Phase transitions induced by confinement of ferroic nanoparticles

TL;DR

This paper develops a phenomenological theory to analyze how confinement and surface effects induce phase transitions in ferroic nanoparticles, predicting size-dependent transition temperatures and conditions for ferroelectricity enhancement.

Contribution

It introduces an analytical model incorporating surface stress and other effects to predict size-induced phase transitions in ferroic nanoparticles, including ferroelectricity in nanorods and nanospheres.

Findings

Transition temperature can be higher than bulk due to size effects.

Optimal ferroelectric properties occur at nanoparticle radii of 5-50 nm.

Size-induced ferroelectricity predicted in KTaO3 nanorods below 20 nm radius.

Abstract

General approach for consideration of primary ferroic (ferroelectric, ferromagnetic, ferroelastic) nanoparticles phase transitions was proposed in phenomenological theory framework. The surface stress, order parameter gradient, striction as well as depolarization and demagnetization effects were included into the free energy. The strong intrinsic surface stress under the curved nanoparticle surface was shown to play the important role in the shift of transition temperature (if any) up to the appearance of new ordered phase absent in the bulk ferroic. The approximate analytical expression for the size-induced ferroelectric transition temperature dependence on cylindrical or spherical nanoparticle sizes, polarization gradient coefficient, correlation radius, intrinsic surface stress and electrostriction coefficient was derived. It was shown that the transition temperature of nanoparticle could be higher than the one of the bulk material. The best conditions of ferroelectric properties conservation and enhancement in nanowires correspond to the radius 5-50nm and compressive surface stress. Under the favorable conditions size effects (spatial confinement) induces ferroelectric phase in incipient ferroelectrics nanowires and nanospheres. The prediction of size-induced ferroelectricity in KTaO3 nanorods with radius less then 5-20 nm at room temperatures could be important for the next step of device miniaturization based on 3D nanostructures.