Domain morphology, electrocaloric properties, and negative capacitance states of ferroelectric nanowires array: role of size and interfacial effects

We analyzed the domain morphology, electrocaloric properties, and negative capacitance states in a one-dimensional array of uniformly oriented, radial symmetric ferroelectric nanowires, whose spontaneous polarization is normal to their symmetry axis. The wires are densely packed between flat electrodes. Using finite element modeling based on the Landau-Ginzburg-Devonshire approach, we calculated the distributions of spontaneous polarization, domain structures, dielectric permittivity, and electrocaloric coefficient of the dense nanocomposite. Due to the interfacial and size effects, the paraelectric and ferroelectric states of the wires can be stable, depending on their radius and the dielectric permittivity of the surrounding medium. It is demonstrated that interfacial and collective effects determine the stability of the polar (or anti-polar) state in the array when the wire radius is significantly smaller than the critical size of the paraelectric transition in an isolated wire. We reveal that a large region of a mixed state, characterized by poly-domain ferroelectric states with nonzero average polarization inside each wire and zero average polarization of the whole array, can be stable. By selecting the dielectric permittivity of the surrounding medium and the nanowire radius, one can maximize the negative capacitance effect in the capacitor with densely packed wires. It is also possible to achieve maximal enhancement of the electrocaloric coefficient due to interfacial and size effects in the wires. We predicted the local increase in spontaneous polarization near the physical contact of the nanowires with electrodes, which is essentially an interfacial effect.