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MA: Fachverband Magnetismus
MA 10: Poster I
MA 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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