The Fe2P type Mn-Fe-P-Si alloys exhibit a giant magneto -elastic first -order transition, but the large hysteresis limits their performance. Crystal structure evolution and magnetocaloric performance were investigated by varying the Mn and Fe contents at a constant V substitution of 0.02 in Fe2P-type (Mn1.17-xFe0.73-yV0.02) (P0.5Si0.5) (where x + y = 0.02). The V substitution of Fe content shows a larger reduction of hysteresis compared with the same substitution amount of Mn content. During magnetoelastic phase transition, V -substitution reduces the volume change and the volumetric stresses, providing a superior mechanical stability. Compound with the V substitution of Fe (y = 0.02) shows the best magnetocaloric effect with a low thermal hysteresis of 0.6 K. Our developed Mn1.17-xFe0.73-yV0.02P0.5Si0.5 alloys are excellent materials for room -temperature magnetic heatpumping applications by using a permanent magnet.

Large thermal hysteresis in the (Mn,Fe)(2)(P,Si) system hinders an efficient heat exchange and thus limits the magnetocaloric applications. Substitution of manganese by vanadium in the Mn1-x1Vx1Fe0.95P0.593Si0.33B0.077 and Mn1-x2Vx2Fe0.95P0.563Si0.36B0.077 compounds enable a significant reduction in the thermal hysteresis without losing the giant magnetocaloric effect. For the composition closest to the critical one, where first-order crossovers to second-order phase transition in the series of x(2) = 0.02, Mn0.98V0.02Fe0.95P0.563Si0.36B0.077 exhibits a thermal hysteresis that is reduced from 1.5 to 0.5 K by 67%, yielding an adiabatic temperature change of 2.3 K and magnetic entropy change of 5.6 J/kgK for an applied field of 1 T, which demonstrates its potential for highly efficient magnetic heat pumps utilizing low-cost permanent magnets. (C) 2022 The Author(s). Published by Elsevier B.V.

The phase diagram of the magnetocaloric MnxFe2−xP1−ySiy quaternary compounds was established by characterising the structure, thermal and magnetic properties in a wide range of compositions (for a Mn fraction of 0.3 ≤ x < 2.0 and a Si fraction of 0.33 ≤ y ≤ 0.60). The highest ferromagnetic transition temperature (Mn0.3Fe1.7P0.6Si0.4, TC = 470 K) is found for low Mn and high Si contents, while the lowest is found for low Fe and Si contents (Mn1.7Fe0.3P0.6Si0.4, TC = 65 K) in the MnxFe2−xP1−ySiy phase diagram. The largest hysteresis (91 K) was observed for a metal ratio close to Fe:Mn = 1:1 (corresponding to x = 0.9, y = 0.33). Both Mn-rich with high Si and Fe-rich samples with low Si concentration were found to show low hysteresis (≤2 K). These compositions with a low hysteresis form promising candidate materials for thermomagnetic applications.

The first-order magneto-elastic transition in the Mn-Fe-P-Si alloys can be tailored by vanadium substitution. Alloys with a suitable V substitution provide an excellent magnetocaloric effect with minor hysteresis in low magnetic fields up to 1.2 T. Mossbauer measurements show that the hyperfine field is reduced by V substitution. Neutron diffraction reveals that Fe is substituted by V on the 3f site and the magnetic moment on the 3f site is enhanced by the V substitution. The modified magnetic exchange field around the 3f and 3g positions in the lattice can be utilized to design suitable magnetocaloric materials that operate in low magnetic fields. (C) 2019 The Authors. Published by Elsevier B.V.

Ni-Mn-$X$ ($X=\mathrm{In}$, Sn, and Sb) based Heusler alloys show a strong potential for magnetic refrigeration owing to their large magnetocaloric effect (MCE) associated with first-order magnetostructural transition. However, the irreversibility of the MCE under low field change of 0--1 T directly hinders its application as an efficient magnetic coolant. In this work, we systematically investigate thermal and magnetic properties, crystalline structure and magnetocaloric performance in $\mathrm{N}{\mathrm{i}}_{51\ensuremath{-}x}\mathrm{M}{\mathrm{n}}_{33.4}\mathrm{I}{\mathrm{n}}_{15.6}{\mathrm{V}}_{x}$ alloys. With the introduction of V, a stable magnetostructural transition near room temperature is observed between martensite and austenite. An extremely small hysteresis of 2.3 K is achieved for the composition $x=0.3$. Due to this optimization, the magnetic-field induced structural transition is partially reversible under 0--1 T cycles, resulting in a reversible MCE. Both magnetic and calorimetric measurements consistently show that the largest value for the reversible magnetic entropy change can reach about $5.1\phantom{\rule{0.16em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}\mathrm{k}{\mathrm{g}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ in a field change of 0--1 T. A considerable and reversible adiabatic temperature change of \ensuremath{-}1.2 K by the direct measurement is also observed under a field change of 0--1.1 T. Furthermore, the origin of this small hysteresis is discussed. Based on the lattice parameters, the transformation stretch tensor is calculated, which indicates an improved geometric compatibility between the two phases. Our work greatly improves the MCE performance of Ni-Mn-$X$-based alloys and make them suitable as realistic magnetic refrigeration materials.

Approaching the border of the first order transition and second order transition is significant to optimize the giant magnetocaloric materials performance. The influence of vanadium substitution in the Mn1.2-xVxFe0.75P0.5Si0.5 alloys is investigated for annealing temperatures of 1323, 1373 and 1423 K. By tuning both the annealing temperature and the V substitution simultaneously, the magnetocaloric effect can be enhanced without enlarging the thermal hysteresis near the border of the first to second order transition. Neutron diffraction measurements reveal the changes of site occupation and interatomic distances caused by varying the annealing temperature and V substitution. The properties of the alloy with x = 0.02 annealed at 1323 K is comparable to those found for the MnFe0.95P0.595Si0.33B0.075 alloy, illustrating that Mn1.2-xVxFe0.75P0.5Si0.5 alloys are excellent materials for magnetic heat-pumping near room temperature. (C) 2019 The Authors. Published by Elsevier B.V.

Large thermal hysteresis in the MnFe(P, Si, B) system hinders the heat exchange rate and thus limits the magnetocaloric applications at high frequencies. Substitution of Mn by V in Mn1-xVxFe0.95P0.593Si0.33B0.077 and Mn1-xVxFe0.95P0.563Si0.36B0.077 alloys was found to reduce the thermal hysteresis due to a decrease in the latent heat. Introducing V increases both the field-induced transition temperature shift and the magnetic moment per formula unit. Thus, a decease in the thermal hysteresis is obtained without losing the giant magnetocaloric effect. In consequence, an ultralow hysteresis (0.7 K) and a giant adiabatic temperature change of 2.3 K were achieved, which makes these alloys promising candidates for commercial magnetic refrigerator using permanent magnets.