Field-sensitivity and reversibility of the inverse magnetocaloric effect at martensitic transformations

Taake C, Samanta T, Caron L. Field-sensitivity and reversibility of the inverse magnetocaloric effect at martensitic transformations. Applied Physics Letters. 2024;124(5): 052403.The magnetic field-sensitivity of martensitic phase transitions (MPTs) responsible for magnetocaloric effects has been examined in B-substituted Ni50Mn34.8In15.2-xBx Heusler alloys (x = 1, 2, 3, and 4). Increasing boron substitution acts as a positive chemical pressure similar to the effect of hydrostatic pressure (p) and shifts the martensitic phase transition temperature (T-M) toward higher temperature. The observed structural compatibility of the MPT results in a lower thermal hysteresis (Delta T-hyst<5 K at low field). Delta T-hyst remains almost unchanged; however, the field sensitivity of T-M decreases significantly with increasing B content or application of p. As a result, the reversibility of the isothermal entropy change (|Delta S-rev|) reduces for higher B concentration or under hydrostatic pressure p. The experimental observation reveals that the lower field-sensitivity of the MPT with increasing B or p is associated with the simultaneous increase in the magnetocrystalline anisotropy energy (MAE) and decrease in the Zeeman energy (ZE). The relatively larger ZE and smaller MAE for x = 1 result in the improved reversibility of the entropy change (|Delta S-rev| = 21.48 J/kg K for Delta mu H-0 = 5 T), which is comparable to or even larger than the values reported for similar Heusler alloys

Entropy change reversibility in MnNi1−x Co x Ge0.97Al0.03 near the triple point

The nature of the phase transition has been studied in MnNi _1− _x Co _x Ge _0.97 Al _0.03 ( x = 0.20–0.50) through magnetization, differential scanning calorimetry and x-ray diffraction measurements; and the associated reversibility in the magnetocaloric effect has been examined. A small amount of Al substitution for Ge can lower the structural phase transition temperature, resulting in a coupled first-order magnetostructural transition (MST) from a ferromagnetic orthorhombic to a paramagnetic hexagonal phase in MnNi _1− _x Co _x Ge _0.97 Al _0.03 . Interestingly, a composition-dependent triple point (TP) has been detected in the studied system, where the first-order MST is split into an additional phase boundary at higher temperature with a second-order transition character. The critical-field-value of the field-induced MST decreases with increasing Co concentration and disappears at the TP ( x = 0.37) resembling most field-sensitive MST among the studied compositions. An increase of the hexagonal lattice parameter a _hex near the TP indicates a lattice softening associated with an enhancement of the vibrational amplitude in the Ni/Co site. The lattice softening leads to a larger field-induced structural entropy change (structural entropy change≫ magnetic entropy change, for this class of materials) with the application of a lower field, which results in a larger reversibility of the low-field entropy change (|Δ S _rev | = 6.9 J kg ^−1 K for Δ μ _0 H = 2 T) at the TP

Magnetocaloric effect in the (Mn,Fe)₂(P,Si) system: From bulk to nano

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 (Ms) is mainly attributed to the increased concentration of an atomically disordered shell, and with a decreased particle size, both the thermal hysteresis and Tc 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 Ms from 120 to 148 Am2kg−1 and the magnetic entropy change (ΔSm) 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.RST/Fundamental Aspects of Materials and EnergyInstrumenten groe