The Magnetocaloric Effect Accompanied With Metamagnetic Phase Transition

This issue mainly investigates and tunes the critical magnetic field and phase transition temperature by material components and process design. Through these, we aim to tailor the work temperature span of magnetocaloric effect and enhance the refrigerate capacity.For magnetostructural phase alloys, the big different magnetization between phases determines the magneto-induced capacity. But due to the nature of first-order transition, to some extent, the narrow phase transition temperature span and big thermal/magnetic hysteresis prevents the application of magnetocaloric effect. Based on this, we investigate and analyze two characteristic magnetostructural phase alloys: Ni-Mn-Sn ferromagnetic shape memory alloy and metamagnetic MnCoSi alloy.1) Investagation about the metamagnetism of MnCoSi and its relevant propertiesMnCoSi alloy has a layered double helical antiferromagnetic structure at a low temperature. Under Neel temperature, MnCoSi experiences a metamagnetism from an antiferromagnetic state to a ferromagnetic state with an application of high magnetic field. While the critical field of metamagnetism is high （>2 T）, this restricts its application. When we introduce 5% Mn/Co vacancy into MnCoSi alloy (Mn_0.95CoSi, MnCo_0.95Si), the alloys display a metamagnetism. And due to the vacancy on Mn sites, the critical field of magnetization is about 1.3 T at room temperature. This transition is accompanied with a magnetocaloric effect. Compared to stoichiometric MnCoSi alloy, the critical field is about half of that. This will develop the magnetic-induced capacity in a low field. Moreover, the transition is second order without magnetic hysteresis, has a processing irreversibility and a wide working temperature span. The introduction of Co vacancy makes the alloy show a more stable ferromagnetic state and the critical field of metamagnetism increase. The phase transition temperature decreases to 260 K （270 K in MnCoSi alloys） and magnetic hysteresis increases.Besides, we study the quaternary CoMn_0.99Fe_0.01Si alloy under different heating treatments （annealing and quenching）. The critical field decreases to 1.3 T with the quenching treatment. This is about 35% lower than samples with annealing treatment. And the phase transition temperature decreases to 230 K （250 K in annealing sample）. Meanwhile, the magnetic entropy increases under a low field.2) Investigation on structure and phase transition temperature of Ni-Mn-Sn alloyThe lattice parameter and martensitic transition temperature of Ni-Mn-Sn will change with doping other elements （4d,4f）. After the substitution of Mn by Sm atom, the phase transition temperature increases in Ni-Mn-Sn alloy and the alloy shows complex coexist phases. And the thermal hysteresis decreases with increasing the Sm-content. In Mn₃₉Ni₅₀-xMoxSn₁₁ alloy, with increasing Mo-content, the phase transition temperature decreases. With different Mo-contents, the samples show L2₁, L1₀（austenite）,10M and 40 （martensite） structures.