Giant direct and inverse electrocaloric effects in multiferroic thin films

Abstract

Refrigeration systems based on compression of greenhouse gases are environmentally threatening and cannot be scaled down to on-chip dimensions. In the vicinity of a phase transition caloric materials present large thermal responses to external fields, which makes them promising for developing alternative solid-state cooling devices. Electrocaloric effects are particularly well-suited for portable refrigeration applications; however, most electrocaloric materials operate best at non-ambient temperatures or require the application of large electric fields. Here, we predict that modest electric fields can yield giant room-temperature electrocaloric effects in multiferroic BiCoO$_{3}$ (BCO) thin films. Depending on the orientation of the applied field the resulting electrocaloric effect is either direct (heating) or inverse (cooling), which may enable the design of enhanced refrigeration cycles. We show that spin-phonon couplings and phase competition are the underlying causes of the disclosed caloric phenomena. The dual electrocaloric response of BCO thin films can be effectively tuned by means of epitaxial strain and we anticipate that other control strategies like chemical substitution are also possible.