Multicaloric materials and effects
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ntroduction Caloric materials show reversible thermal changes in response to field-induced changes of a ferroic order parameter. Specifically, one may achieve magnetocaloric,1 electrocaloric,2 and mechanocaloric3,4 (barocaloric and elastocaloric) effects using magnetic fields, electric fields, and stress fields, respectively. In practice, caloric responses are quantified in two thermal extremes, as either isothermal entropy change or adiabatic temperature change.5–7 In recent years, interest has focused on the development of materials that display large caloric effects near room temperature, specifically for application in solid-state refrigeration devices.8 Giant caloric responses are expected to occur in the vicinity of phase transitions, where the properties of the materials are strongly temperature dependent.5,6,9 Materials that display ferromagnetic, ferroelectric, or ferroelastic phase transitions of the first order 6,7 can show particularly large caloric effects because the driving field can access a large latent heat. Multiferroic materials necessarily display two or more of the aforementioned ferroic orders, and these may be strongly coupled10 such that each ferroic order parameter can respond to more than one type of applied field. Consequently, different ferroic properties can emerge at nearby temperatures, or even
simultaneously. Large caloric effects can thus be driven by the simultaneous or sequential application of more than one type of external field, in what are known as multicaloric materials.7 The main limitation of materials displaying caloric effects near first-order phase transitions is thermal and field hysteresis.11 While hysteresis typically arises as a consequence of nucleation, in caloric materials it occurs primarily due to domain-wall pinning, which is the net result of long-range elastic strain associated with phase transitions of interest. It was recently shown that hysteresis can be bypassed by exploiting the response of multicaloric materials to more than one type of driving field,12 but work must then be done to vary the secondary field. Most giant magnetocaloric and electrocaloric materials (undergoing first-order phase transitions) are expected to also show mechanocaloric effects, as the ferroic order parameter is likely to be strongly coupled to the lattice (Figure 1).1–3,13–26 Over the last few years, there has been growing interest in studying caloric effects while varying the stress field along with the magnetic or electric field.27–29 So far, most of this type of work has employed hydrostatic pressure as the primary control parameter. On the other hand, there has been much less research on multicaloric effects that employ magnetic and electric fields, in spite of the ongoing interest in multiferroic materials and the
Enric Stern-Taulats, Department of Materials Science, University of Cambridge, UK; [email protected] Teresa Castán, Departament de Física de la Matèria Condensada, Universitat de Barcelona, Catalonia; [email protected] Lluís Mañosa, Departament de Física de
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