Modulated phases occur in numerous functional materials like giant ferroelectrics and magnetic shape memory alloys. To understand the origin of these phases, we review and generalize the concept of adaptive martensite. As a starting point, we investigate the coexistence of austenite, adaptive 14M phase and tetragonal martensite in Ni-Mn-Ga magnetic shape memory alloy epitaxial films. The modulated martensite can be constructed from nanotwinned variants of a tetragonal martensite phase. By combining the concept of adaptive martensite with branching of twin variants, we can explain key features of modulated phases from a microscopic view. This includes phase stability, the sequence of 6M-10M-NM intermartensitic transitions, and magnetocrystalline anisotropy.DOI:

No abstract

No abstract

No abstract

No abstract

Diffusionless phase transitions are at the core of the multifunctionality of (magnetic) shape memory alloys, ferroelectrics and multiferroics. Giant strain effects under external fields are obtained in low symmetric modulated martensitic phases. We outline the origin of modulated phases, their connection with tetragonal martensite and consequences for their functional properties by analysing the martensitic microstructure of epitaxial Ni-Mn-Ga films from the atomic to macroscale.Geometrical constraints at an austenite-martensite phase boundary act down to the atomic scale. Hence a martensitic microstructure of nanotwinned tetragonal martensite can form. Coarsening of twin variants can reduce twin boundary energy, a process we could observe from the atomic to the millimetre scale. Coarsening is a fractal process, proceeding in discrete steps by doubling twin periodicity. The collective defect energy results in a substantial hysteresis, which allows retaining modulated martensite as a metastable phase at room temperature.In this metastable state elastic energy is released by the formation of a 'twins within twins' microstructure which can be observed from the nanometre to millimetre scale. This hierarchical twinning results in mesoscopic twin boundaries. Our analysis indicates that mesoscopic boundaries are broad and diffuse, in contrast to the common atomically sharp twin boundaries of tetragonal martensite. We suggest that observed extraordinarily high mobility of such mesoscopic twin boundaries originates from their diffuse nature which renders pinning by atomistic point defects ineffective.

No abstract

We study the temperature dependence of the magnetic anisotropy of three different martensites known to exist in the Ni–Mn–Ga alloys. The anisotropy constants were determined from magnetization curves measured at different temperatures. The anisotropy of five-layered modulated tetragonal martensite is uniaxial with easy magnetization direction along short crystallographic axis. At room temperature K1(rt)=1.65×105 J m−3 and K2 is negligible. Seven-layered modulated orthorhombic martensite exhibits easy magnetization direction along the shortest crystallographic axis. K1(rt)=1.7×105 J m−3 and K2(rt)=0.9×105 J m−3 referring to hard and mid-hard magnetization axes. Nonmodulated tetragonal martensite possesses a uniaxial anisotropy with easy plane and hard magnetization direction along the long crystallographic axis with K1(rt)=−2.3×105 J m−3 and K2(rt)=0.55×105 J m−3. The temperature dependence of K1(T) of five-layered martensite follows magnetization power law with exponent n=3 suggesting a single ion origin of the magnetic anisotropy in Ni–Mn–Ga martensite.