Giant barocaloric effect in all-$d$-metal Heusler shape memory alloys

We have studied the barocaloric properties associated with the martensitic transition of a shape memory Heusler alloy ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{31.5}{\mathrm{Ti}}_{18.5}$ which is composed of all-$d$-metal elements. The composition of the sample has been tailored to avoid long-range ferromagnetic order in both austenite and martensite. The lack of ferromagnetism results in a weak magnetic contribution to the total entropy change, thereby leading to a large transition entropy change. The combination of such a large entropy change and a relatively large volume change at the martensitic transition gives rise to giant barocaloric properties in this alloy. When compared to other shape memory Heusler alloys, our material exhibits values for adiabatic temperature and isothermal entropy changes significantly larger than values reported so far for this class of materials. Furthermore, our ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{31.5}{\mathrm{Ti}}_{18.5}$ also compares favorably to the best state-of-the-art magnetic barocaloric materials.

Figure 1

(a) Differential scanning calorimetry curves on cooling (bottom curve) and heating (upper curve) runs. (b) Temperature dependence of the magnetization measured under different applied magnetic fields. (c) Inverse of the magnetic susceptibility as a function of temperature (line is a linear fit to the data in the austenitic phase). (d) Isothermal magnetization as a function of magnetic field in austenite (green) and martensite (blue). Black solid line corresponds to the predicted Curie-Weiss behavior computed using $T_c$ and $\mu$ determined from susceptibility data.

Figure 2

(a) Diffraction patterns in the austenite (299 K, black line) and in the martensite (200 K, red line), with indexation of the peaks corresponding to the cubic (black labels) and orthorhombic (red labels) phases for austenite and martensite, respectively. Peaks from NaCl and MnO are also indicated. Isobaric evolution of selected diffraction patterns obtained on cooling (b) at atmospheric pressure and (c) under 9 kbar, respectively. (d) Normalized number of counts of the 220 peak of the cubic phase as a function of temperature for atmospheric pressure (black squares) and for 9 kbar (red circles). Lines are guides to the eye. For clarity only a typical error bar is shown for each run.

Figure 3

Calorimetric curves at selected values of applied hydrostatic pressure recorded during (a) heating and (b) cooling runs. Entropy (with respect to the entropy at $T_0=220$ K) as a function of temperature for selected values of applied hydrostatic pressure for (c) heating and (d) cooling runs.

Figure 4

(a) Transition temperature and (b) transition entropy change as a function of pressure. Blue symbols stand for the forward transition and red symbols stand for the reverse transition. Lines are linear fits to the data.

Figure 5

Isothermal entropy changes corresponding to the (a) removal and (b) application of selected values of hydrostatic pressure. Vertical dashed lines indicate the reversibility region. Adiabatic temperature changes corresponding to the (c) removal and (d) application of selected values of hydrostatic pressure.

Figure 6

(a) Maximum isothermal entropy change and (b) maximum adiabatic temperature change. Red symbols correspond to the reverse transition upon application of pressure and blue symbols correspond to the forward transition upon removal of pressure.

Figure 7

Isothermal entropy and adiabatic temperature changes for state-of-the-art magnetic barocaloric materials. For the sake of clarity each material is designated by an indicative label and not by its actual composition. All data are obtained from calorimetric measurements (quasidirect method) except ${\left|\mathrm{\Delta }T\right|}_{\max }$ for GdSiGe and LaFaSi which correspond to thermometric measurements. The minimum pressure required to obtain ${\left|\mathrm{\Delta }S\right|}_{\max }$ is indicated for each compound. Data correspond to the first application (or first removal) of pressure, and are extracted from Refs. [24, 25, 27, 28, 29, 30, 31, 32, 33, 34].