Characteristic length scales of Heusler alloy films, including film thickness and grain size, affect the transformation hysteresis by altering internal energy barriers to interphase boundary motion. Previous studies have illustrated strong film thickness effects on transformation temperatures ofnanoscale TiNiCu thin filmwith film thickness < 100 nm, and that stress hysteresis and temperature hysteresis of CuAlNi microwires increased with decreasing wire diameter with diameter < 100 μm. However, length-scale dependent hysteresis has yet to be determined for other classes of caloric materials, including Heusler alloys. Understanding such transformation behavior at small length scales is critical for microelectronic and micromechanical applications, for promoting rapid heat transfer through caloric alloy thin films and thin wires, in strain-coupled magnetoelectric composites, and in microstructured multifunctional composites and foams.
By annealing electrochemically deposited multi-layer monatomic (Ni, Mn, Sn) films, Ni0.5Mn0.386Sn0.114 Heusler alloy films with decreasing thicknesses, 14.5, 8.7, and 2.9 μm, were synthesized. Phase transformation temperatures and sizes of nearly four hundred grains on each film were collected optically while the samples were heated or cooled. These data showed that the average grain areas/volumes decrease with decreasing film thicknesses. For grains within a single film (constant thickness), there is no statistically significant correlation between grain area or volume and hysteresis width. At the same time, film hysteresis increases with decreasing film thicknesses (from 4.9 oC at 14.5 μm to 15.7 oC at 2.9 μm). Previously, Chen and Schuh (2011) attributed the size effect in the hysteresis of small CuAlNi alloy microwires to the enhanced internal frictional work during transformation, associated with an increase in surface area and volume ratio. Our thickness dependent size effects could be similarly explained by internal friction-induced energy dissipation, whereby the thinner the film is, the stronger the interactions between interphase boundaries and the film-substrate interface, and the more energy is dissipated by frictional work. A power law model is fit to hysteresis width-film thickness data, and is used to elucidate scaling relationships which govern size dependence of hysteresis in Heusler alloy thin films, which are compared against previously observed size-dependent hysteresis in other thermoelastic martensitic transformations.