Phase transitions induced by confinement of ferroic nanoparticles

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.Comment: 35 pages, 11 figures, 2 appendices, to be submitted to Physical Review

New multiferroics based on EuxSr1−xTiO3 nanotubes and nanowires

“…The coupling was considered as the reason of magnetization appearance inside the ferromagnetic domain wall in non-ferromagnetic media [51]. Biquadratic coupling tensor and higher order expansion coefficients are regarded composition dependent: For tetragonal ferroelectric, (anti)ferromagnetic and cubic elastic symmetry groups, coefficients α are renormalized by the surface tension [15][16][17], misfit strains [53] and biquadratic coupling with a structural order parameter [27][28][29]. For considered geometry, the renormalization is: …”

“…The intrinsic surface stress can induce ferroelectricity, ferromagnetism and increase corresponding phase transition temperatures in conventional ferroelectrics and quantum paraelectric nanorods, nanowires [15,16,17,18,19] and binary oxides [20].The surface stress is inversely proportional to the surface curvature radius and directly proportional to the surface stress tensor (similar to Laplace surface tension). Thus, the intrinsic surface stress should depend both on the growth conditions and the surface termination morphology [21,22].…”

General approach for the description of size effects in ferroelectric nanosystems

“…Pade approximations of the solution of Eq. 18a for transition temperature T CR could be written as [17]:…”

“…It is worth to note the enormous achievements of both the phenomenological [14] and microscopic [15] theories, their recent advances in different fields like the description of nanorods [16,17], size effects in thin films [18,19], ferroelectric nanoparticles [20][21][22]; flexoelectric effect influence on the intrinsic properties [23,24] and response [25][26][27] of the nanosystems; the developed analytical model accounting for depolarization field as well as the formation of misfit dislocations [28][29][30]. However, despite this progress, the phenomenological theory lacks a general method, suitable for the solution of vast variety of different problems of ferroics description.…”

Surface effect on domain wall width in ferroelectrics

“…1,2,3,4,5,6,7 Laminar domain structure formation in the thick films with free surfaces was considered in the classic papers by Kittel 8 for ferromagnetic media and Mitsui and Furuichi 9 for ferroelectric media. In the classical papers of Cao and Cross 10 and Zhirnov 11 , a single boundary between two domains was considered in the bulk ferroelectrics, allowing for electrostriction contribution.…”