November 07, 2022
Minneapolis, United States
Magneto structural coupling and skyrmions lattice phase field in the lacunar spinel GaMo4Se8
The prospect of controlling topologically protected spin textures such as skyrmions lattices in high-density race-track memories offers an alternative to conventional domain wall based spintronic devices (1, 2). A variety of skyrmion spin structures have been observed in non-centrosymmetric compounds and understanding the phase field stability and size of the magnetic texture is of fundamental importance to control the desired functionality in the materials.
The lacunar spinel family with the net formula AB4X8 (A = Al, Ga, Ge; B = V, Mo; X = S, Se) has gained much attention in this respect as several compounds crystallize in the non-centrosymmetric polar rhombohedral structure (R3m, C3v) at low temperature and are magnetically ordered, with skyrmion lattice observed in GaV4S8 (3), GaV4Se8 (4) and GaMo4S8 (5). So far only polycrystalline samples of GaMo4Se8 have been studied (6).
We have performed extensive structural and magnetic characterisation on both single and poly-crystals of GaMo4Se8. Using high resolution powder neutron diffraction we confirms the phase separation below the phase transition from cubic to mixture of rhombohedral and orthorhombic phases and a significant magneto-structural coupling. Among lacunar spinels, the phenomenon of phase separation is unique to GaMo4Se8 and likely to result from the high compressive strain relaxation through the formation of the minority phase (Fig. 1a). Our small angle neutron scattering study on single crystals reveals a cycloid spin structure with a period of 15.4 nm (Fig. 1b) and upon the application of a magnetic field along the polar axis, pattern compatible with a skyrmion lattice is observed (Fig. 1c). The temperature-magnetic field phase field is established and shows that the spin texture is preserved down to the lowest temperature and under a large magnetic field of 1 T.
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(5) A. Butykai et al., npj Quantum Materials, 2022, 7, 26
(6) E. C. Schueller et al., Phys. Rev. Materials, 2020, 4, 064402