Hexagonal spin structure of A-phase in MnSi: densely packed skyrmion quasiparticles or two-dimensionally modulated spin superlattice?

Authors:
S. V. Grigoriev, N. M. Potapova, E. V. Moskvin, V. A. Dyadkin, Ch. Dewhurst, S. V. Maleyev
The year of the publication:
2014
Journal:
JETP Letters N 3 vol. 100 238-243
Abstract:

We have studied in detail the A-phase region in the field-temperature (H−T ) phase diagram of the cubic helimagnet MnSi using small angle neutron diffraction. The A-phase revealed itself as a two-dimensional hexagonal pattern of Bragg spots with k h(1,2,3) ⊥ H . The directions and value of wave-vectors k h(1,2,3) are well preserved over the whole crystal of the size of 100 mm 3 , but in the small room of the (H−T ) phase diagram just below T c = 29 K. The droplets of the orientationally disordered, presumably hexagonal, spin structure with k h ⊥ H are observed in the wide range beyond the A-phase boundaries in the field range from B T 1 ≈ 0.1 T to B T 2 ≈ 0.25 T at temperatures down to 15 K. No melting of these droplets into individual randomly located skyrmions is observed for all temperatures and magnetic fields. The wavevector of two dimensional modulations k h is equal to the wavevector of the cone phase k c . We conclude that observable is a two dimensionally modulated hexagonal spin superlattice built on the same competion of interactions (ferromagnetic exchange and Dzyaloshinsky–Moriya interaction) similar to a case of one-dimensionally modulated simple spin spiral.

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