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Regensburg 2010 – wissenschaftliches Programm

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KR: Fachverband Kristallographie

KR 10: Poster: Multiferroics (with MA, DF, KR, DS)

KR 10.60: Poster

Dienstag, 23. März 2010, 10:45–13:45, Poster A

Phenomenology of the magnetic shape memory effect in modulated and non-modulated Ni-Mn-Ga and FePd alloys — •Aristide T. Onisan and Ulrich K. Rößler — IFW Dresden

Large magnetic shape memory effects in ferromagnetic martensites are observed only in modulated phases, but recently such effects are also demonstrated in the non-modulated (NM) phase of Ni-Mn-Ga with tetragonal crystal structure and c/a > 1. The modulated structures have been identified with adaptive, ultra-finely twinned martensite structures of the same tetragonal structure [1]. We develop a phenomenological theory of magnetic martensites based on geometric continuum theory of martensites, linear elasticity, and micromagnetism [2]. A cubic to tetragonal martensitic transition underlies the twinned microstructures, and magnetic anisotropy is modelled by easy-axis or easy-plane uniaxial anisotropy with four-fold in-plane anisotropy. Equilibrium phase diagrams for the distribution of crystallographic variants and magnetic domains are calculated in dependence on external magnetic fields and stresses. Applications are presented for the easy-axis system with materials parameters for 5M-type Ni2MnGa, and for FePd and NM Ni2MnGa as easy-plane systems. Modulated phases like 5M are constructed by second-order twinning within the concept of adaptive martensites [1]. Their magnetic properties depend on the relation between magnetic exchange length and modulation period.
[1] S. Kaufmann et al., arXiv:0906.5365. [2] A.N. Bogdanov, A. DeSimone, S. Müller, U.K. Rößler, J. Magn. Magn. Mater. 261 (2003) 204-209. Supported by DFG, SPP 1239.

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