Depending on composition and chemical order, Ni-Mn-based Heusler alloys exhibit interesting functional properties, which render them useful for magnetic shape memory applications or as magnetocaloric materials. This is linked to the presence of hierarchically twinned modulated structures in martensite, which can be interpreted as adaptive, self-organized arrangement of [110]-aligned nanotwins consisting of non-modulated tetragonal building blocks as was shown previously for the paradigmatic case of stoichiometric Ni₂MnGa [1]. A band-Jahn-Teller-type reconstruction of the Fermi surface which in particular softens the [110] transversal acoustic phonons leads to a downhill transformation path from cubic austenite to nanotwinned martensite [2]. The twin interfaces are subject to competing repulsive and attractive interactions related to the frustrated antiferromagnetic coupling between neighboring Mn atoms [3].
Based on recent first-principles calculations in the framework of density functional theory, the present contribution explores the signatures of the interdependence of magnetism, chemical order and nanotwinning in Ni-Mn-based Heusler systems beyond Ni-Mn-Ga and their relevance for the functional properties. Particular emphasis will be made on off-stoichiometric compositions suitable for magnetocaloric purposes.

[1] R. Niemann, U. K. Rößler, M. E. Gruner, O. Heczko, L. Schultz, S. Fähler, Adv. Eng. Mater. 14 (2012), 562-581
[2] M. E. Gruner, S. Fähler, P. Entel, Phys. Stat. Sol. (b) 251 (2014), 2067-2079
[3] M. E. Gruner, R. Niemann, P. Entel, R. Pentcheva, U. K. Rössler, K. Nielsch, S. Fähler, Sci. Rep. 8 (2018), 8489.