Self-thermometry measurements of the adiabatic temperature change in first-order phase transition magnetocaloric materials

Abstract

Accurately measuring the magnetocaloric effect is necessary to foster the development of magnetic refrigeration devices. However, current methods are inconvenient, requiring different instruments to measure each individual property or a custom-made setup. By measuring the time-varying magnetization in a commercially available VersaLab\textsuperscript{\textregistered} PPMS\textsuperscript{\textregistered} from Quantum Design, we have determined the adiabatic temperature change ($Δ$T$_{\textrm{ad}}$) of the first-order phase transition material Gd$_5$Si$_2$Ge$_2$, for a magnetic field change of 0 to 1 T, under high vacuum ($<$ 0.1 mTorr). For each temperature and magnetic field, the equilibrium magnetization is used as the magnetization-to-temperature conversion curve, allowing us to extend the validity of a previously proposed technique to the first-order phase transition material Gd$_5$Si$_2$Ge$_2$, which exhibits significant hysteresis. Our method thus enables full characterization (magnetic entropy change, adiabatic temperature change, and heat capacity) of any magnetocaloric material, whether it has a first-order or a second-order phase transition, using a single instrument. Comparing to a directly measured $Δ$T$_{\textrm{ad}}$, our method resulted in a peak $Δ$T$_{\textrm{ad}}$ value of 4.47 K, within 1\% of the directly measured value for a sample of the same composition.

Figures (4)

• Figure 1: Schematic representation of the behavior of a magnetocaloric material during a magnetic field application. Figure (a) shows the magnetic field profile and respective magnetization variation in time, and Figure (b) shows the correspondent temperature behavior.
• Figure 2: (a) Magnetization relaxations after a magnetic field application of 1 T, at T = 268 K, 269 K and 270 K initial temperatures, and respective equilibrium magnetizations. (b) Directly measured magnetization variation with temperature in cooling and in heating for a constant magnetic field of 1 T, and constructed M$_{\textrm{eq}}$(T) curve from magnetization relaxation data with respective cubic interpolation. The arrows near the directly measured curves indicate the direction of temperature variation during measurement.
• Figure 3: (a) Magnetization relaxation after a magnetic field of 1 T is applied and kept constant at a T = 268 K initial temperature. (b) Equilibrium magnetization measurements and measured magnetization variation with temperature under heating and for a constant magnetic field of 1 T. In (a) the relaxation amplitude, $\Delta$M, is schematically shown and its respective conversion to temperature shown in (b). Note how different conversion curves can lead to quite different $\Delta$T$_{\textrm{ad}}$ values.
• Figure 4: Results of $\Delta$T$_{\textrm{ad}}$ using the proposed method for field applications from 0 to 1 T. The different curves correspond to the different conversion curves used. A reference curve of direct measurements made with a thermocouple in a sample of the same composition is shown.