Crystal structures and phase diagram of Ni-Mn-Sn. The shadowed area in the phase diagram is the region where Ni-Mn-Sn, on cooling, first becomes ferromagnetic and later transforms martensitically. (A) The L21 structure showing also the relationship with the tetragonal unit cell. (B) Tetragonal (L10) unit cell. (C) The tetragonal unit cell viewed from top. (D) The martensitic structure constructed from nanotwinned variants of the L10 structure.