Getting a grip on skyrmions

Information carrier

Figure 1: Magnetisation configuration of an individual skyrmion showing the storage of binary information.

Crystalline magnetic skyrmions are one of the most promising replacements for conventional ferromagnetic memory, which is prone to the ‘Alzheimer’s disease’ of information technology, caused by the so-called superparamagnetic effect, at small length scales of the magnetic bits.

In the topological scheme, ‘1’ and ‘0’ are encoded by the topological charge, which allows for spintronics memories with much higher storage densities, higher stability, as well as a greatly reduced energy cost for the bit manipulation (see Fig. 1). However, crystalline skyrmions normally form a single-domain, long-range-ordered state. The correlation length even reaches the scale of millimetres. This prevents the spatial separation of the state into useful bits. In other words, breaking up the single-domain skyrmion lattice into domains is a fundamental issue that has to be solved before taking skyrmion-based applications to the next level.

Characterising the magnetic structures

Recently, a team of scientists from the University of Oxford, Technische Universität München, Technische Universität Dresden, Ecole Polytechnique Fédérale de Lausanne, and Diamond Light Source carried out a study on skyrmion domains in the magnetoelectric material copper oxoselenite, Cu₂OSeO₃. This material is particularly promising for two reasons. First, it is a ferroelectric material, meaning that a net electrical polarisation can be induced by the magnetic order, and vice versa. Second, the magnetic anisotropy is very delicately balanced in this system, which can weaken the orientation in which the skyrmion lattice is locked.

The greatest challenge for the team was the characterisation of the magnetic structures. An individual skyrmion vortex has the diameter of around 60 nm. Such a small length scale requires high spatial resolution, so that most of the magnetic microscopy techniques are unsuitable. Therefore, the team choose Resonant Elastic X-ray Scattering (REXS) to characterise the domain structures. REXS has unique advantages for the study of skyrmions: it is element-selective, has a very high sensitivity for magnetic order, provides a variable sampling area, and allows for fast and efficient data acquisition. An experiment like this requires an intense soft X-ray source as the magnetic signal is very weak, with a well-controlled low temperature and flexible external magnetic field in ultrahigh vacuum.

Figure 2: a) REXS setup and single-crystal x-ray diffraction; b) Real-space multidomain skyrmion state; c) Reciprocal space REXS pattern.

Discovering the multidomain lattice state

This is why the scientists picked Diamond’s I10 beamline, known as BLADE (Beamline for Advanced Dichroism Experiments), as these challenging conditions can only be met using I10’s RASOR diffractometer (Reflectivity and Advanced Scattering from Ordered Regimes) (Fig. 2a). With RASOR’s ultra-high resolution, they have successively characterised all magnetic states in Cu₂OSeO₃, including the helical, conical, and skyrmion phase. Most significantly, they discovered the existence of a multidomain skyrmion lattice state – a new form of skyrmion order. This state reveals itself in the form of a necklace-like magnetic diffraction pattern, as shown in the figure. Moreover, the scientists found a controlled way to create the skyrmion lattice domain state, as well as manipulating the domain size and distribution.