Magnetic frustration as a source of topologically non-trivial spin textures

Figure: a) Stacking of Mn2+ triangular layers along the [111] direction. b) The non-collinear spin configuration of Mn2+ can be divided into three sublattices as illustrated in the insets. c) Spin configurations in the same layers lead to cylinders of fractional skyrmions along [111], d) the wrapping of fractional skyrmions on one layer are described by two spheres with opposite winding directions (red and blue hexagons).

Antiferromagnetic (AFM) materials with non-collinear spin configurations and non-trivial topological features are presently in the focus of intense research. They have great potential as magnetic information carriers, reveal novel non-trivial states of matter and identify new underlying interaction mechanisms. We search for such materials within frustrated magnets with magnetic ions located on triangular or Kagome lattices by means of neutron scattering.

One celebrated example in an AFM spinel, MnSc2S4. The system was studied in the context of a spiral spin liquid [1] which undergoes a multi-step ordering below TN = 2.3 K. In zero magnetic field, the magnetic structure can be described with a single propagation vector k= (3/4 3/4 0) forming multiple domains. In magnetic field the helical structure transforms into a triple-k state, which is the first fractional antiferromagnetic skyrmion lattice (SkL) [2] (Figure).

Another potential candidate for non-trivial topology is a MGa2S4 family (M=Ni, Fe). A set of unconventional magnetic properties observed experimentally boosted a large number of theoretical work including the triangular lattice AFM –, the spin-nematic quadrupolar – and the Z2-vortex – models. For a certain set of exchange parameters a skyrmion crystal multi-k state was predicted. Recently [3] it was possible to unfold the magnetic properties, as originating from ideal and non-ideal structure of the MGa2S4 and the exploration of the ideal system is now tackled.

YbAgGe contains a magnetic geometrically frustrated kagome-like lattice of Yb3+ ions with significant local single-ion anisotropy. Its electronic state arises from hybridization of 4f and conduction electrons leading to heavy electronic masses. Entanglement of these interactions leads to non-trivial behavior under external magnetic field [4] and topologically non-trivial states.