Intriguing spin quasiparticles discovered

Merons are a type of topological spin texture, with relevance for both fundamental and technological problems. In this theoretical work, scientists show that a new family of van der Waals (vdW) CrCl3 ferromagnet can host merons and anti-merons, and goes on to explore their dynamics and interactions.

Magnetic moments in magnetic materials can organise themselves in different forms of spin structures. From usual ferromagnetic and anti-ferromagnetic materials where spins of neighbouring atoms tend to align parallel or antiparallel to each other, respectively, up to more intricate configurations where spins can assume complex orientations composing a quasiparticle on their own (Figure 1). 

Such spin quasiparticles hold promises in several aspects of information technologies from cheaper, lighter devices, up to ultra-compact spin-based electronic (spintronics). In particular, these quasiparticles offer energy-efficient current-driven phenomena useful in the next generation of low-energy device applications (i.e. computer memory, logic, etc.). Despite their importance, direct observation of spin textures, such as merons and anti-merons, is rare and has been limited to femtosecond transient states, and a few complex chiral magnets. On a recent paper led by the School’s Dr Elton Santos published in Nature Communications, and featured at the Editor’s Highlight section, reports evidence that recently exfoliated vdW CrCl3 hosts merons and antimerons in its magnetic structure. This finding may revolutionise how to integrate fundamental spin-textures in two-dimensional device platforms. 

The concept of merons and antimerons originated in classical field theory dated from the 1970s, and later applied to particle physics, and more recently in condensed matter physics. In merons and antimerons, which are also called vortex and antivortex, the spins at the core region point either up or down, but those around the perimeter align parallel to the plane of the material with a gradual variation of the orientation of the spins as they move towards or backwards the centre (Figure 1).  Some studies have confirmed the existence of merons and antimerons in chiral magnets, but their complex chemistry and synthesis require additional protocols that ramp up further progress. With the discovery of meron quasiparticles in a relatively simpler vdW layered material, it opens the prospect of explorations of a number of other 2D magnets where such spin textures could be observed. Indeed, the researchers also give some guidelines for looking into materials that may develop topological non-trivial spin textures such as weak out-of-plane anisotropy, and potential competitions between next-nearest neighbour interactions. With the discovery of graphene in 2004 via a handy scotch-tape exfoliation technique, different nanosheets have been isolated up to date. This has generated a broad and fast-pace increased library of 2D compounds for applications. Similar methods have been applied to layered magnetic materials which have contributed to rapidly popularise the field.

The multiscale theoretical methods developed also provide a general framework for a rapid computational screening ahead of lab experimentation on compounds that may stabilise merons and antimerons. The authors unveil the complete evolution of these spin quasiparticles from pair creation, their subsequent motion over magnetic domains (Figure 2), and annihilation via collisions. These results push the boundary to what is currently known about the most concise and fundamental localized spin structures in 2D magnets and open exciting opportunities to explore magnetic domain control via topological spin textures.