Influencing Martensitic Transition in Epitaxial Ni-Mn-Ga-Co Films with Large Angle Grain Boundaries

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 Al2O3(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.

Integration of Multifunctional Epitaxial (Magnetic) Shape Memory Films in Silicon Microtechnology

“…For optimum properties, NiTi requires different deposition and heat treatment conditions compared to Ni–Mn–Ga, as described in the Experimental Section. However, we could directly use the growth parameters optimized for MgO substrates [ 40 ] for the growth on STO/SOI, which illustrates that our approach requires a minimum adaption of existing processes. The 500 nm thick film, depicted in Figure , is a) epitaxial as evidenced by the (110) pole figure measurement, b) having a twinned martensitic microstructure.…”

“…The epitaxial NiTi film was grown by sputter deposition, as described in. [ 40 ] A 50 nm thick Cr buffer layer was deposited on the STO/SOI substrate at a deposition temperature of 500 °C, followed by the 500 nm thick NiTi film deposited at 525 °C. The 4″ NiTi target had a composition of Ni 48.6 Ti 51.4 .…”

“…Furthermore, multifunctional Heusler alloys follow the concept “the material is the machine,” [ 9 ] and thus are ideally suited for miniaturization since no additional parts like levers and valves are required, which would complicate microfabrication substantially. The required epitaxial growth has been achieved on various single crystal oxidic substrates like Al 2 O 3 , [ 10,11 ] SrTiO 3 , [ 12 ] and MgO, [ 13 ] as well as other materials like NaCl [ 14 ] and GaAs. [ 15 ] However, these substrates are both expensive and incompatible with standard silicon microtechnology, which hampered the application of multifunctional Heusler alloys within microsystems up to now.…”

Giant Vertical Magnetization Shift Caused by Field-Induced Ferromagnetic Spin Reconfiguration in Ni50Mn36Ga14 Alloy

“…Ni-Mn-Z (Z = Ga, Sn, In, Sb) Heusler alloys have attracted much attention and have been extensively studied in the past decade [19][20][21], because off-stoichiometric Ni-Mn-Z Heusler alloys contain two mixed phases: (i) the conventional interaction of Mn atoms produces ferromagnetism; (ii) an additional interaction of the Mn atoms produces antiferromagnetism. In some regions, the 2 of 12 antiferromagnetic (AFM) interaction becomes stronger than the ferromagnetic (FM) interaction, thereby producing AFM regions, while in other regions, an SG state will occur due to the interaction of the two types of magnetic states at low temperatures [22,23].…”