Magnetocaloric materials: Strained relations.

An area of modern life in which thermodynamics plays a central role is refrigeration. Its widespread use in household and industrial applications has driven a need for cheap and environmentally friendly processes that might replace the costly approaches based on mechanical work. One promising strategy is to take advantage of the thermodynamic changes associated with the varying magnetization of materials exposed to a magnetic field, a phenomenon known as the magnetocaloric effect. This requires magnetic materials with suitable intrinsic properties that maximize the thermal response associated with the change in magnetization1. Although magnetic refrigeration has become a standard technique in low-temperature physics, the relatively few materials with large magnetocaloric properties at higher temperatures has restricted its use in commercial applications. Writing in Nature Materials, Xavier Moya and colleagues now describe experiments that potentially overcome this limitation and open the door to a new range of materials with giant magnetocaloric effects2. The researchers study epitaxial films of La0.7Ca0.3MnO3 (LCMO) grown on BaTiO3 (BTO) substrates. Under ordinary conditions, LCMO becomes ferromagnetic below 260 K, but its magnetocaloric properties are modest. However, when placed next to BTO the picture changes: BTO undergoes a structural transition at around 200 K, which in turn strains the LCMO film and induces a change in its lattice parameters. On cooling, this structural change causes a jump in the magnetization of LCMO, which results in a substantial magnetocaloric effect, albeit extrinsic in origin. The magnetocaloric effect is an adiabatic process that occurs without an exchange of heat with the surrounding environment taking place. Consequently, when an applied magnetic field is cut off, the entropy associated with the magnetic moments of a material increases and, in turn, its lattice entropy must decrease, resulting in a drop in temperature of the material. This effect is largest close to the transition temperature of ferromagnets, where the magnitude of the magnetization changes most rapidly with temperature. Gadolinium has been widely studied as a magnetocaloric material because it is ferromagnetic at room temperature and has a sizeable magnetic moment. However, as for all materials undergoing continuous (or in the language of thermodynamics, second order) phase transitions, its magnetocaloric effect is rather modest: the governing parameter is the change of the magnetization with temperature, and this is limited by the fundamental fact that the magnetization decays continuously to zero as the transition temperature is approached (Fig. 1a). The effect is significantly enhanced in materials undergoing discontinuous (or first order) phase transitions. Gd5Si2Ge2 (ref. 3) was the first of a series of gadolinium-based materials found to display a first-order magnetic phase transition and, as a result of the abrupt jump in the magnetization near the transition temperature (Fig. 1b), a giant magnetocaloric effect. Other examples of ferromagnets with tunable first-order transition temperatures and large saturation magnetizations are based on Fe2P and La(Fe,Si)13 (ref. 4). Typically the magnetization jump at first-order phase transitions is accompanied by a change in the lattice parameters, which may cause a structural transition. This magnetoelastic coupling complicates matters, and makes the indirect derivation of the magnetocaloric effect unreliable: the adiabatic temperature change associated with the applied magnetic field is not merely a function of the magnetization. A drastic example of this ambiguity is given by the Heusler alloy NiMnIn(Co) (refs 5,6), in which the magnetization increases on heating as a result of a structural transition, leading to an inverse magnetocaloric effect. Nevertheless, direct measurements of the magnetocaloric adiabatic temperature change are possible, and this structural contribution can be used to enhance it5. By demonstrating that structural transitions in a substrate can induce a discontinuous jump in a magnetic overlayer (Fig. 1c), Moya et al. have emphasized the importance of the magnetoelastic coupling, and in doing so they have indicated a route for designing new material combinations with giant magnetocaloric effects. There may be great potential in nanostructured mixtures such as core–shell nanoparticles or bilayer films, provided they consist of a ferromagnetic material for which the magnetization depends strongly on variations in the lattice parameter, and a material undergoing a first-order structural phase transition. The strained relationship between BTO and LCMO may be a template for cooler things to come. Download references