Abstract
Magnetocaloric materials based on field-induced first order transformations such as Ni-Mn-Ga-Co are promising for more environmentally friendly cooling. Due to the underlying martensitic transformation, a large hysteresis can occur, which in turn reduces the efficiency of a cooling cycle. Here, we analyse the influence of the film microstructure on the thermal hysteresis and focus especially on large angle grain boundaries. We control the microstructure and grain boundary density by depositing films with local epitaxy on different substrates: Single crystalline MgO(0 0 1), MgO(1 1 0) and AlO(0 0 0 1). By combining local electron backscatter diffraction (EBSD) and global texture measurements with thermomagnetic measurements, we correlate a smaller hysteresis with the presence of grain boundaries. In films with grain boundaries, the hysteresis is decreased by about 30% compared to single crystalline films. Nevertheless, a large grain boundary density leads to a broadened transition. To explain this behaviour, we discuss the influence of grain boundaries on the martensitic transformation. While grain boundaries act as nucleation sites, they also lead to different strains in the material, which gives rise to various transition temperatures inside one film. We can show that a thoughtful design of the grain boundary microstructure is an important step to optimize the hysteresis.
Highlights
Solid state cooling with magnetocaloric materials is a promising way for more efficient and environmentally friendly cooling techniques [1]
Though our experiments clearly reveal a strong influence of the substrate on the transition temperatures, a detailed analysis of stress is beyond the scope of this paper
We presented results of the martensitic transition for epitaxial Ni-Mn-Ga-Co films grown on MgO(0 0 1), MgO(1 1 0) and Al2 O3 (0 0 0 1) substrates
Summary
Solid state cooling with magnetocaloric materials is a promising way for more efficient and environmentally friendly cooling techniques [1]. A major drawback of this first order phase transition is the hysteresis, which can reduce the efficiency of a cooling system drastically. To understand these transition processes in general and the origin of the occurring hysteresis in particular, epitaxial Ni-Mn-based thin films with a defined crystallographic relationship between film and substrate can help [3]. The high energy barrier for the martensite nucleation makes a large undercooling necessary and contributes significantly to the transformation hysteresis [3]. The hysteresis can be reduced by lowering the energy barriers for the formation of nuclei and their growth.
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