The spins of a layer of manganese atoms on a tungsten surface form a spiral pattern with a unique turning sense. Such 'chiral magnetic order' might exist in other, similar contexts, and could have many useful applications.
Objects that differ from their mirror image — human hands, for instance — have a turning sense. This phenomenon of handedness, or chirality, is found in many natural contexts, from the elementary particles participating in electroweak interactions, via organic molecules and hurricanes, all the way to galaxies. Solids with a magnetic order of unique chirality could have many useful practical applications, because their peculiar symmetry allows the mixing of electronic, optical, magnetic and structural properties. On page 190 of this issue, Bode et al.1present compelling evidence for chiral magnetic order in a strikingly simple solid-state system: a single layer of manganese atoms on a tungsten substrate.
The authors achieved this by combining highly sophisticated, spin-sensitive scanning tunnelling microscopy (STM) with an equally sophisticated first-principles calculation of the electronic structure of the manganese surface. The amount of electrical current tunnelling from the manganese sample to the authors' STM tip, which was coated with chromium or iron, depended on the electrons' direction of spin. What Bode et al. observed was a long-period, spiral-shaped magnetic modulation of the STM intensity distribution, superimposed on a basic antiferromagnetic structure (one in which adjacent spins point in opposite directions) (Fig. 1). When a magnetic field was applied, the pattern shifted in a given direction, identifying its unique chirality.
The pattern of the manganese spins is an example of what is known as a Dzyaloshinskii spiral, after the Soviet physicist Igor Dzyaloshinskii. In a pioneering effort some 40 years ago2, he showed that magnetic order can become twisted into long-period spirals in crystals lacking inversion symmetry, provided that the interaction between spin and orbital angular momentum (spin–orbit coupling) in the constituent atoms is large enough. Dzyaloshinskii's theory was for a long time regarded as an oddity, because such spiral magnetic states destroy the prized homogeneity of condensed-matter systems. But many ordered crystalline phases have since been found that do have long-range modulations in their structure that are of a non-integer periodicity compared with their underlying lattices3. Most prominently, a chiral modulated state provided by the helical twisting of cholesteric liquid crystals4 underpins modern display technologies. Dzyaloshinskii's magnetic spirals were the first simple example of a whole class of complex inhomogeneities in condensed matter. Ironically, however, it was only in 1980 that a magnetic Dzyaloshinskii spiral was first identified5, in manganese silicide.
The simplest systems lacking inversion symmetry are, in fact, not bulk materials with rare crystal structures, but the surfaces of solids. Surfaces offer a universal experimental setting without inversion symmetry, even for crystalline materials that otherwise have full inversion symmetry — such as the manganese and tungsten used by Bode and colleagues in their investigations1.
These authors' results imply that the magnetism of surface-dominated nanoscale objects will be strongly influenced by chiral interactions. Assessing the quantitative strength of these interactions is a challenge whose results could determine what novel applications the materials might have. In their first-principles electronic-structure calculations1, the authors establish that the chiral interactions in the atomic layer of manganese are boosted by the spin–orbit coupling of the tungsten substrate — a result in quantitative agreement with their experiment. Besides this, the agreement also nicely illustrates that first-principles electronic-structure calculations can now accurately predict complex magnetic effects6.
Mirror symmetry is also broken in so-called multiferroic materials, in which the coexistence of magnetic order and ferroelectric order means that the electronic, optical and magnetic properties of the material are interlinked. Spiral magnetic order is known to occur in these materials, but the identification and controlled exploitation of multiferroic effects in artificial nanoscale systems is still in its infancy (see ref. 7 for a review). The effect of surface-induced chiral interactions in thin layers, or at the interfaces of multilayers or granular heterostructures, adds a new twist to the complexity of these materials.
The importance of chiral interactions at the surfaces of magnets is further augmented by the use of spin-polarized electric currents to switch magnetic states in 'spintronic' devices. Spin-polarized currents can exert a torque on magnetic states that is formally related to the chiral spin–orbit coupling observed by Bode et al.1. This similarity will motivate the study of new classes of system, such as magnetic semiconductors whose chiral interactions are artificially boosted by the choice of substrate.
Finally, Bode and colleagues' results shed new light on unusual mesoscopic-scale magnetic textures. More than a decade ago, for example, it was shown theoretically that chiral interactions support metastable vortex-like excitations, so-called skyrmions8. These excitations are the smallest possible micromagnetic objects — just the size of a single magnetic-domain wall9 — and identifying them experimentally is a tough call for magnetic imaging techniques.
Now that we know that chiral interactions at surfaces can be very strong, many earlier results will have to be revisited. Chiral interactions might, for example, be responsible for a magnetic-superlattice modulation recently found in an iron monolayer on an iridium substrate10. These questions are more than an academic challenge: understanding and controlling the twists and turns of thin-film magnetic states could well be handy for new applications such as ultra-high-density magnetic recording media.
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Korean Journal of Materials Research (2019)
Physical Review B (2014)
Nature Physics (2011)
Physical Review B (2009)
Physik in unserer Zeit (2008)