Multiferroic Phases and Transitions in Ferroelectric Lead Titanate Nanodots.

Discovery of novel phases and their associated transitions in low-dimensional nanoscale systems is of central interest as the origin of emergent phenomena and new device paradigms. Although typical ferroelectrics such as PbTiO3 exhibit diverse phase transition sequences, the conventional incompatible mechanisms of ferroelectricity and magnetism keep them as simply nonmagnetic phases, despite the immense practical prospective of multiferroics in novel functional devices. Here, we demonstrate using density function theory that PbTiO3 nanodots exhibit unconventional multiferroic phase transitions. The nanosize and nonstoichiometric effects intrinsic to nanodots bring about the coexistence of ferromagnetism with the host electric polarization, mediated by the termination and surface morphology. We also predict the key features of the size-dependent phase diagram of nanodots that involve a rich sequence of ferroelectric-multiferroic-ferromagnetic/nonmagnetic (FE-MF-FM/NM) multiferroic phase transitions. The present work thus provides an avenue to realizing multiferroics and multifunctional oxides in low-dimensional systems.

Unusual Multiferroic Phase Transitions in PbTiO3 Nanowires.

Unconventional phases and their transitions in nanoscale systems are recognized as an intriguing avenue for both unique physical properties and novel technological paradigms. Although the multiferroic phase has attracted considerable attention due to the coexistence and cross-coupling of electric and magnetic order parameters, mutually exclusive mechanism between ferroelectricity and ferromagnetism leaves conventional ferroelectrics such as PbTiO3 simply nonmagnetic. Here, we demonstrate from first-principles that ultrathin PbTiO3 nanowires exhibit unconventional multiferroic phases with emerging ferromagnetism and coexisting ferroelectric/ferrotoroidic ordering. Nanometer-scale and nonstoichiometric effects intrinsic to the nanowires bring about nonzero and nontrivial magnetic moments that coexist with the host ferroelectricity. The multiferroic order is susceptible to surface termination and nanowire morphology. Furthermore, calculations suggest that the nanowires undergo size-dependent ferroelectric-multiferroic-ferromagnetic phase transitions. This work therefore provides a route to multiferroic transitions in conventional nonmagnetic ferroelectric oxides.

Multiferroic grain boundaries in oxygen-deficient ferroelectric lead titanate.

Ultimately thin multiferroics arouse remarkable interest, motivated by the diverse utility of coexisting ferroelectric and (anti)ferromagnetic order parameters for novel functional device paradigms. However, the ferroic order is inevitably destroyed below a critical size of several nanometers. Here, we demonstrate a new path toward realization of atomically thin multiferroic monolayers while resolving a controversial origin for unexpected "dilute ferromagnetism" emerged in nanocrystals of nonmagnetic ferroelectrics PbTiO3. The state-of-the-art hybrid functional of Hartree-Fock and density functional theories successfully identifies the origin and underlying physics; oxygen vacancies interacting with grain boundaries (GBs) bring about (anti)ferromagnetism with localized spin moments at the neighboring Ti atoms. This is due to spin-polarized defect states with broken orbital symmetries at GBs. In addition, the energetics of oxygen vacancies indicates their self-assembling nature at GBs resulting in considerably high concentration, which convert the oxygen-deficient GBs into multiferroic monolayers due to their atomically thin interfacial structure. This synthetic concept that realizes multiferroic and multifunctional oxides in a monolayered geometry through the self-assembly of atomic defects and grain boundary engineering opens a new avenue for promising paradigms of novel functional devices.