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VOLUME 100 | ISSUE 3 | PAGE 238
Hexagonal spin structure of A-phase in MnSi: densely packed skyrmion quasiparticles or two-dimensionally modulated spin superlattice?
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 {\bf k}_{h(1,2,3)} \perp \bf{H}. The directions and value of wave-vectors  kh(1,2,3) are well preserved over the whole crystal of the size of 100 mm3, but in the small room of the (H-T) phase diagram just below Tc = 29 K. The droplets of the orientationally disordered, presumably hexagonal, spin structure with {\bf k}_h \perp \bf{H} are observed in the wide range beyond the A-phase boundaries in the field range from B_{T1} \approx 0.1 T to B_{T2} \approx 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 kh is equal to the wavevector of the cone phase kc. 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.