Complete thermodynamic characterization of second-order phase transition magnetocaloric materials exclusively through magnetometry

Abstract

To fully assess and improve the performance of near room-temperature magnetic refrigerants, it is crucial to measure and compare their three most relevant thermodynamic properties, and their dependence on temperature (T) and applied magnetic field (H): the isothermal entropy change, ∆Siso(T,H); the adiabatic temperature change, ∆Tad(T,H); and the heat capacity, Cp(T,H). Typically, each thermodynamic property requires its own specialized measurements, which are very time-demanding and can be challenging to setup. In this work, we report a complete thermomagnetic characterization of a benchmark magnetocaloric material, gadolinium, using a single and commercially available SQUID magnetometer. By improving a recently reported method with incremental field ramping steps for measuring temperature through magnetization, we obtained a ∆Tad(T) curve under a 1 T field change with its peak amplitude and maximizing temperature respectively within 2 % and 0.8 % of previously reported values for gadolinium. We were also able to estimate the temperature dependent heat capacity, Cp(T,μ0H=0.85T), using the ∆Tad(T) measurements from magnetometry combined with magnetization versus temperature curves at different field values. This estimate of Cp around the transition temperature of gadolinium is within a relative error of 11 % of the experimental and reported values. The reported methodology allows the complete characterization of a second-order magnetocaloric material (∆Siso(T,H), ∆Tad(T,H), Cp(T,H)) around its Curie temperature using a single and widely available device, which can accelerate studies of different magnetocaloric materials’ performance, and approximate their implementation in magnetic refrigeration and/or waste heat energy harvesting industries.