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 (Mn_1.17-xFe_0.73-yV_0.02) (P_0.5Si_0.5) (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 Mn_1.17-xFe_0.73-yV_0.02P_0.5Si_0.5 alloys are excellent materials for room-temperature magnetic heat-pumping applications by using a permanent magnet.

In the field of nanoscale magnetocaloric materials, novel concepts like micro-refrigerators, thermal switches, microfluidic pumps, energy harvesting devices and biomedical applications have been proposed. However, reports on nanoscale (Mn,Fe)₂(P,Si)-based materials, which are one of the most promising bulk materials for solid-state magnetic refrigeration, are rare. In this study we have synthesized (Mn,Fe)₂(P,Si)-based nanoparticles, and systematically investigated the influence of crystallite size and microstructure on the giant magnetocaloric effect. The results show that the decreased saturation magnetization (M_s) is mainly attributed to the increased concentration of an atomically disordered shell, and with a decreased particle size, both the thermal hysteresis and T_c are reduced. In addition, we determined an optimal temperature window for annealing after synthesis of 300–600 °C and found that gaseous nitriding can enhance M_s from 120 to 148 Am²kg⁻¹ and the magnetic entropy change (ΔS_m) from 0.8 to 1.2 Jkg⁻¹K⁻¹ in a field change of Δμ₀H = 1 T. This improvement can be attributed to the synergetic effect of annealing and nitration, which effectively removes part of the defects inside the particles. The produced superparamagnetic particles have been probed by high-resolution transmission electron microscopy, Mössbauer spectra and magnetic measurements. Our results provide important insight into the performance of giant magnetocaloric materials at the nanoscale.

MnMX (M = Co or Ni, X = Si or Ge) alloys with strong magnetostructural coupling exhibit giant magnetic entropy change and are currently extensively studied. However, large thermal hysteresis results in serious irreversibility of the magnetocaloric effect in this well-known system. In this work, we report a low thermal hysteresis and large reversible magnetocaloric effect in a MnNiGe-based system. The introduction of Fe into both Ni and Mn sites can establish stable magnetostructural transitions from paramagnetic hexagonal to ferromagnetic orthorhombic phases. Fascinatingly, a low thermal hysteresis of 5.2 K is achieved in Mn_0.9Fe_0.2Ni_0.9Ge alloy with a large magnetization difference of 62.1 A m²/kg between the two phases. These optimized parameters lead to a partially reversible phase transformation under a magnetic stimulus and bring about a large reversible magnetic entropy change of −18.6 Jkg⁻¹K⁻¹ under the field variation of 0–5 T, which is the largest value reported in MnMX system up to now. Moreover, this low-hysteresis magnetostructural transformation and large reversible magnetocaloric effect can be tuned by doping with Si in a wide temperature range covering room temperature. We also introduce geometrically nonlinear theory to discuss the origin of low hysteresis in MnMX alloys. A strong relation is found between thermal hysteresis and the change of c axis in the orthorhombic structure during the transition. Our work greatly develops the potential of MnMX alloys as magnetocaloric materials and is meaningful to seek or design a MnMX system with low thermal hysteresis.