Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session L43: Focus Session: Phase Complexity and Enhanced Functionality in Magnetic Oxides I |
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Sponsoring Units: DMP GMAG Chair: David Mandrus, University of Tennessee Room: LACC 150C |
Tuesday, March 22, 2005 2:30PM - 2:42PM |
L43.00001: Evidence of magnon phonon interaction in La$_{0.70}$Ca$_{0.30}$Mn$O_3$ Jaime Fernandez-Baca, Mark Hagen, Pengcheng Dai, Jiri Kulda, Feng Ye, Y. Tomioka, Y. Tokura One of the unresolved issues in the understanding of the spin dynamics of the CMR manganites is the anomalous softening and damping of the magnons near the zone boundary observed in A$_{1-x}$B$_{x}$MnO$_{3}$ (A=La, Pr, Nd, etc; B=Ca, Sr, etc. x $\sim $ 0.30). [1], [2]. While many theoretical explanations have been formulated to explain this phenomena we proposed that these anomalous features are related to a strong magnon-phonon coupling expected from the strong magnetoelastic interactions in the CMR manganites.[2] In this talk we present neutron scattering evidence for the coupling of the magnons and a severely damped branch of optical phonons in La$_{0.70}$Ca$_{0.30}$MnO$_{3}$. ORNL is managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725. [1] H. Y. Hwang et al, Phys. Rev. Lett 80, 1316 (1998) [2] P. Dai et al, Phys. Rev. B 61, 9553 (2000) [Preview Abstract] |
Tuesday, March 22, 2005 2:42PM - 2:54PM |
L43.00002: Electron-phonon coupling of La2-2xSr1+2xMn2O7 revealed by ARPES Z. Sun, Y.-D. Chuang, J.F. Douglas, A.V. Fedorov, H. Zheng, J.F. Mitchell, T. Kimura, Y. Tokura, D.S. Dessau Angle-resolved photoemission experiments were performed on Bi-layer manganite La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ in the doping regions where strong CMR effects are observed. The low temperature electronic band structure near the (pi, 0) point shows a clear kink structure with an energy scale near 65 meV. We attribute this kink structure to electrons coupling to the LO bond-stretching phonons with a coupling constant $\sim $ 1. Moving along the straight nested segments of Fermi surface near the zone boundary, we didn't find any change of the kink energy or coupling strength. This indicates the nested Fermi surface is strongly correlated to electron-phonon coupling, which should play an important role in the rich physics of manganites. [Preview Abstract] |
Tuesday, March 22, 2005 2:54PM - 3:06PM |
L43.00003: Acoustic phonon instabilities in La$_{1-x}$Ca$_{x}$MnO$_{3}$ Y.H. Ren, R. Merlin, C.S. Hong, N.H. Hur, Y.F. Hu, Qi Li, G. Luepke We report on time-resolved ultrafast optical measurements on a La$_{1-x}$Ca$_{x}$MnO$_{3}$ (LCMO) single crystal ($x$= 0.3) and thin film ($x$= 0.33). The differential reflectivity shows coherent GHz oscillations due to the excitation of longitudinal acoustic phonons. The wavelength dependence of the period of such oscillations allow us to determine the sound velocity. The measured GHz sound velocity is 30\% - 40\% larger than the sound velocity in the MHz range as determined by resonant ultrasound measurements [Jin et al., PRL 90, 036103 (2003)]. Temperature- dependent data show that the sound velocity anomaly disappears at T \symbol{126}350 K, a value that is close to the temperature for which charge-ordered clusters form. [Preview Abstract] |
Tuesday, March 22, 2005 3:06PM - 3:42PM |
L43.00004: Spatially Resolved Photoemission Spectroscopy to Probe Electronic Phase Separation in Manganites and Related Compounds Invited Speaker: Manganese oxides that exhibit colossal magnetoresistance (CMR) are often characterised by a competition of different electronic phases that critically influence their properties and leads to the coexistence of spatially separated competing phases. Despite extensive experimentation, characteristic length-scales associated with phase coexistence remains an important open question. While theoretical work has pointed to a nanometric length-scale, experiments have uncovered multiple length-scales ranging from the atomic to the sub-micron, covering many orders of magnitude. The role of chemical inhomogeneity in driving this phenomenon is not well understood. Moreover, these early experiments were carried out on polycrystalline and thin film specimens. Here we use a spatially resolved, direct spectroscopic probe for electronic structure with an additional unique sensitivity to chemical compositions to investigate high quality single crystal sample of La$_{1/4}$Pr$_{3/8}$Ca$_{3/8}$MnO$_{3}$. The formation of distinct electronic domains is observed in absence of any perceptible chemical inhomogeneity, where the relevant length-scale is at least an order of magnitude larger than all previous estimates. The present results, exhibiting memory effects in the domain morphology, suggest that electronic domain formation is intimately connected with long-range strains, often thought to be an important ingredient in the physics of this effect. Additionally, we have also applied this technique to a variety of related materials, such as (LuMnO$_{3})_{0.79}$(La$_{5/8}$Sr$_{3/8}$MnO$_{3})_{0.21}$, and Sr$_{2}$Fe$_{x}$Mo$_{1-x}$O$_{6}$. Our preliminary results in all these cases suggest that the existence of spatially inhomogeneous electronic phases plays important roles in determining many of the interesting properties of such systems. This work is carried out in collaboration with M. Bertolo, G. Cautero, S-W. Cheong, A. Fujimori, T. Y. Koo, S.R. Krishnakumar, U. Manju, S. Ray, S. La Rosa P. A. Sharma and D. Topwal. [Preview Abstract] |
Tuesday, March 22, 2005 3:42PM - 3:54PM |
L43.00005: Magnetic and Electronic Properties of Ln$_{1-x}$Sr$_x$CoO$_3$ D. Stauffer, C. Leighton The magnetic and electronic properties of bulk, polycrystalline, Ln$_{1-x}$Sr$_{x}$CoO$_{3}$ (Ln=La, Nd and Pr, 0$\le $x$\le $0.6) were investigated. The La and Nd systems show a crossover from insulating glassy behavior to metallic ferromagnetism at x=0.18. The only differences between the two systems are the lower transition temperatures for Nd (due to reduced bandwidth) and the antiferromagnetic alignment of Nd and Co, inducing ferrimagnetism. Pr$_{1-x}$Sr$_{x}$CoO$_{3}$ however, is radically different. A similar cluster glass to ferromagnet crossover occurs at low doping but at higher doping the system exhibits a second magnetic transition [1], well below $T_{C}$. Through a systematic investigation as a function of composition we have discovered that this effect is maximized at half-doping, vanishing completely at x$<$0.40 and x$>$0.60. Possible explanations (including ferrimagnetism, spin-state transitions, phase competition, and charge/orbital ordering) are discussed in light of the neutron diffraction, transport and magnetometry results. We acknowledge support from the ACS Petroleum Research Fund. [1] R. Mahendiran and P. Schiffer, Phys. Rev. B. \textbf{68} 024427 (2003). [Preview Abstract] |
Tuesday, March 22, 2005 3:54PM - 4:06PM |
L43.00006: Transverse susceptibility as a probe of spin and charge dynamics in LSMO single crystals H. Srikanth, P. Poddar, G. T. Woods, Y. Mukovskii AC and RF susceptibility measurements are excellent probes to investigate the charge and spin dynamics in novel materials such as half-metals, exchange bias materials, intermetallic Kondo systems and manganites. We will present temperature and field-dependent transverse susceptibility (TS) measurements on {\$}La$_{1-x}$ Sr$_{x}$MnO$_{3}${\$}; (x=0.1, 0.15, 0.25) single crystals at radio frequency (10 MHz) using a precise resonant tunnel-diode oscillator technique. These dynamic experiments probe the coupled electronic, magnetic and structural transitions in LSMO. The effective magnetic anisotropy fields are directly probed over a wide range in temperature, from the ferromagnetic Curie temperature to low temperatures well into the charge-ordered state in the insulating samples. Our experiments reveal distinct features at characteristic temperatures within the charge-ordered state that indicate a change in magnetic anisotropy associated with the dynamics of charge ordering itself. Overall, we demonstrate the unique advantages offered by RF susceptibility measurements in probing magnetic phase transitions in CMR oxides and other novel materials. [Preview Abstract] |
Tuesday, March 22, 2005 4:06PM - 4:18PM |
L43.00007: Optical and magneto-optical studies of ferromagnetic manganese oxides thin films H.L. Liu, Kenneth Y. -J. Zhang, K.S. Lu, L. Uba, S. Uba, W. -J. Chang, J.- Y. Lin, L.M. Wang We report on the optical and magneto-optical (MO) properties of ferromagnetic La$_{0.7}$Sr$_{0.3}$MnO$_3$, La$_{0.7}$Ca$_{0.3} $MnO$_3$, La$_{0.7}$Ce$_{0.3}$MnO$_3$, and La$_{0.7}$MnO$_3$ thin films epitaxially grown on SrTiO$_3$ substrate. The optical reflectance and transmittance of the samples were measured over a broad frequency range (50-52000 cm$^{-1}$) and at temperatures between 20 and 340 K. To extract the optical constants of the films, we analyzed all of the layers of this thin-film structure using a Drude-Lorentz model. The MO polar Kerr spectra of the samples were measured in an applied magnetic field of 1.5 Tesla between 0.74 and 5.6 eV. The off-diagonal components of the dielectric tensor were calculated by analyzing Kerr rotation, ellipticity, and the determined diagonal elements of the dielectric tensor. These data clearly show that accurate values of diagonal and off-diagoanl components of the dielectric tensor are important for the spin-polarized band-structure studies in the ferromagnetic manganese oxides thin films. [Preview Abstract] |
Tuesday, March 22, 2005 4:18PM - 4:30PM |
L43.00008: Phase diagram of La1-xCaxMnO3 thin films in 0.40 = x = 0.45 D. Sanchez, L.E. Hueso, J.C. Chapman, N.D. Mathur The $x-T$ phase diagram of La$_{1-x}$Ca$_{x}$MnO$_{3}$ films is expected to be very different from that of polycrystalline samples, due to epitaxial strain. Two series of films differing in composition by $x$=0.01 were prepared in the range 0.40 $\le $ x $\le $ 0.45 by pulsed laser deposition on SrTiO$_{3}$ (001) and NdGaO$_{3}$ (001) substrates. Ferromagnetism was found in all samples, with the ferromagnetic fraction decreasing with increasing $x$. Low temperature metallic behaviour was only observed for $x \quad \le $ 0.41. [Preview Abstract] |
Tuesday, March 22, 2005 4:30PM - 4:42PM |
L43.00009: TEM Study of Two-Dimensional Incommensurate Modulation in La(2-2x)Ca(1+2x)Mn2O7 (0.6 Leonid A. Bendersky, Ian D. Fawcett, Martha Greenblatt Ruddlesden-Popper n = 2 compounds La$_{2-2x}$Ca$_{1+2x}$Mn$_{2}$O$_{7}$ with 0.6 x 0.8 were synthesized by a citrate gel technique. Electron diffraction identified the presence of two-dimensional incommensurate modulations. The modulation is sensitive to electron-beam irradiation and rapidly degrades under a focused beam. In-situ heating experiments indicated the existence of possible tetragonal-to-incommensurate phase transition around 350 $^{o} $C. High-resolution imaging proves that the modulation in the real space is truly two-dimensional and incommensurate. Such modulation has never been observed before for La-Ca-Mn-O or any other perovskite system. The best approximation of the incommensurate superstructure is by a commensurate lattice \textbf{a}$_{x}$' = \textbf{a}$_{xt }$+ 2\textbf{a}$_ {yt}$; \textbf{a}$_{y}$' = -2\textbf{a}$_{xt}$ + \textbf{a}$_{yt}$ ; \textbf{c}' = \textbf{c}$_{t}$ (t-tetragonal I4/mmm).$_{ }$ Such superstructure could be satisfied with a 4:1 two-dimensional ordering in a (001) plane. For the studied compositions, such 4:1 ordering is plausibly described by charge ordering between Mn$^{4+}$ and Mn$^{3+ }$ions. Structural models of the charge ordering between Mn$^{4+}$ and Mn$^{3+ }$ions are suggested. [Preview Abstract] |
Tuesday, March 22, 2005 4:42PM - 4:54PM |
L43.00010: Variable temperature magnetic force microscopy of patterned La$_{0.6}$Ca$_{0.4}$MnO$_3$ devices Casey Israel, Changbae Hyun, Alex de Lozanne, B.B. van Aken, D. S\'{a}nchez, L. Granja, L.E. Hueso, N.D. Mathur Patterned La$_{0.6}$Ca$_{0.4}$MnO$_3$ films have been fabricated into planar devices with the intention of studying the properties of domain walls in colossal magnetoresistive films doped near phase boundaries. The phase balance is very sensitive near phase boundaries and may lead to interesting phenomena associated with domain walls. The field dependent transport data from these devices show features which suggest domain configuration changes that can be imaged by magnetic force microscopy (MFM). We present variable temperature MFM images of these devices in applied fields and correlate the field dependent micromagnetic structure to the field dependent transport data. [Preview Abstract] |
Tuesday, March 22, 2005 4:54PM - 5:06PM |
L43.00011: Transport Noise in Ordered Layered LCMO M.B. Weissman, Akilan Palanisami, Maitri Warusawithana, J. Eckstein Both intrinsic alloy disorder and strain effects have proposed as important sources of the mixed-phase colossal magnetoresistive regime in manganites We have measured transport properties in films of La$_{2/3}$Ca$_ {1/3}$MnO$_{3}$ prepared by atomic-layer-by-layer epitaxy on nearly lattice- matched NGO substrate, both as standard random alloys and as nominally ordered multilayers of LaMnO$_{3}$ and Ca$_{1/3}$MnO$_{3}$. The ordered multilayer samples show a similar transition to the alloys, but with a slightly reduced T$_{C}$. The width in T of the coexistence regime, either determined form the standard R(T) or from resistance noise, is actually slightly greater for the ordered samples than for the alloy. In both cases it is much less than found for alloys under tensile strain on STO or films on STO with e-beam damage. The apparent interpretation is that pointlike disorder plays a smaller role than do strain effects and correlated disorder. Other measurements of ac magnetoresistance, non-linear transport, and noise statistics also indicate the importance of strain interactions. [Preview Abstract] |
Tuesday, March 22, 2005 5:06PM - 5:18PM |
L43.00012: High magnetic field phase diagram of multiferroic DyMn$_{2}$O$_{5}$ up to 45 T K. H. Kim, T. H. Kim, S. Y. Haam, N. Hur, S. Park, S-W. Cheong, Y. Jo, J.-G. Park, A. Migliori Strong magnetoelectric coupling in multiferroic crystals such as ReMn$_{2}$O$_{5}$ (Re=rare earth) has provided unprecedented opportunity to manipulate ferroelectric (FE) polarization using magnetic fields. Most of investigations to dates have yet been limited to a rather low magnetic field region. Herein, we present the first high magnetic field (B) versus temperature phase diagram of DyMn$_{2}$O$_{5}$ in magnetic fields up to 45 T and temperatures below 50 K, determined from the dielectric constant, pyroelectric, and magnetoelectric current measurements using various magnets; superconducting magnets up to 17 T, a dc resistive magnet up to 33 T, and a mid-pulse magnet up to 45 T. Our phase diagram reveals that at least 4 different kinds of FE domains, which show strong temperature-and field-history- dependence, develop at low temperatures below 40 K, and exhibit dramatic evolution under B. For example, as B increases at 4 K, FE polarization shows successive flopping at B$\sim$2, 7 T and 22 T, producing large dielectric constant changes, $\Delta$$\epsilon$(B)/$\epsilon$(0 T) $\sim$15, 70, 20\%, respectively. We discuss the complex phase diagram in the context of strong spin-lattice coupling that is linked to the exchange interaction between Dy f- and Mn d-spins. [Preview Abstract] |
Tuesday, March 22, 2005 5:18PM - 5:30PM |
L43.00013: Magnetic and charge correlations in the 2D Hubbard model on a triangular lattice Matthew Enjalran The high temperature superconductors have motivated numerous theoretical studies of strongly correlated many-body systems for nearly two decades. The richness of the phase diagram of these materials belies their relatively simple quasi-two-dimensional structure of stacked CuO$_2$ planes, where copper ions form a square lattice. With the recent discovery of superconductivity in Na$_x$CoO$_2$ $\cdot$ yH$_2$O, the physics community has an experimental system where strong electron correlations in a quasi-two-dimensional environment are further complicated by geometric frustration arising from the triangular lattice structure of the cobalt ions. Motivated by the interesting experimental phase diagram of Na$_x$CoO$_2$ $\cdot$ yH$_2$O, we investigate the 2D Hubbard model on a triangular lattice at different filling fractions. We report preliminary results from a numerical Hartree-Fock calculation for the magnetic and charge correlations in our model. We also discuss the potential application of the constrained path quantum Monte Carlo (CPQMC) method to the study of frustrated 2D Fermi systems. [Preview Abstract] |
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