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

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 ${\mathbf k}_{h(1,2,3)}\perp\mathbf{H}$. The directions and value of wave-vectors ${\mathbf k}_{h(1,2,3)}$ 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 ${\mathbf k}_h\perp\mathbf{H}$ 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.