Abstrakt
Shape memory alloys are Heusler alloys in which we find interesting properties strongly dependent on crystall structure. Easy straining of these materials is allowed by magnetic shape memory and magnetically induced reorientation.
Magnetic field which is needed is often only <1 T. Martensitic transformation is the key concept for deep understanding of these efects.
Therefore, its study is necessary for future applications. Ni2MnGa is a model system where the above mentioned effects can be well observed.
The preparation of single-crystals is neccessary for the characterisation of these materials, especially by the x-ray diffraction [1]. The properties of Ni-Mn-Ga systems can be easily varied by the changes in composition.
Moreover, these changes can be very subtle. It has been discovered that the temperature of martensitic transformation can be shifted by 100 K [2] with the composition change from 50:25:25 to 50:28:22 (Ni:Mn:Ga).
Therefore, the careful preparation of single-crystals is necessary, when we want samples with a homogenous composition and microstructure. It will also play the role in the doping of samples (i.e.
In instead of Ga). In this contribution we will compare the samples prepared by Bridgman method and in floating zone.
The speeds of growth for which we obtained samples of the best quality differ drastically - the speed was 80 mm/h in floating zone and 0.5 mm/h in Bridgman method. The most substantive features of the prepared samples are grooves in the sample from Bridgman method which are caused by boron contamination from BN crucible used within preparation.
The sample prepared in floating zone contained composition inhomogeneities of the size about 80 micrometers which were eliminated with the annealing. The most important result is that all single-crystals prepared in floating zone exhibit MIR down to low temperatures. [1] Oleg Heczko, et al., Structure and microstructure of Ni-Mn-Ga single crystal exhibiting magnetic shape memory effect analysed by high resolution X-ray diffraction, Acta Materialia 115 (2016) 250-258 [2] Xiao Xu, et al., Magnetic properties and phase diagram of Ni50Mn50-xGax ferromagnetic shape memory alloys, Acta Materialia 61 (2013) 6712-6723