Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session W23: Focus Session: Manganites II |
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Sponsoring Units: DMP GMAG Chair: Ichiro Takeuchi, University of Maryland Room: Morial Convention Center 215 |
Thursday, March 13, 2008 2:30PM - 2:42PM |
W23.00001: Parameters controlling magnetic transitions in manganites B. Dabrowski, S. Kolesnik, O. Chmaissem, J. Mais Using description of structural and physical properties of perovskites in terms of the tolerance factor t(x,T,d), which is dependent on composition, temperature, and oxygen-content, we have established synthetic methods and studied composition-structure-properties for new manganites La$_{1-x-y}$Sr$_{x}$Ba$_{y}$MnO$_{3-d}$ far beyond solubility limits normally achieved during the synthesis in air. Parameters controlling magnetic transitions have been identified through examples of single-valent compounds of RMnO$_{3}$ (the Mn-O-Mn bond angles that can be equivalently described in terms of the tolerance factor) and Sr$_{1-x}$Ca$_{x}$MnO$_{3}$ (the tolerance factor and the variance of A-site ion sizes), and the mixed-valent La$_{0.5}$Sr$_{0.5-y}$Ba$_{y}$MnO$_{3}$ (the tolerance factor, variance of sizes, and the local strains described in terms of the elongated Mn-O bonds). By using an example of kinetically stable, atomically-ordered layered-perovskites RBaMn$_{2}$O$_{6}$ we show that the increase of Curie temperature T$_{c}$ and enhancement of colossal magneto-resistive effect at room temperature, can be achieved through reduction of variance of sizes and local strains. Work at NIU was supported by the NSF-DMR-0302617 and at ANL by the U.S. DOE under contract No. DE-AC02-06CH11357. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W23.00002: Phase separation in (001) and (110) La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ epitaxial films J. Fontcuberta, I.C. Infante, F. S\'anchez, M. Wojcik, E. Jedryka, S. Estrad\'e, J. Arbiol, F. Peir\'o Recent reports on the formation of a highly conducting layer at the interface between two insulators have driven a strong interest to interface physics. To explore strain and polarity effects on electronic phase separation (PS), we have grown epitaxial films of La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ (LCMO) on (001) and (110) SrTiO$_{3}$ (STO) substrates. (001) LCMO/STO and (110) LCMO/STO interfaces differ on the polarity sequence. Measurements of the LCMO layers show that the (110) LCMO films display always ``better'' magnetic properties that their (001) LCMO counter-partners and do not show any traces of PS. Moreover, we have analyzed the properties of (001) and (110) thin LCMO films capped with a thin STO barrier and compared with those of bare LCMO films. It turns out that STO capping induces PS in (001)LCMO films but not in (110) films. Reasons for this asymmetric behaviour will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W23.00003: Crystal Structure and Physical Properties of Oxygen Composition Controlled La$_{1-x}$Sr$_{x}$MnO$_{3+\delta }$ Single Crystals. Yuui Yokota, Jun-ichi Shimoyama, Tetsuro Ogata, Atsushi Nakamura, Hiraku Ogino, Shigeru Horii, Kohji Kishio The La$_{1-x}$Sr$_{x}$MnO$_{3+\delta }$ is known to have various magnetic and crystal structures as functions of $x$. Although this system has relatively large oxygen nonstoichiometry, the effects of excess oxygen on the crystal structure and physical properties have not been well understood. In the present study, the crystal structure and physical properties of excess oxygen controlled La$_{1-x}$Sr$_{x}$MnO$_{3+\delta }$ single crystals were systematically studied. Single crystals with nominal compositions of La$_{1-x}$Sr$_{x}$MnO$_{3+\delta }$ (0.05 $\le \quad x \quad \le $ 0.2) were grown by the floating zone method. Thin plate-like crystals were obtained from the grown boules and controlled the oxygen content by post-annealing in various atmospheres. With increasing \textit{$\delta $}, the orthorhombic of the as-grown La$_{0.95}$Sr$_{0.05}$MnO$_{3+\delta }$crystal changed to the rhombohedral through pseudo-cubic orthorhombic. In addition, ferromagnetic behaviors appeared accompanying the CMR effect and $T_{C}$ increased as a function of \textit{$\delta $}. These are attributable to an increase of valence of Mn by excess oxygen. Relationships among the Sr substitution level, excess oxygen content, crystal structure and magnetic behaviors of La$_{1-x}$Sr$_{x}$MnO$_{3+\delta }$ will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W23.00004: Improved CMR properties of RE-doped (La,Sr)MnO$_{3}$ single crystals Jun-ichi Shimoyama, Tetsuro Ogata, Yuui Yokota, Hiraku Ogino, Shigeru Horii, Kohji Kishio The relationships among crystal structure, $T_{C}$ and CMR effect have been eagerly studied for (La,Sr)MnO$_{3}$ system mainly as a functions of the Sr composition, $x$ thus far. In the present study, we have attempted to improve the CMR properties near room temperature of the present system by optimizations of $T_{C}$ and phase transition temperature between orthorhombic and rhombohedral through RE mixing and elimination of excess oxygen, $i.e.$ cation vacancies, for (La$_{1-x}$Sr$_{x})$MnO$_{3}$ single crystals with $x$ = 0.2 and 0.25, which have higher$ T_{C}$ than room temperature and essentially high electronic conductivity. Crystal boules with nominal compositions of La$_{0.8-z}$RE$_{z}$Sr$_{0.2}$MnO$_{y}$ and La$_{0.75-z}$RE$_{z}$Sr$_{0.25}$MnO$_{y}$ (RE = Pr, Nd, Sm : $z$ = 0 $\sim $ 0.3) were grown by the floating zone method. Crystal structure of La$_{0.75-z}$Pr$_{z}$Sr$_{0.25}$MnO$_{3}$ at $\sim $300 K changed from rhombohedral ($z$ = 0, 0.15, 0.25) to orthorhombic ($z$ = 0.3) due to a decrease in mean ionic radius of A site. In addition, Pr-doping systematically decreased $T_{C}$. Similar tendencies were confirmed for Nd- or Sm-doped samples. The RE-doped samples exhibited large CMR ratio at $\sim $300 K comparable to that of La$_{0.825}$Sr$_{0.175}$MnO$_{3}$ and much higher conductivity reflecting high Sr concentration when phase transition temperature and $T_{C}$ were optimized. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W23.00005: Unconventional spin-dynamics in a phase separate, weakly disordered perovskite manganite Feng Ye, Jaime Fernandez-Baca, Pengcheng Dai, Hye-Jung Kang, Jeffrey Lynn, Chenglin Zhang, S.-W. Cheong The intense investigation of perovskite manganites has revealed a variety of fascinating properties. The phenomena known as phase separation, the coexistence at different length scales of ferromagnetic, charge/orbital order has been recently recognized as an intrinsic feature of several strongly correlated electron systems. Using inelastic neutron scattering, we have studied the spin dynamics of the archetypical material (La,Pr)$_{7/8}$Ca$_{3/8}$MnO$_{3}$, where competing ground states coexist at low temperature. The low-$T$ spin wave excitations at H = 0 and 2T are drastically different. We discuss this difference in terms of magnetic excitations from ferromagnetic clusters of different length scales. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W23.00006: Phase separation in Pr$_{0.55}$Ca$_{1.45}$MnO$_{4}$ evidenced by magnetic excitations Songxue Chi, Pengcheng Dai, Feng Ye, Jaime Fernandez-Baca, Hye Jung Kang, Jeffrey W. Lynn, Ying Chen, Yoshio Kaneko, Yoshinori Tokura At doping levels x$<$0.5, a coexistence of commensurate (CM) and incommensurate (ICM) magnetic peaks are observed in single-layered manganites Pr$_{1-x}$Ca$_{1+x}$MnO$_{4}$ with elastic neutron scattering. Temperature dependence measurements of the magnetic intensities with different energy resolutions indicate a glassy nature of the magnetic moments. The magnetic excitation measurements using inelastic neutron scattering on the x=0.45 system reveal both symmetric and asymmetric behaviors about the CM peak positions. This strongly suggests two types of magnetic excitations originated from separated phases: the CE-type magnetic phase and an additional electronic phase caused by extra electrons introduced into the CE template. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W23.00007: Signature of Magnetic Phase Separation in Pr$_{1-x}$Ca$_{x}$MnO$_{3}$. Dalgis Mesa, Hao Sha, Jiandi Zhang, F. Ye, P.C. Dai, J. A. Fernandez-Baca, J. W. Lynn, Y. Tomioka, Y. Tokura Elastic neutron scattering has been used to study the evolution of the long-/short-range charge-orbital (CO-OO), ferromagnetic (FM), and antiferromagnetic (AF) correlations in the single crystals Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ (x = 0.3, 0.35 and 0.4). In contrast with Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (PCMO30), both the long-range CO-OO and AF ordering show precipitous decrease in intensity below 30K, where the short-range FM clusters are formed in the Pr$_{0.65}$Ca$_{0.35}$MnO$_{3}$ (PCMO35). Those results provide clear evidence of magnetic phase separation. The doping dependence results of the short-range magnetic correlations indicate that there is a critical doping $x_{cr}$ (close to $x$ = 0.35) for the existence of phase separation in the CO-OO ground state. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W23.00008: Anomalous Short Range Charge Ordering in La$_{1-x}$Ca$_{x}$MnO$_{3}$ Jing Tao, S.J. Pennycook, Y. Zhu Long range charge ordered (CO) phases have been observed at low temperatures in manganites in certain doping ranges. Structural characterization of the long-range CO phase showed that the CO superlattice is always along the Pnma a axis in bulk samples [1]. Here we report the observations of a short range CO phase in La$_{1-x}$Ca$_{x}$MnO$_{3}$ samples as a function of cation concentration $x$, temperature and magnetic field using in-situ electron microscopy. We find short range CO nanoclusters with the CO superlattice in both perpendicular directions (a and c axes) in single crystal domains. The a and c axis nanoclusters have different densities, and show different dependences on temperature and magnetic field. Time evolution of the CO nanoclusters with the CO superstructure along the anomalous direction (c axis) is also recorded, which implies that the energy barrier between the two types of the CO structure is very small [2]. [1]. P. G. Radaelli \textit{et al}., PRB 59, 14440 (1999) [2]. Research sponsored by the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering and by appointment to the ORNL Postdoctoral Research Program administered jointly by ORNL and ORISE. Work at BNL was supported by the U.S. DOE/BES under Contract No. DE-AC02-98CH10886. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W23.00009: Spatially-Resolved X-Ray Microstructural Studies of Bulk Phase Separation in Manganites J.D. Budai, D.D. Sarma, W. Liu, J.Z. Tischler, B.C. Larson, G. Shenoy, D. Topwal, S-W. Cheong We have used spatially-resolved, 3D x-ray microdiffraction to study phase separation in two directionally-solidified, transition-metal oxide systems: (Y,Eu)MnO and (Lu,LaSr)MnO. Both systems exhibit micron-scale coexistence of separate single-crystal lamellar domains (hex/ortho and hex/rhomb respectively) when grown by a floating zone method. Micron-resolution 3D x-ray microscopy reveals the domain morphologies, lattice orientations, and local strain fields within the phase-separated eutectic systems. The orientations of the lamellae are consistent with energetic predictions and the formation of low-energy, semi-coherent interfaces. In addition, we observe a bias for larger strain fluctuations in one phase. More generally, the microstructural features observed experimentally in these well-defined, micron-scale eutectic domains provide clues to the domain interactions believed to exist in similar, more homogeneous, nanoscale manganite systems. Structural studies at the nanoscale will be enabled in the future by advances in x-ray focusing optics. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W23.00010: Strain and Curent Induced Multiphase Coexistence in (La$_{0.5}$Pr$_{0.5}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ Probed by Magnetic Force Microscopy Frank Ruzicka, Changbae Hyun, Junwei Huang, Alfred Lee, Alex de Lozanne, Tara Dhakal, Jacob Tosado, Amlan Biswas The ferromagnetic (FM) domain structure of a thin film (La$_{0.5}$Pr$_{0.5})_{0.67}$Ca$_{0.33}$MnO$_{3}$ sample grown on a (110) NdGaO$_{3}$ (NGO) substrate was investigated using low-temperature magnetic force microscopy with temperature control, an external magnetic field and an electric field across the sample. We observed that the FM domains form stable patterns with in-plane magnetization at 78K. At higher temperatures the system enters a fluid phase separated (FPS) state. The FM domains change as the voltage across the sample increases. We believe that competition between a charge-ordered insulating (COI) phase and a ferromagnetic metallic (FMM) phase exists in this FPS state and the COI phase may be driven to an FMM phase by an electric field. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W23.00011: Domain Mapping of a Ca-doped Manganite Joshua Turner, K. Jessica Thomas, Mary Upton, John Hill, Jean Jordan-Sweet, Yoshinori Tokura, Yasuhide Tomioka, Stephen Kevan In the last few years, disorder has emerged as a key without which the colossal magnetoresistance (CMR) effect would not exist. Single crystals represent the simplest arenas to observe and study the peculiarities central to the manganites. This is in contrast to polycrystalline compositions for instance, where rampant strain fields can veil fundamental physics. By micro-focusing x-rays through a glass capillary, we have performed a microtopography technique to map the crystallographic domain structure of the Ca-doped manganite crystal, PCMO. This technique serves as a domain-mapping alternative tool to TEM that does not require challenging sample preparation procedures. Surprisingly, we find micron size domain structure. We suggest that the separation of crystallographic domains by twin boundaries is more relevant than previously believed. These boundaries could affect the large-scale, sub-micron size disorder that has been observed recently, and also may play a role in the heterogeneous nature of the CMR effect. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W23.00012: Temperature-Dependent Electronic Structure of the Colossal Magnetoresistive Manganite La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ via Hard X-Ray Photoemission Francesco Offi, Norman Mannella, Giancarlo Panaccione, Tommaso Pardini, Andrea Fondacaro, Piero Torelli, Simo Huotari, Mark West, John Mitchell, Charles Fadley We have studied single-crystal La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ with hard x-ray photoemission (HXPS) at an excitation energy of 7.7 keV. These more bulk-sensitive measurements reveal low-binding-energy satellites in the Mn 2p$_{3/2}$, 3s, and 3p core spectra that are consistent with previously observed satellites in Mn 2p$_{3/2}$ for other strongly-correlated materials, and which have been interpreted in terms of non-localized screening effects. The Mn 3s spectrum is agreement with recent soft x-ray measurements (Mannella et al., P.R.L. \textbf{92}, 166401 (2004)) in showing an increased multiplet splitting at temperatures 100 K or more above T$_{C}$, although the effect is here reduced. Core-normalized valence-band spectra exhibit enhancement of intensity at high temperature that is evidence of localization of Mn 3d-derived charge, in agreement with prior soft x-ray work. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W23.00013: Unusual temperature dependence of the oxygen-isotope effect on the exchange-energy of La$_{1-x}$Ca$_{x}$MnO$_{3}$ Guo-meng Zhao, John Mann We report magnetic susceptibility $\chi(T)$ measurements on oxygen-isotope exchanged La$_{1-x}$Ca$_{x}$MnO$_{3+y}$ up to 700 K. The $1/\chi(T)$ data show that the ferromagnetic exchange-energy $J$ depends strongly on the oxygen-isotope mass. The isotope effect on $J$ decreases with temperature up to 400 K and then increases again with temperature above 400 K. This unusual temperature dependence cannot be explained by existing theories of the colossal magnetoresistance effect for doped manganites. We suggest that a correct model for description of the physics of manganites should be based on double-exchange and the formation of localized bipolarons in the paramagnetic state. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W23.00014: Origin of the Non-Linear Pressure Effects in Perovskite Manganites Zhiqiang Chen, Trevor Tyson, Ken Ahn, Zhong Zhong, Jinzhu Hu High-pressure resistivity and x-ray diffraction measurements were conducted on La$_{0.85}$MnO$_{3-\delta }$ to $\sim $6 GPa and $\sim $7 GPa, respectively. At low pressures the metal-insulator transition temperature (T$_{MI})$ increases linearly up to a critical pressure, P* $\sim $ 3.4 GPa, followed by reduction of T$_{MI}$ at higher pressure. Analysis of the bond distances and bond angles reveal that a bandwidth increase drives the increase of T$_{MI}$ below P*. The reduction of T$_{MI}$ at higher pressures is found to result from Jahn-Teller distortions of the MnO$_{6 }$octahedra. The role of anharmonic interatomic potentials is discussed. [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W23.00015: Phonon-induced magnetic deflagration and detection of very fast CMR in manganites Ferran Macia, Joan Manel Hernandez, Guillem Abril, Alberto Hernandez-Minguez, Francisco Parisi, Paulo V. Santos, Javier Tejada In this work we describe experiments in which we have used surface acoustic waves to induce controlled magnetic avalanches in (La, Pr)-based manganites. The avalanches propagate inside the sample following the law of the magnetic deflagration, and occur at well determined values of the temperature and the applied magnetic field, that depend on the phase separation fraction. Another important point is that the magnetic avalanche is accompanied by colossal variation of the electrical resistance in about 0.1 ms. F. Macia \textit{et al.}, Phys. Rev. B \textbf{76}, 174424 (2007). F. Macia \textit{et al}., Phys. Rev. B, submitted [Preview Abstract] |
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