Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N25: Focus Session: Novel and Complex Oxides: Cobaltites and Manganites |
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Sponsoring Units: DMP Chair: Rongying Jin, ORNL Room: LACC 501A |
Wednesday, March 23, 2005 8:00AM - 8:12AM |
N25.00001: Anisotropic Magnetic and Magnetotransport Properties of $EuBaCo_{2}O_{5.5}$ Single Crystals Zhixian Zhou, Pedro Schlottmann Magnetization, resistivity, and magnetoresistance were measured on detwinned EuBaCo$_{2}$O$_{5.5}$ single crystals over a wide range of dc magnetic fields (up to 33 T) and temperature (from 4 K to 300 K). EuBaCo$_{2}$O$_{5.5}$ has a layered structure (along the c-axis) with all the Co-ions in a trivalent state. The isothermal magnetization increases gradually until a critical field, where it undergoes a transition towards saturation for H $\vert \vert $ a-axis, while no similar transition is observed for H along b or c-axes. The critical field increases linearly with decreasing temperature, reaching 25 T at 4 K. From the M(H) data, a saturation moment of approximately 0.8$\mu _{B}$/Co is determined. Coinciding with the field induced transition in the magnetization, the isothermal resistivity shows a steep decrease for H $\vert \vert $ a-axis. The correlations between the magnetic order and the large negative MR will be discussed. We are grateful to the late Jack E. Crow who strongly influenced this work. *This work was carried out at the NHMFL, which is partially supported by the National Science Foundation through Cooperative Agreement No. DMR-0084173 and the State of Florida. [Preview Abstract] |
Wednesday, March 23, 2005 8:12AM - 8:24AM |
N25.00002: Three Dimensional Magnetism in Na$_x$CoO$_2$ Michelle Johannes, Igor Mazin, David Singh Recent neutron studies reveal an underlying A-type antiferromagnetic order and a surprisingly three dimensional magnetism in Na$_x$CoO$_2$ for x$>$0.7. We look carefully at interplanar hopping in this compound, comparing supercell and virtual crystal approximation (VCA) calculations, and find that the formation of an sp$^2$ hybrid on the Na ion plays an important role in the magnetic coupling between Co ions in different layers. The specific ($2b$ vs. $2d$) position of Na ions can change the hopping integral, and therefore the superexchange, between planes indicating that Na ordering may be related to magnetic order. By reformulating a linear spin wave model to account for more than one interplanar neighbor, we show that the isotropy of magnetic interactions is due not to isotropic exchange constants but rather to the ability of each Co ion to interact with seven other Co ions in the next plane. [Preview Abstract] |
Wednesday, March 23, 2005 8:24AM - 8:36AM |
N25.00003: Antiferromagnetic Spin Waves and Pr Crystal Field Excitations in Pr$_{0.5}$Sr$_{0.5}$MnO$_{3}$ V. V. Krishnamurthy, J. L. Robertson, M. D. Lumsden, G. J. Mankey, J. F. Mitchell Neutron scattering investigations reveal three interesting features of antiferromagnetism in the doped manganite Pr$_{0.5}$Sr$_{0.5}$MnO$_{3}$. The intensity of the (0.5 0 0.5) antiferromagnetic (AF) Bragg peak shows that the AF domains exist between 150 and 190 K in the ferromagnetic state. The spin wave dispersion of the Mn sub-lattice measured at 20 K in the wave vector range of (0.5 0 0.5) to (2 0 2) along the AF coupling direction could be well described by the Heisenberg model with nearest neighbor exchange interactions and single-ion anisotropy. The AF coupling and the single ion anisotropy energy of Pr$_{0.5}$Sr$_{0.5}$MnO$_{3 }$ are comparable to those found in LaMnO$_{3}$, suggesting the same of order of magnitude of the gap in the dispersion at the zone center. The ferromagnetic coupling of Pr$_{0.5}$Sr$_{0.5}$MnO$_{3}$ is smaller by a factor of 3 as compared to that of LaMnO$_{3 }$ resulting in a smaller amplitude of the spin waves in the former. Pr crystalline field (CF) excitations in the AF state are found to be different from those in the ferromagnetic state suggesting the renormalization of at least one CF excitation of Pr due to an interaction with the spin waves of Mn near the zone boundary. Funded by DOE. [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 9:12AM |
N25.00004: Doped Cobaltites: Phase Separation, Intergranular Giant Magnetoresistance, and Glassy Transport Invited Speaker: We have used magnetometry, transport, Nuclear Magnetic Resonance (NMR), Small Angle Neutron Scattering (SANS), and Transmission Electron Microscopy (TEM) to investigate magnetoelectronic phase separation in La$_{1-x}$Sr$_{x}$CoO$_{3}$. This material shows a crossover from a glassy phase at low doping to ferromagnetism (F) above x = 0.18, as well as a simultaneous transition from insulator to metal. NMR confirms magnetic phase inhomogeneity with low spin non-magnetic, glassy, and F regions coexisting spatially. SANS reveals 25 {\AA} F clusters forming in a matrix of non-F insulator at low doping, eventually leading to a percolation transition to long-range F order at x $>$ 0.18. In single crystals, this formation of isolated clusters leads to a hysteretic negative MagnetoResistance (MR) at low temperatures, which has field, temperature, and doping dependencies consistent with an intergranular Giant MagnetoResistance (GMR) effect. We argue that this system is a naturally forming analog to the artificial structures fabricated by depositing nanoscale F particles in a metallic or insulating matrix, i.e. this material displays an intergranular GMR effect without the deliberate introduction of chemical interfaces. The formation of nanoscopic F clusters also gives rise to glassy transport phenomena that are reminiscent of relaxor ferroelectrics. The transport properties show a bifurcation of field cooled and zero field cooled temperature traces, slow response to changes in magnetic fields, and, most notably, a ``waiting time'' effect that can be observed directly in the resistivity. \textbf{Acknowledgements:} ACS Petroleum Research Fund, UMN NSF MRSEC. \textbf{Co-Authors:} J. Wu, J. Lynn, C. Glinka, J. Burley, H. Zheng, J. Mitchell, W. Moulton, M. Hoch, P. Kuhns, A. Reyes, C. Perrey, N. Munoz, R. Thompson and B. Carter. [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:24AM |
N25.00005: Electron-hole excitations in CaMnO$_{3}$ and LaMnO$_{3}$(*) Oscar D. Restrepo, Adolfo G. Eguiluz The electron-hole excitations in CaMnO$_{3}$ and LaMnO$_{3}$ are investigated via ab initio techniques (time-dependent density-functional theory). The ground state is described within the LDA+U “correlated-band structure” method. The electron dynamics is handled within the random-phase approximation (RPA). The loss spectrum in both materials is dominated by a striking “collective” excitation; the same is directly related to the underlying electronic structure, as its energy is a signature of the relative location of the upper and lower Hubbard bands. The physics of the dynamical screening (the spectral weight of the leading excitation, its remarkable dependence on wave vector (both on the magnitude and direction of q, etc.) is controlled by d-d transitions and the microscopic crystal local fields. Our predictions can be readily verified via measurements of the dynamical structure factor with inelastic scattering of hard x-rays --providing a direct test of the quality of the LDA+U ground state and the RPA dynamics. In fact, there is qualitative agreement with a recent investigation for LaMnO$_{3}$ with resonant x-ray scattering (**). (*) Research supported by NSF Grant ITR-DMR 0219332 (**) S.Grenier et al., cond-mat/0407326. [Preview Abstract] |
Wednesday, March 23, 2005 9:24AM - 9:36AM |
N25.00006: Doping Evolution of the Electronic Structure of Bilayer Colossal Magnetoresistive Manganites C. M. Jozwiak, G.-H. Gweon, J. Graf, S.Y. Zhou, H. Zheng, J. F. Mitchell, A. Lanzara Manganites have been the subject of great current interest not only because they exhibit the colossal magnetoresistance (CMR) effect, but also because they display a wide variety of magnetic properties and undergo several phase transitions from paramagnetic insulator (PMI) to ferromagnetic metals (FMM). Here we present a detailed momentum, doping and temperature dependent study of the electronic properties of bilayer manganites La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$, by means of angle resolved photoemission spectroscopy. Differences and similarities between different doping are discussed. In particular we will address how the electronic structure evolves from the PMI to the FMM phase as well as the emergence of the CMR phase from the doping evolution. [Preview Abstract] |
Wednesday, March 23, 2005 9:36AM - 9:48AM |
N25.00007: Extracting d-orbital information from Magnetic Compton experiments in bilayer manganites B. Barbiellini, P.E. Mijnarends, S. Kaprzyk, A. Bansil, Yinwan Li, P.A. Montano, J. F. Mitchell Magnetic Compton profiles (MCPs) have been measured for the colossal magnetoresistance double layer manganite La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ along various crystallographic directions over a wide range of temperatures and magnetic fields. The experimental results are interpreted via first-principles computations of the magnetic momentum density and the MCPs. The usefulness of the so called $B({\bf r})$ function, obtained by a one-dimensional Fourier transform of the MCP, is emphasized [1]. In particular, the form of $B({\bf r})$ for momentum transfer along the [110] direction is found to contain a prominent dip at around 1 a.u., whose depth is shown to provide a sensitive measure of the population of $e_g$ electrons of $d_{x^2-y^2}$ symmetry in the system. Work supported in part by the USDOE.\\ $\mbox{[1]}$ Yinwan Li, P. A. Montano, J.F. Mitchell, B. Barbiellini, P. E. Mijnarends, S. Kaprzyk and A. Bansil, Phys. Rev. Lett. {\bf 93}, 207206 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:24AM |
N25.00008: Orbital effects in cobaltites by neutron scattering Invited Speaker: The orbital degree of freedom can play a central role in the physics of transition metal perovskite oxides because of its intricate coupling with other degrees of freedom such as spin, charge and lattice. In this talk the case of La$_{1-x}$Sr$_{x}$CoO$_{3}$ will be presented. Using elastic and inelastic neutron scattering, we investigated the thermal evolution of the local atomic structure and lattice dynamics in the pure sample and with the addition of charge carriers as the system crosses over from a paramagnetic insulator to a ferromagnetic metal. In LaCoO$_{3}$, the thermal activation of the Co ions from a nonmagnetic ground state to an intermediate spin state gives rise to orbital degeneracy. This leads to Jahn-Teller distortions that are dynamical in nature. Doping stabilizes the intermediate spin configuration of the Co ions in the paramagnetic insulating phase. Evidence for local static Jahn-Teller distortions is observed but without long-range ordering. The size of the JT lattice is proportional to the amount of charge. However, with cooling to the metallic phase, static JT distortions disappear for x $\le $ 30 {\%}, the percolation limit. This coincides with narrowing of two modes at $\hbar \omega =22\,and\,24\,meV$ in the phonon spectrum in which we argue is due to localized dynamical JT fluctuations$^{1}$. The implications of the orbital effects to the structural and magnetic properties will be discussed. $^{1}$D. Louca and J. L. Sarrao, Phys. Rev. Lett. \textbf{91,} 155501 (2003). [Preview Abstract] |
Wednesday, March 23, 2005 10:24AM - 10:36AM |
N25.00009: Direct crystallographic evidence of charge ordering in the novel double perovskite mixed-valence (NaMn3)Mn4O12 A. Prodi, B. Schmitt, F. Gozzo, Q. Huang, J.W. Lynn, F. Bolzoni, E. Gilioli, F. Licci, M. Marezio, A. Gauzzi By means of high-resolution synchrotron X-ray powder diffraction measurements, we studied in detail the temperature-dependent crystal structure of the mixed-valence manganese oxide, (NaMn$_{3})$Mn$_{4}$O$_{12}$. At 176 K, we observed a static ordering of the Jahn-Teller distortion of the Mn$^{3+}$O$_{6}$ octahedra that drives a cubic-monoclinic structural transition concomitant to the Mn$^{3+}$-Mn$^{4+}$ charge ordering of the octahedral B-sites of the double-perovskite structure AA'$_{3}$B$_{4}$O$_{12}$. This transition is followed by a CE-type magnetic ordering of these sites at 125K and by an independent antiferromagnetic ordering of the Mn A' sites at 90 K. Remarkably, both neutron [1] and X-ray data show that the charge ordering is intrinsic to the low symmetry phase, resulting in the setting up of two distinct MnO$_{6}$ octahedra with very different average Mn-O distances. A bond valence sum analysis shows that these two Mn sites exactly correspond to 3+ and 4+ formal valence states. This direct evidence of charge disproportionation has never been reported in half-doped manganites, where charge order has been believed to occur only from controversial analysis of structural modulations. Ref: [1] A. Prodi \textit{et al., Nature Materials} \textbf{3}, 48 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 10:36AM - 10:48AM |
N25.00010: On the Microstructure of the Charge Density Wave Observed in La1-xCaxMnO3 James Loudon, Susan Cox, Neil Mathur, Paul Midgley We have used low temperature (90~K) transmission electron microscopy to investigate the `charge ordering' modulation in the mixed valent manganite, La$_{1{\-}x}$Ca$_{x}$MnO$_{3}$. It has been stated that Mn$^{3+}$ and Mn$^{4+}$ ions order at low temperature to produce a structural modulation composed of supercells whose size is an integer multiple of the unmodulated unit cell. Here, we use convergent beam electron diffraction to show that the periodicity of the modulation need not be an integer multiple of the undistorted cell, even on the smallest scales. We therefore suggest that this modulation is a charge density wave with a uniform periodicity. We show that the modulation wavevector lies close to the \textbf{a*} axis of the crystal but need not be exactly collinear. A typical grain of size 0.5~$\mu $m in La$_{0.48}$Ca$_{0.52}$MnO$_{3}$ had a wavevector which varied on a scale of tens of nanometres with an average of $<$\textbf{q}$>$~=~0.450\textbf{a}$*$ and a standard deviation \textit{$\Delta $q}~=~0.004$a*$ in its magnitude and \textit{$\Delta \theta $}~=~0.56\r{ } in its direction at 90~K. The magnitude of the wavevector in this composition fell by 20{\%} as the temperature was increased from 90~K to room temperature. This change occurred by nucleation and growth. Although weak, the modulation was still present at room temperature, some 30~K above the `charge ordering temperature'. [Preview Abstract] |
Wednesday, March 23, 2005 10:48AM - 11:00AM |
N25.00011: Structural and magnetic properties of A-site ordered manganites RBaMn$_2$O$_6$ (R=Pr, Nd, Pr$_{1/2}$Nd$_{1/2}$) Yang Ren, H. Churchill, B. Dabrowski, J. Mais, S. Kolesnik, O. Chmaissem Temperature and magnetic-field dependent structural and physical properties of A-site ordered manganites RBaMn$_{2}$O$_{6}$ (R = Pr, Nd, Pr$_{1/2}$Nd$_{1/2})$ were studied using high-resolution high-energy X-ray powder diffraction and magnetic and transport measurements. The ferromagnetic (FM) to antiferromagnetic (AF) phase transitions of all three materials are accompanied by first-order structural changes. Both the FM and AF phases of PrBaMn$_{2}$O$_{6}$ and Pr$_{1/2}$Nd$_{1/2}$BaMn$_{2}$O$_{6}$ have tetragonal structures, though the FM phase of the latter shows significant broadening of the (200) peak, suggesting a slight in-plane orthorhombic distortion. NdBaMn$_{2}$O$_{6}$ is tetragonal in the AF phase and orthorhombic in the FM phase. The FM-AF transition temperature T$_{c}$ increases with decreasing R$^{3+}$ ionic radius, while decreases with applied magnetic fields. The T$_{c}$ can be shifted by 15$\sim $25 K for H=6 T. Use of the Advanced Photon Source was supported by the~U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. W-31-109-Eng-38 and work at NIU by NSF- DMR-0302617. [Preview Abstract] |
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