RESUMO
The signature of correlated electron materials (CEMs) is the coupling between spin, charge, orbital and lattice resulting in exotic functionality. This complexity is directly responsible for their tunability. We demonstrate here that the broken symmetry, through cubic to orthorhombic distortion in the lattice structure in a prototype manganite single crystal, La(0.69)Ca(0.31)MnO(3), leads to an anisotropic magneto-elastic response to an external field, and consequently to remarkable magneto-transport behavior. An anomalous anisotropic magnetoresistance (AMR) effect occurs close to the metal-insulator transition (MIT) in the system, showing a direct correlation with the anisotropic field-tuned MIT in the system and can be understood by means of a simple phenomenological model. A small crystalline anisotropy stimulates a "colossal" AMR near the MIT phase boundary of the system, thus revealing the intimate interplay between magneto- and electronic-crystalline couplings.
Assuntos
Compostos de Cálcio/química , Magnetismo , Compostos de Manganês/química , Óxidos/química , Titânio/química , Algoritmos , Anisotropia , Simulação por Computador , Cristalização , Impedância Elétrica , Microscopia Eletrônica de Transmissão , Modelos Químicos , Difração de Raios XRESUMO
Using in situ magneto-optical Kerr effect measurements and phenomenological modeling, we study the tunability in both the magnetization anisotropy and magnetic coupling of Fe nanodots on a curved Cu(111) substrate with varying vicinity. We observe that, as the terrace width w decreases, the magnetization anisotropy increases monotonically, faster when w is smaller than the nanodot size d. In contrast, the magnetic coupling strength also increases until w approximately d, after which it decreases steeply. These striking observations can be rationalized by invoking the counterintuitive dimensionality variation of the surface electrons mediating the interdot coupling: the electrons are confined to be one dimensional (1D) when w > or = d, but become quasi-2D when w < d due to enhanced electron spillover across the steps bridged by the nanodots.
RESUMO
The polarization screening mechanism and ferroelectric phase stability of ultrathin BaTiO(3) films exposed to water molecules is determined by first principles theory and in situ experiment. Surface crystallography data from electron diffraction combined with density functional theory calculations demonstrate that small water vapor exposures do not affect surface structure or polarization. Large exposures result in surface hydroxylation and rippling, formation of surface oxygen vacancies, and reversal of the polarization direction. Understanding interplay between ferroelectric phase stability, screening, and atomistic processes at surfaces is a key to control low-dimensional ferroelectricity.
RESUMO
Spin glass (SG) is a magnetic state with spin structure incommensurate with lattice and charge. Fundamental understanding of its behavior has a profound impact on many technological problems. Here, we present a novel case of interface-induced spin glass behavior via self-assembly of single-crystalline NiO microcolumns in a single-crystalline NiFe2O4 matrix. Scanning transmission electron microscopy indicates that the hexagonal-shaped NiO columns are along their [211] direction and oriented along the [111] direction of the NiFe2O4 matrix. Magnetic force microscopy reveals magnetic anisotropy between NiO columns (antiferromagnetic transition temperature TN â¼ 523 K) and NiFe2O4 matrix (ferrimagnetic transition temperature TFI â¼ 860 K). This leads to spin disorder/frustration at atomically sharp NiFe2O4/NiO interfaces responsible for spin glass behavior below TSG â¼ 28 K. Our results demonstrate that self-assembly of magnetically distinct microstructures into another crystalline and magnetically ordered matrix is an effective way to create novel spin states at interfaces.
RESUMO
Atomically engineered oxide multilayers and superlattices display unique properties responsive to the electronic and atomic structures of the interfaces. We have followed the growth of ferroelectric BaTiO3 on SrRuO3 electrode with in situ atomic scale analysis of the surface structure at each stage. An oxygen-induced surface reconstruction of SrRuO3 leads to formation of SrO rows spaced at twice the bulk periodicity. This reconstruction modifies the structure of the first BaTiO3 layers grown subsequently, including intermixing observed with cross-section spectroscopy. These observations reveal that this common oxide interface is much more interesting than previously reported and provide a paradigm for oxygen engineering of oxide structure at an interface.