Search

Abstract Using high resolution powder x-ray and neutron diffraction experiments, we determined the off-centered displacement of the ions within a unit cell and magnetoelectric coupling in nanoscale BiFeO3 (≈20–200 nm). We found that both the off-centered displacement of the ions and magnetoelectric coupling exhibit nonmonotonic variation with particle size. They increase as the particle size reduces from bulk and reach maximum around 30 nm. With further decrease in particle size, they decrease precipitously. The magnetoelectric coupling is determined by the anomaly in off-centering of ions around the magnetic transition temperature (T N ). The ions, in fact, exhibit large anomalous displacement around the T N which is analyzed using group theoretical approach. It underlies the nonmonotonic particle-size-dependence of off-centre displacement of ions and magnetoelectric coupling. The nonmonotonic variation of magnetoelectric coupling with particle size is further verified by direct electrical measurement of remanent ferroelectric hysteresis loops at room temperature under zero and ∼20 kOe magnetic field. Competition between enhanced lattice strain and compressive pressure appears to be causing the nonmonotonic particle-size-dependence of off-centre displacement while coupling between piezo and magnetostriction leads to nonmonotonicity in the variation of magnetoelectric coupling.

We report size reduction induced modification of magnetic property leading to phase separation and its correlation with crystallographic structure in one dimensional nanostructure of manganite system (La0.5Sr0.5MnO3)—which has not previously been reported. Structural study has been undertaken using synchrotron x-ray and neutron diffraction. These studies show the coexistence of crystallographic (tetragonal and orthorhombic) and magnetic phases (ferromagnetic (FM) and antiferromagnetic (AFM) respectively) below the Néel temperature (<200 K). Neutron depolarization experimentation also corroborates the neutron diffraction results. Our results demonstrate that the coexistence of the FM and AFM phases arises due to structural phase separation of tetragonal and orthorhombic phases. In contrast, bulk La0.5Sr0.5MnO3, showing FM (tetragonal phase) throughout the measured temperatures (6–300 K), does not show any phase separation. Our study will provide in-depth understanding of the underlying physics and basic mechanism of size induced phase separation in nanowires, and the role of crystal structure in stabilizing a specific ground state of these manganite nanowires. [ABSTRACT FROM AUTHOR]

We perform a comprehensive study of the structural, lattice dynamical and ferroic properties of the room temperature phase of AgNbO3 using a combination of neutron diffraction, Raman spectroscopy and first-principles density functional theory. The Rietveld analysis of neutron diffraction data indicates that AgNbO3 at room temperature crystallizes in an orthorombic structure with noncentrosymmetric Pmc21 space group symmetry. The zone-center phonon mode frequencies of Pmc21 and Pbcm phases of AgNbO3 are computed and are found to be in good agreement with experimentally obtained Raman mode frequencies. Computed frequencies for both space groups are found to be very close. However, computed modes with frequencies 488 and 846 cm−1 are found to be Raman-active for the Pmc21 phase while Raman inactive for Pbcm phase. The computed potential energy surfaces and Born effective charges of the ions suggest that the primary contribution to ferroic properties of AgNbO3 come from Nb and O ions only. Furthermore the calculated potential energy profiles suggest competing ferroic and antiferrodistortive instabilities in AgNbO3 are primarily due to the off-centering of Nb ions and rotations of NbO6 octahedra, respectively. [ABSTRACT FROM AUTHOR]