Calorimetric study of the giant magnetocaloric effect in (MnNiSi)0.56(FeNiGe)0.44

Abstract

MnNiSi) 0.56 (FeNiGe) 0.44 belongs to a new family of alloys, similar to MnAs, showing a magnetostructural first-order transition near room temperature with large latent heat and magnetocaloric effect (MCE). From isothermal magnetization, remarkable values of the entropy change have been reported, such as | S T | = 11.5 J/kg K at 290 K for the small field change of 1 T, or | S T | = 70.1 J/kg K for 5 T on a slightly different composition. Strangely, this last value almost doubles that obtained from the Clausius-Clapeyron equation.Therefore, a very detailed set of calorimetric determinations have been made including heat capacity by adiabatic and differential scanning calorimetry (DSC), and direct measurement of the heat absorbed on isothermal demagnetization for several magnetothermal histories of the sample. We found high values of | S T | = 45.7 J/kg K at 289.4 K for a field change of 7 T, for a sample previously cooled to a low temperature and then heated under magnetic field to the target temperature. This value is high but very far from previously reported data, which happened to be nonphysical but obtained from a very frequently used, but incorrect, experimental protocol to determine S T from magnetization, via the Maxwell relation. The spurious contribution has been analyzed and computed, explaining the nonphysical reported values. The strong thermal and magnetic hysteresis of 8.4 K and 8.3 T, respectively, make this alloy useless for magnetic refrigeration, but the results encourage searching for other derivatives with lower hysteresis which could have even higher | S T |.

Rare-earth tuned magnetism and magnetocaloric effects in double perovskites R 2NiMnO6

“…MCE remains a topic of immense interest as it opens pathway toward realization of clean and energy efficient magnetic refrigeration technology with clear advantage over the conventional vapor-compression techniques that use potentially harmful chlorofluorocarbon gases [20]. Recent studies show a giant and reversible MCE in various magnetic materials such as MnFeP 0.45 As 0.55 [21] (magnetic entropy change −ΔS M = 18.0 J kg −1 K −1 for ΔH = 0-5 T), (MnNiSi) 0.56 (FeNiGe) 0.44 (−ΔS M = 70 J kg −1 K −1 for ΔH = 0-5 T) [22], nano-crystalline films of EuTiO 3 (−ΔS M = 24 J kg −1 K −1 for ΔH = 0-2 T) [23], GdCrTeO 6 (−ΔS M ≈ 42 J kg −1 K −1 for ΔH = 0-7 T) [24], Gd 5 (Si x Ge 1−x ) [25], Ni-Mn-In [26] (adiabatic temperature change ΔT ad = 6.2 K), Gd 2 NiMnO 6 [27] (−ΔS M = 35.5 J kg −1 K −1 for ΔH = 0-7 T). The materials which show MCE at low temperatures can be advantageous for cryogenic magnetic cooling to obtain a sub-kelvin temperature as an alternative option of He3/He4 liquid whose prices are constantly increasing.…”