Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session L39: Focus Session: Intrinsic Inhomogeneity in Multiferroic Materials |
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Sponsoring Units: DMP Chair: Neil D. Mathur, University of Cambridge Room: LACC 514 |
Tuesday, March 22, 2005 2:30PM - 3:06PM |
L39.00001: Nanoscale Structural Correlations in Magnetoresistive Manganites Invited Speaker: The colossal magnetoresistance (CMR) effect in perovskite manganites is a magnetic-field-induced transition form a paramagnetic insulating (PI) to a ferromagnetic metallic state. The large electrical resistivity of the PI state lies at the heart of the CMR effect. This enhanced resistivity stems, in part, from strong electron-lattice coupling and the associated local lattice distortions. Both uncorrelated local distortions (Jahn-Teller polarons) and correlated distortions are present in the PI state. The latter are believed to signal the presence of nanoscale orbital correlations. In this talk, we describe recent x-ray and neutron scattering studies of the orbital correlations in pseudocubic manganites Ln$_{1-x}$B$_x$MnO$_3$. Possible microscopic structures giving rise to these correlations are discussed. Dynamical properties of the correlated and uncorrelated distortions are presented. It is found that the correlations are ubiquitous in the orthorhombic PI phase of hole-doped manganites, and that their properties are defined by a single parameter - the doping level $x$. The correlations, however, are absent in the other paramagnetic phase exhibited by the manganites - the rhombohedral phase. The latter phase is metallic in the doping range in which the CMR effect is observed. Since the uncorrelated lattice distortions are present in the both of these phases, the insulating character of the PI state in CMR manganites results from the presence of the correlated lattice distortions. The orbital correlations, therefore, play the key role in the CMR effect. [Preview Abstract] |
Tuesday, March 22, 2005 3:06PM - 3:42PM |
L39.00002: Electrical and structural investigations, and ferroelectric domains in nanoscale structures Invited Speaker: Generally speaking material properties are expected to change as the characteristic dimension of a system approaches at the nanometer scale. In the case of ferroelectric materials fundamental problems such as the super-paraelectric limit, influence of the free surface and/or of the interface and bulk defects on ferroelectric switching, etc. arise when scaling the systems into the sub-100 nm range. In order to study these size effects, fabrication methods of high quality nanoscale ferroelectric crystals as well as AFM-based investigations methods have been developed in the last few years. The present talk will briefly review self-patterning and self- assembly fabrication methods, including chemical routes, morphological instability of ultrathin films, and self-assembly lift-off, employed up to the date to fabricate ferroelectric nanoscale structures with lateral size in the range of few tens of nanometers. Moreover, in depth structural and electrical investigations of interfaces performed to differentiate between intrinsic and extrinsic size effects will be also presented. [Preview Abstract] |
Tuesday, March 22, 2005 3:42PM - 3:54PM |
L39.00003: Magneto-electric phase diagrams in Tb$_1-x$Gd$_x$MnO$_3$ Takeshi Goto, Yuuichi Yamasaki, Hideyuki Watanabe, Tsuyoshi Kimura, Yoshinori Tokura {\it R}MnO$_3$ with distorted perovskite structure has the ferroelectric (FE) ground state with long-period antiferromagnetic spin order for {\it R}=Tb and Dy [1], while showing the A-type (layered antiferromagnetic) paraelectric (PE) state for R=Gd. The A-type PE state in GdMnO$_3$ is associated with the appreciable ferromagnetic component (~0.2$\mu$B) along the c axis due to the Dyaloshinskii-Moriya interaction. When an external field of about 7T is applied along the c axis, the FE state of TbMnO$_3$ is observed to turn into PE. This is likely because the A-type PE state with canted spin component is induced by the magnetic field via Zeeman coupling. In this study, we have prepared single crystals of solid solutions Tb$_1-x$Gd$_x$MnO$_3$, whose both end materials are the PE and FE with different magnetic orders. Due to the exchange interaction between the Heisenberg like Gd moments and Mn spins, the series of compounds show the rich phase diagrams as functions of Gd composition x and magnetic field H. Enhanced magnetic-field response of the ferroelectric phase is demonstrated in the intermediate x-region. [1] T. Goto et al. Phys. Rev. Lett.92,257201 (2004). [Preview Abstract] |
Tuesday, March 22, 2005 3:54PM - 4:06PM |
L39.00004: Observation of a Griffiths phase in paramagnetic La$_{1-x}$Sr$_x$MnO$_3$ Joachim Deisenhofer, Hans-Albrecht Krug von Nidda, Joachim Hemberger, Alois Loidl, Daniel Braak, Rushana Eremina, Vladimir Ivanshin, Anatoli Balbashov, Tsuyoshi Kimura, Yoshinori Tokura We report on the discovery of a new phase boundary above the magnetic ordering temperature in low doped La$_{1-x} $Sr$_x$MnO$_3$ by means of ESR and susceptibility measurements. The observed triangular phase regime in the paramagnetic state is identified as a realization of a Griffiths phase, where disorder in the ferromagnetic bonds leads to the existence of a temperature scale above $T_C$. The influence of quenched disorder to allow for the occurrence of a Griffiths phase becomes evident by its appearance within the Jahn-Teller distorted orthorhombic structure. [Preview Abstract] |
Tuesday, March 22, 2005 4:06PM - 4:18PM |
L39.00005: Search for uniform periodicity around the chemical window in manganites Neil Mathur, Susan Cox, Tony Williams, Paul Attfield, Paul Midgley We have recently shown that the low temperature superstructure in La$_{1-x}$Ca$_{x}$MnO$_{3}$ ($x\ge $0.5) possesses a uniform modulation that is not consistent with the charge-ordered stripe picture [cond-mat/0308581]. Here we investigate whether this phenomenon is confined to (La,Ca)MnO$_{3}$ or whether it arises in other manganites where the tendency to charge localisation is stronger. [Preview Abstract] |
Tuesday, March 22, 2005 4:18PM - 4:54PM |
L39.00006: Complex Phenomena in Nanostructured Transition Metal Oxides Invited Speaker: When the spatial dimension of a material becomes comparable or even smaller than the characteristic length scale of the relevant cooperative (collective) phenomena, it is expected that all related physical properties including phase transitions of this material will be dramatically changed. In this work, we focus on the discovery, understanding, and design of low-dimensional 3d transition metal oxides (TMO). We use both physical and chemical methods including laser MBE growth and hydrothermal synthesis to grow TMO thin films and nanostructures. The electronic and magnetic properties of the TMO films have been investigated by in-situ scanning tunneling microscopy/spectroscopy and ex-situ SQUID magnetometer. We have observed both large-scale (large than a few tens nanometers) and nano-scale electronic phase separation (PS) in epitaxially grown thin films of (La$_{5/8-0.3}$Pr$_{0.3})$Ca$_{3/8}$MnO$_{3}$. While the large PS domains are present only below the Curie temperature (T$_{c})$, the nano-scale PS clusters exist at temperatures both below and above T$_{c}$, which implies that the small clusters may originate from doping-related disorder. [Preview Abstract] |
Tuesday, March 22, 2005 4:54PM - 5:06PM |
L39.00007: Spin and Orbital Ordering of Y$_{1-x}$La$_x$VO$_3$ (0 $\leq$ x $\leq$ 1) J.-Q. Yan, Y. Ren, J.-S. Zhou, J.B. Goodenough At lowest temperatures, LaVO$_{3}$ exhibits G-type orbital ordering (OO) and C-type spin ordering (SO), while YVO$_{3}$ has C-type OO and G-type SO. To study the transition of different ground states with the variation of the size of the A-site cation, single-phase Y$_{1-x}$La$_{x}$VO$_{3}$ (0 \textbf{$\le $ }x \textbf{$\le $ }1) samples were melt-grown with the aid of an image furnace. The structural change accompanying the spin and orbital ordering was studied by high-energy, high-resolution x-ray powder diffraction. The temperature dependence of magnetic susceptibility and thermal conductivity was also studied. The results show that C-type OO is stabilized over a wider temperature range as the La content increases in the compositional range 0 \textbf{$\le $ }x \textbf{$\le $ }0.18; Too drops from 200 K for YVO$_{3}$ to 145 K for Y$_{0.82}$La$_{0.18}$VO$_{3}$, while T$_{N}$ decreases slightly from 116 K to 112 K. An abrupt change of the ground state from C-type OO to G-type OO was observed at a critical composition x = 0.20. At x $\ge $ 0.20, all compositions show G-type OO as the ground state and no long-range OO above T$_{N}$ was observed. *Y. Ren, Experimental Facilities Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439 [Preview Abstract] |
Tuesday, March 22, 2005 5:06PM - 5:18PM |
L39.00008: Thermal expansion measurements in multiferroic HoMn$_{2}$O$_{5}$ Clarina dela Cruz, Fei Yen, Bernd Lorenz, C.W. Chu, M.M. Gospodinov Thermal expansion measurements were done on HoMn$_{2}$O$_{5}$ along the a,b,and c axes at zero applied magnetic field. Distinctive anomalies in the linear expansivities along the principal axes were seen at T$_{N}$=44K,T$_{C}$=39K,T$_{N\mbox{'}}$=20K and T$_{C\mbox{'}}$=15K with a notable negative c-axis thermal expansivity below 100K.All three axes were observed to shrink at T$_{N}$ and T$_{N\mbox{'}}$ while a and b expand as c shrinks when it passes through T$_{C}$ and T$_{C\mbox{'}}$ upon cooling. These anomalies are intimately correlated with anomalies in the dielectric constant and the specific heat at the phase transition temperatures. Our observations suggest that the coupling of the magnetic orders with the dielectric properties are mediated by strong magnetoelastic effects and the lattice anomalies play a crucial role in understanding the ferroelectricity in the compound. The anomalies associated with the ferroelectric transitions at T$_{C}$ and T$_{C\mbox{'}}$ show a thermal hysteresis revealing the first order nature of the transitions. [Preview Abstract] |
Tuesday, March 22, 2005 5:18PM - 5:30PM |
L39.00009: Strains and rotations in cubic-tetragonal ferroelastics Allan Jacobs Many ferroelastic materials are also ferroelectric or ferromagnetic; a classic example is barium titanate which transforms from cubic to tetragonal with decreasing temperature. The various low-temperature variants in ferroelastics are described by different values of the order-parameter (deviatoric) strain(s). In order to maintain a coherent interface free of disclinations, dislocations and other defects, the variants are rotated as well as separated by domain walls; the rotation might be observable by birefringence imaging. Non-order-parameter strains (dilatational and shear) are generally present in walls but are usually small there. In regions where walls collide however, the rotations are confused, generating dilatational and shear strains of order of the deviatoric strains. This talk describes various kinds of collision regions observed in simulations of cubic-tetragonal systems. [Preview Abstract] |
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