Anomalous heat transfer near the martensite-austenite phase transition in Ni50Mn28Ga22-x(Cu, Zn)x (x = 0; 1.5) alloys

Abstract

The paper presents a detailed investigation of the thermal transport of the Ni50Mn28Ga22-x(Cu, Zn)x (x = 0; 1.5) alloys through specific heat, thermal diffusivity and thermal conductivity measurements. It is shown that the partial replacement of Ga atoms by Zn or Cu atoms leads to a convergence of magnetic and martensitic phase transition temperatures. Our measurements of the thermal diffusivity and thermal conductivity revealed that the behavior of these parameters in the phase transitions region is anomalous due to the coexistence of martensite-austenite phases and the effect of the latent heat of the phase transition. Moreover, the latent heat plays a crucial role in the appearance of a deep minimum of thermal diffusivity in the region of martensitic transformation for the Cu- and Zn-doped samples. Estimates of the phonon mean free path lph in martensitic and austenitic phase gave values of lph,mart = 1.5 nm and lph,aust = 1.9 nm, respectively. This implies that the main mechanism of phonon scattering is local structural inhomogeneities, which decrease sharply upon transition from martensite phase to the highly symmetric austenitic phase.