Giant magnetocaloric effect from reverse martensitic transformation in Ni-Mn-Ga-Cu ferromagnetic shape memory alloys

TL;DR

This study demonstrates a giant magnetocaloric effect near room temperature in Ni-Mn-Ga-Cu alloys by fine-tuning the transformation and Curie temperatures through Cu substitution, achieving significant entropy change and refrigerant capacity.

Contribution

First demonstration of giant magnetocaloric effect in Ni-Mn-Ga-Cu alloys near room temperature via reverse martensitic transformation with Cu tuning.

Findings

Achieved delta-SM of -81.75 J/Kg-K at 302.5 K for 9 T field change.

Maximum refrigerant capacity of 327.01 J/Kg in heating cycle.

Cu addition tunes transformation temperatures, optimizing magnetocaloric properties.

Abstract

In an effort to produce Giant Magnetocaloric effect (GMCE) near room temperature, in a first ever such study, the austenite transformation temperature (As) was fine tuned to ferromagnetic Curie temperature (TC) in Ferromagnetic Shape Memory Alloys (FSMA) and a large GMCE of delta-SM = - 81.75 J/Kg-K was achieved in Ni50Mn18.5Cu6.5Ga25 alloy during reverse martensitic transformation (heating cycle) for a magnetic field change of 9 T at 302.5 K. Fine tuning of As with TC was achieved by Cu substitution in Ni50Mn25-xCuxGa25 (0< x < 7.0)-based FSMAs. Characterizations of these alloys were carried out using Optical and Scanning Electron Microscopy, X-ray Diffraction (XRD), Differential Scanning Calorimetry (DSC) and DC magnetization measurements. Addition of Cu to stoichiometric Heusler type Ni2MnGa increases the martensitic transformation temperatures and decreases TC. Concurrently, DSM increases with Cu addition and peaks at 6.5 at. % Cu for which there is a virtual overlap between TC and As. Maximum Refrigerant Capacity (RCP) of 327.01 J/Kg was also achieved in the heating cycle for 9 T field change at 302.5 K. Corresponding values for the cooling cycle measurements (measured during forward transformation) were 30.4 J/Kg-K and 123.52 J/Kg respectively for the same 6.5 at. % Cu sample and same thermo-magnetic conditions.