Getting a grip on skyrmions

A new skyrmion state is discovered paving the way for magnetic memory applications.

Magnetic skyrmions – swirls of magnetic moments – have unique topological properties that make them promising candidates for the next generation of computer storage devices. So far, the long-range ordering of the skyrmion lattice phase prevented the materials from spatially breaking up into domains, which are the ultimate elements of magnetic memory bits.
 
In a recent resonant X-ray scattering study published in Nano Letters, scientists report a controlled way to create a multidomain skyrmion lattice state. This important discovery allows for the separation of these topologically-protected quasiparticles into smaller domains, ultimately permitting the local manipulation of skyrmions. This is an essential step towards skyrmion-based spintronics in data information and telecommunication technologies.

Information carrier

Modern information technology has a simple underlying concept, namely that of binary information where bits can have values of ‘1’ or ‘0’. The ideal material provides a carrier for information in the smallest possible size, allowing for the fastest processing speed with the best energy economy for information processing. Over the past decades, ferromagnetic materials climbed to the top and still are the most promising materials system for future spin-based electronics, or ‘spintronics’. In magnets, the binary information (‘1’ or ‘0’) is encoded by imposing the magnetisation to point either up or down. The fundamental reason why this concept works in ferromagnets is the presence of magnetic domains.

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.

“This is an exciting moment for the skyrmion community”, said Shilei Zhang, the lead scientist from Oxford. “For years we’ve been trying to find a way to adapt the skyrmion lattice phase to fit into the framework of spintronics memory devices. It would be impossible to use a millimetre-sized, long-range-ordered skyrmion lattice as a memory bit. However, breaking them up into smaller domains is the crucial first step before any skyrmion-based memory is to be devised. And now, we can not only create such a state, but also precisely manipulate the domains.”
 
To discuss potential applications for BLADE (beamline I10), please contact Principal Beamline Scientist Dr Paul Steadman: paul.steadman@diamond.ac.uk.
 
For more information on Diamond’s Magnetic Spectroscopy Group, please contact group leader Professor Gerrit van der Laan: gerrit.vanderlaan@diamond.ac.uk.
 
For more information on the research topic, please contact group leader Professor Thorsten Hesjedal: Thorsten.Hesjedal@physics.ox.ac.uk.
 
 

Related publication:

Zhang SL et al., Multidomain Skyrmion Lattice State in Cu2OSeO3. Nano Letters (2016) DOI:10.1021/acs.nanolett.6b00845