The recent literature concerning the magnetocaloric effect (MCE) has been reviewed. The MCE properties have been compiled and correlations have been made comparing the behaviours of the different families of magnetic materials which exhibit large or unusual MCE values. These families include: the lanthanide (R) Laves phases (RM2, where M = Al, Co and Ni), Gd5(Si1−xGex)4 ,M n(As1−xSbx), MnFe(P1−xAsx), La(Fe13−xSix) and their hydrides and the manganites (R1−xMxMnO3, where R = lanthanide and M = Ca, Sr and Ba). The potential for use of these materials in magnetic refrigeration is discussed, including a comparison with Gd as a near room temperature active magnetic regenerator material. (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0034-4885/68/6/R04/pdf

Recent developments in magnetocaloric materials

The magnetocaloric effect (MCE) in paramagnetic materials has been widely used for attaining very low temperatures by applying a magnetic field isothermally and removing it adiabatically. The effect can also be exploited for room-temperature refrigeration by using giant MCE materials. Here we report on an inverse situation in Ni-Mn-Sn alloys, whereby applying a magnetic field adiabatically, rather than removing it, causes the sample to cool. This has been known to occur in some intermetallic compounds, for which a moderate entropy increase can be induced when a field is applied, thus giving rise to an inverse magnetocaloric effect. However, the entropy change found for some ferromagnetic Ni-Mn-Sn alloys is just as large as that reported for giant MCE materials, but with opposite sign. The giant inverse MCE has its origin in a martensitic phase transformation that modifies the magnetic exchange interactions through the change in the lattice parameters.

Inverse magnetocaloric effect in ferromagnetic Ni-Mn-Sn alloys.

A magnetically, electrically or mechanically responsive material can undergo significant thermal changes near a ferroic phase transition when its order parameter is modified by the conjugate applied field. The resulting magnetocaloric, electrocaloric and mechanocaloric (elastocaloric or barocaloric) effects are compared here in terms of history, experimental method, performance and prospective cooling applications.

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Caloric materials near ferroic phase transitions

Martensitic and magnetic transformations of the Heusler Ni50Mn50−yXy (X=In, Sn and Sb) alloys were investigated by differential scanning calorimetry measurement and the vibrating sample magnetometry technique. In all these alloy systems, the austenite phase with the ferromagnetic state was transformed into the martensite phase, which means that these Heusler alloys have potential as Ga-free ferromagnetic shape memory alloys (FSMAs). Furthermore, multiple martensitic transformations, such as two- or three-step martensitic transformations, occur in all these alloy systems. It was confirmed by transmission electron microscopy observation that the crystal structure of the martensite phase is an orthorhombic four-layered structure which has not been reported in other FSMAs. Therefore, the present Ga-free FSMAs have the great possibility of the appearance of a large magnetic-field-induced strain.

Magnetic and martensitic transformations of NiMnX(X=In,Sn,Sb) ferromagnetic shape memory alloys

Magnetocaloric effect: From materials research to refrigeration devices

Commensurate and incommensurate “5M” modulated crystal structures in Ni–Mn–Ga martensitic phases

We have studied the isothermal entropy change around a first-order structural transformation and in correspondence to the second-order Curie transition in the ferromagnetic Heusler alloy Ni2.15Mn0.85Ga. The results have been compared with those obtained for the composition Ni2.19Mn0.81Ga, in which the martensitic structural transformation and the magnetic transition occur simultaneously. With a magnetic field span from 0 to 1.6 T, the magnetic entropy change reaches the value of 20 J/kg K when transitions are co-occurring, while 5 J/kg K is found when the only structural transition occurs.

Giant entropy change at the co-occurrence of structural and magnetic transitions in the Ni2.19Mn0.81Ga Heusler alloy

Crystal structure of 7M modulated Ni–Mn–Ga martensitic phase

From direct to inverse giant magnetocaloric effect in Co-doped NiMnGa multifunctional alloys

The composition dependence of magnetic and structural transformations, as well as the temperature and composition dependence of the magnetocrystalline anisotropy, were studied in the martensitic Ni2+xMn1+yGa1+z (x+y+z=0) Heusler alloys. Moving away from the stoichiometric composition, a slight variation, a large reduction, and a marked enhancement, were, respectively, observed for the Curie temperature, the magnetic anisotropy, and the structural (martensitic–austenitic) transformation temperatures.

Antonio Paoluzi

Papers

Commensurate and incommensurate “5M” modulated crystal structures in Ni–Mn–Ga martensitic phases

Giant entropy change at the co-occurrence of structural and magnetic transitions in the Ni2.19Mn0.81Ga Heusler alloy

Crystal structure of 7M modulated Ni–Mn–Ga martensitic phase

From direct to inverse giant magnetocaloric effect in Co-doped NiMnGa multifunctional alloys

Composition and temperature dependence of the magnetocrystalline anisotropy in Ni2+xMn1+yGa1+z (x+y+z=0) Heusler alloys