JETP Letters: issues online

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 k_h(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 T_c = 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 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.