Abstract
Magnetic shape memory Heuslers have a great potential for their exploitation in next-generation cooling devices and actuating systems, due to their “giant” caloric and thermo/magnetomechanical effects arising from the combination of magnetic order and a martensitic transition. Thermal hysteresis, broad transition range, and twinning stress are among the major obstacles preventing the full exploitation of these materials in applications. Using Ni-Mn-Ga seven-modulated epitaxial thin films as a model system, we investigated the possible links between the phase transition and the details of the twin variants configuration in the martensitic phase. We explored the crystallographic relations between the martensitic variants from the atomic-scale to the micro-scale through high-resolution techniques and combined this information with the direct observation of the evolution of martensitic twin variants vs. temperature. Based on our multiscale investigation, we propose a route for the martensitic phase transition, in which the interfaces between different colonies of twins play the major role of initiators for both the forward and reverse phase transition. Linking the martensitic transition to the martensitic configuration sheds light onto the possible mechanisms influencing the transition and paves the way towards microstructure engineering for the full exploitation of shape memory Heuslers in different applications.
Highlights
Magnetic shape memory Heuslers provide new concepts for magnetic field-driven actuation [1,2], energy harvesting [3], and solid-state cooling technology [4] thanks to the magnetostructural martensitic phase transition and the giant magnetic field-induced strain, which has been reported in bulk single crystals as orders of magnitude higher than piezoelectric and magnetostrictive counterparts [5,6].Ni-Mn-Ga films are a model system for magnetic shape memory materials
By exploiting epitaxial growth on different substrates, suitable growth conditions, and geometrical parameters high quality and suitably oriented epitaxial films can be obtained. They allow accurate studies of structure and magnetism at the different length scales [7,8,9]. Despite their different martensitic configuration with respect to bulk materials, thin films provide a suitable platform to study the role of intrinsic and extrinsic properties in the martensitic transformation process and formulate models that can be extended to bulk materials [10,11]
In situ atomic force microscopy imagining versus temperature were measured by Dimension 3100 equipped with Nanoscope Veeco controller (Veeco Dimension 3100, CA, USA) using
Summary
Magnetic shape memory Heuslers provide new concepts for magnetic field-driven actuation [1,2], energy harvesting [3], and solid-state cooling technology [4] thanks to the magnetostructural martensitic phase transition and the giant magnetic field-induced strain, which has been reported in bulk single crystals as orders of magnitude higher than piezoelectric and magnetostrictive counterparts [5,6]. By exploiting epitaxial growth on different substrates, suitable growth conditions (including stress), and geometrical parameters (e.g., thickness) high quality and suitably oriented epitaxial films can be obtained. They allow accurate studies of structure and magnetism at the different length scales [7,8,9]. In order to find possible solutions to overcome these obstacles, it is necessary to gain a comprehensive view of the configuration of the twin variants at the different length scales and its evolution upon martensitic phase transition. We propose a route for the martensitic forward and reverse transitions of the films, highlighting the major role played by the different martensitic interfaces
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