by simultaneously exploiting the flow of electric charge and of the magnetic moment (spin) in a material. Realizing this level of mastery requires the identification of materials in which each electron's spin and momentum are strongly coupled. Such materials are generally composed of heavy elements, for example gold or bismuth. But on page 492, Sunko et al.1 report surprisingly strong spin–momentum coupling in electrical pathways comprised of oxides of transition metals, such as cobalt, that lack heavy constituents. The authors explain the observed coupling as being due to symmetry properties of electronic states at the material's surface. In addition to its fundamental interest, this discovery could lead to strategies for designing material interfaces that will lie at the heart of tomorrow's spintronic devices.

When a wire is hooked up to the terminals of a battery, electrons flow through the wire. This represents a charge current, because all electrons are charged, but not a spin current, despite the fact that all electrons have spin. The net spin of the electrons is zero on a macroscopic scale because the spin of each electron is randomly oriented. However, a net spin alignment is possible, thanks to Einstein's special theory of relativity, which tells us that an electric field is transformed into a magnetic field in the reference frame of electrons travelling at sufficiently high velocities (close to the speed of light). The spins will align with this magnetic field, in a direction that depends on both the motion of the electrons and the orientation of the electric field through which they propagate.

If the electric field is generated by protons in atoms, the resulting interaction is known as atomic spin–orbit coupling (SOC). The magnitude of this interaction grows with the number of protons and is therefore largest in materials composed of heavy elements. However, SOC alone does not produce a net spin alignment in materials because the average electric field that arises from a periodic arrangement of atoms is zero on a macroscopic scale. A second material property is needed: inversion-symmetry breaking (ISB), whereby opposite orientations of a material can be distinguished. The combination of SOC and ISB produces the spin–momentum coupling required for spintronics applications.

How is inversion symmetry broken? Some crystal structures are naturally inversion-asymmetric, and the resulting spin–momentum coupling is called the Dresselhaus effect2. More generally, inversion symmetry is always broken at a material's surface, producing a spin–momentum coupling known as the Rashba effect3. Because both of these effects require SOC, they should be pronounced in materials composed of heavy elements. Accordingly, the first direct experimental discovery of the Rashba effect was in electronic states near the surface of gold4. However, the necessity for heavy elements should not be overstated, because the magnitude of ISB is equally important5,6.

This is the idea seized on by Sunko et al. to explain their discovery of a surprisingly large Rashba effect in the platinum cobalt oxide PtCoO2. In this material, the heavy element platinum has a passive role, because conduction takes place through a cobalt-related pathway. This fact is backed up by the authors' observation that the spin–momentum coupling increases dramatically when cobalt is replaced with the heavier transition metal rhodium.

How is Sunko and colleagues' discovery possible if cobalt is such a lightweight? The key turns out to be an unusual form of ISB. The structure of PtCoO2 terminates with a sandwich of oxygen–cobalt–oxygen layers at the surface (Fig. 1). Although the conduction electrons derive from cobalt atoms, they cannot jump directly between these atoms because the interatomic distance is too large. Instead, they flow along one of two zigzagging routes between cobalt and oxygen layers. For the electrons, these routes are not equivalent because they are at different distances from the surface. Imagine a two-lane road adjacent to a cliff — even if the lanes are identical, traffic might go more slowly in the lane closer to the edge because drivers are distracted by the view. The substantial asymmetry of the electrons' conduction pathways is a dramatic manifestation of ISB.

Figure 1: Electron pathways that break inversion symmetry.
figure1

The crystal structure of the platinum cobalt oxide PtCoO2 comprises layers of platinum, cobalt and oxygen atoms. At the surface, the structure is terminated by a sandwich of oxygen–cobalt–oxygen layers. Conduction electrons (e) derive from the cobalt atoms, but cannot jump directly between these atoms because the interatomic distance is too large. Instead, they take one of two zigzagging routes (dotted arrows) between cobalt and oxygen layers. Although these two pathways look similar, they have different environments: the lower oxygen layer is directly above a platinum layer, and the upper oxygen layer has nothing but vacuum above it. Consequently, there is a pronounced difference in the electron occupation of the two conduction pathways, breaking what is known as inversion symmetry. Sunko et al.1 suggest that this asymmetry generates strong coupling between the magnetic moment (spin) and the momentum of each electron that flows along the two pathways — a requirement for spin-based devices that offer improved performance over conventional electronics. Solid lines connect neighbouring atoms in the same layer.

According to the authors, in ordinary materials, spin–momentum coupling is typically limited by the smaller energy scale of ISB with respect to that of SOC. Increasing the atomic weight of the material, and consequently the magnitude of SOC, therefore provides diminishing returns. By contrast, the unusually large magnitude of ISB in PtCoO2 unlocks the full atomic SOC of cobalt — which is not so weak, after all, contrary to conventional wisdom.

Sunko and colleagues' study draws a striking contrast between two distinct regimes of the Rashba effect that are characterized by the relative magnitudes of SOC and ISB. Moreover, it is unusual to observe SOC effects in d-electron systems such as PtCoO2. In these systems, the localized nature of the electrons makes them susceptible to electron correlation effects — leading to phenomena such as high-temperature superconductivity in other transition-metal oxides7,8,9. The coexistence of the Rashba effect and correlation effects in PtCoO2 makes the material a fascinating playground for exploring new, potentially exotic physics.

On a more practical level, it remains to be seen what role PtCoO2 might have for spintronics applications. One challenge is that the Rashba effect occurs only for electrons near a material's surface. These states survive only under stringent vacuum conditions, making them unsuitable for real-world devices. Moreover, electrons in the bulk of the material are highly conductive and would overwhelm the comparatively small contribution to conduction provided by surface electrons. Nevertheless, the unusual form of ISB in PtCoO2 suggests design principles for optimizing the Rashba effect. One promising route would be to use modern fabrication techniques to engineer highly asymmetric interfaces on the nanoscale, which could enable unprecedented control over spin currents.Footnote 1

Notes

  1. 1.

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Correspondence to Zhi-Xun Shen or Jonathan Sobota.

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Shen, Z., Sobota, J. Taking control of spin currents. Nature 549, 464–465 (2017). https://doi.org/10.1038/549464a

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