Skyrmions at Magnetic Multilayer Interfaces
In a conventional magnetic material, the spins on neighbouring atomic sites are collinear, due to the scalar product form of the Heisenberg exchange interaction. In principle, vector cross product interactions, called Dzyaloshinkii-Moriya interactions (DMI), are possible, which will attempt to align neighbouring spins at right angles. Most crystal structures have spatial inversion symmetry and under these circumstances DMI interactions at a given atomic site cancel exactly. However, there are lattices that lack inversion symmetry, and in that case chiral magnetic structures arise. Of particular interest are magnetic skyrmions, particle-like twists in the magnetization that have fascinating spintronic properties.They give rise to a so-called topological contribution to the Hall effect, and are very easily moved by the flow of spin-polarised electrons, suggesting the possibility of ultra-low power skyrmion-based spintronic devices such as racetrack memories.
Most materials that exhibits inversion symmetry in their bulk crystal lattice are only magnetic below room temperature. skyrmions at room temperature: the systems studied all require cryogenic temperatures to show magnetic ordering. However, inversion asymmetry also exists at any surface or interface of even a conventional magnetic material, and so a DMI ought also to be found there. Whilst skyrmion formation has been predicted in ultrathin (< 1 nm) magnetic films that have interfaces with heavy metals that give the required strong spin-orbit effects, there are as yet no experimental observations of these interfacial skyrmions. Making such an observation is the goal of this project. We have already observed chiral effects domain walls in systems such as Pt/Co/AlOx, suggesting the presence of a non-zero DMI. We will refine these systems to enhance the interfacial DMI and seek the presence of room-temperature skyrmions through a mixture of magnetotransport, magnetisation dynamics, and high resolution imaging experiments.
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