Giant direct and inverse electrocaloric effects in multiferroic thin films

Refrigeration systems based on the 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 nonambient 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 ${\mathrm{BiCoO}}_3$ (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 such as chemical substitution are also possible.

Figure 1

Structural, ferroelectric, and magnetic properties of energetically competitive bulk BCO polymorphs. (a) Tetragonal $P4mm$ ($\mathcal{T}$), (b) orthorhombic $Pnma$ ($\mathcal{O}$), and (c) monoclinic $Pc$ ($\mathcal{M}$). (d) Sketch of the spin configurations and exchange constants considered for the $\mathcal{T}$-AFM($C$) and $\mathcal{O}$-AFM($G$) Heisenberg spin models. Electrical polarizations $\mathbf{P}$ are referred to the pseudocubic Cartesian axis, and oxygen-octahedra rotation patterns $\mathbf{\theta }$ are expressed in Glazer's notation.

Figure 2

Energy, magnetic, structural, and ferroelectric properties of energetically competitive polymorphs in (100)-oriented ${\mathrm{BiCoO}}_3$ thin films calculated with first-principles methods. (a) Zero-temperature total energy. (b) Antiferromagnetic to paramagnetic transition temperature. (c) Electric polarization along the (100) direction. (d) Antiphase oxygen octahedral rotation angles along the (100) direction. (e) Electric polarization along the (011) direction. (f) Antiphase oxygen octahedral rotation angles along the (011) direction. The vertical lines indicate the strain-induced $\mathcal{T}\to \mathcal{M}$ phase transition occurring at very low temperatures. Electric polarizations are calculated with the Berry phase approach [35].

Figure 3

$T-a_{\mathrm{in}}$ phase diagram and free-energy differences in (100)-oriented BCO thin films. (a) Phase diagram as a function of temperature and in-plane lattice parameter. (b) Helmholtz free-energy differences among competitive polymorphs at $a_{\mathrm{in}}=3.875$ Å, (c) $a_{\mathrm{in}}=3.930$ Å, and (d) $a_{\mathrm{in}}=3.988$ Å. Vertical lines and black arrows indicate $T$-induced magnetic and structural phase transitions, respectively. Blue and green dotted lines correspond to Helmholtz free-energy differences for the $\mathcal{M}$ and $\mathcal{O}$ phases, respectively, calculated without considering spin-phonon coupling effects.

Figure 4

Direct ($\mathcal{O}\to \mathcal{M}$, red) and inverse ($\mathcal{O}\to \mathcal{T}$, blue) electrocaloric effects in (100)-oriented BCO thin films at room temperature. (a) Sketch of the $\mathcal{E}$-induced phase transformations. (b) Critical electric field expressed as a function of in-plane lattice parameter; the two involved electric field orientations are indicated in pseudocubic Cartesian notation. (c) Room-temperature entropy and (d) adiabatic temperature shifts expressed as a function of in-plane lattice parameter.