The crystal structure and phase transitions of the magnetic shape memorycompound Ni2MnGa

Abstract

High resolution neutron powder diffraction and single crystal measurements onthe ferromagnetic shape memory compound Ni2MnGa have been carried out.They enabled the sequence of transformations which take place when theunstressed, stoichiometric compound is cooled from 400 to 20 K to be established.For the first time the crystallographic structure of each of the phaseswhich occur has been determined. At 400 K the compound has the cubicL21structure, and orders ferromagnetically at TC ≈ 365 K. On coolingbelow ∼ 260K a super-structure, characterized by tripling of the repeat in one of the ⟨110⟩cubicdirections, forms. This phase, known as the pre-martensitic phase,persists down to the structural phase transition at TM ≈ 200K and can be described by an orthorhombic unit cell with latticeparameters aortho = 1/√2acubic,bortho = 3/√2acubic, cortho = acubic andspace group Pnnm.Below TMthe compound has a related orthorhombic super-cell withbortho ≈ 7/√2acubic, whichcan be described within the same space group. The new modulation appears abruptlyat TMand remains stable down to at least 20 K.