*4PO8-60 Competition Between Percolative and Band Features in Doped Manganites

Lev P. Gor'kov 1, Maxim O. Dzero 1, and Vladimir Z. Kresin 2. 1 National High Magnetic Field Laboratory, Tallahassee, FL 32310. 2 Lawrence Berkeley Laboratory, Berkeley, CA 94720.

Presenting Author: L.P. Gor'kov

The properties of the manganites A_1-xB_xMnO₃ strongly depend on the level of doping. We discuss the peculiarities of the transition regime from the insulating to the ferromagnetic metallic state which occurs at x~0.16-0.17. Phase separation in this concentration range is interpreted as a percolative phenomenon. In this region all physical properties (conductivity, magnetization, etc.) display peculiar behavior. A further increase in the level of doping results in metallic manganites which display properties consistent with Fermi liquid behavior. Experimental data for x<0.4 will be discussed.

4PO8-61 Phase Separation Scenario for Manganese Oxides

Adriana Moreo, Department of Physics, Florida State University and NHMFL, Tallahassee, FL 32308

Presenting Author: A. Moreo

Recent computational studies of models for manganese oxides have revealed a rich phase diagram [1], not anticipated in early calculations in this context. In particular, the transition between the antiferromagnetic insulator of the hole-undoped limit and the ferromagnetic metal at finite hole-density was found to occur through a mixed-phase process. These phase separation tendencies, also present at low electronic densities, influence the properties of the ferromagnetic region by increasing charge fluctuations. In addition, a pseudogap appears in A(q,w) in most of the Brillouin zone, caused by the coexistence of clusters of two phases. Experimental data reviewed here using several techniques for manganites and other materials are consistent with this scenario.

[1] A. Moreo, S. Yunoki and E. Dagotto, Science {283}, 2034 (1999). See also S. Yunoki et al., Phys. Rev. Lett. {80}, 845 (1998); ibid {81}, 5612 (1998); A. Moreo et al., preprint, cond-mat 9904416, to appear in PRL.

4PO8-62 Ferromagnetism and Superconductivity in La_0.7Sr_0.3MnO₃/YBa₂Cu₃O₇ Superlattices

P. Przyslupski 1, I. Komissarov 1, E. Dynowska 1, T. Skoœkiewicz 1, J. Wosik 2, T. Nishizaki 3, and N. Kobayashi 3. 1 Institut of Physics, Polish Academy of Sciences, 02-668 Warszawa Poland. 2 Texas Center for Superconductivity at University of Houston, Houston,TX 77204. 3 Institute for Materials Research, Tohoku University, Sendai 980-77, Japan.

Presenting Author: P. Przyslupski

We have fabricated perovskite superlattices consisting of ferromagnetic oxides; La_0.7Sr_0.3MnO₃ (LSMO) and YBa₂Cu₃O₇ (YBCO); grown by high pressure sputtering on (100) LaAlO₃ substrates.

We have investigated the magnetotransport and magnetic properties of a series samples, in which YBCO is fixed and LSMO varied from 3 to 20 unit cells. X-ray diffraction analysis of single films and bilayers confirm c-axis epitaxial growth; superlattices show second order satelites peaks. Transport measurements, above superconducting transition show negative magnetoresistance. Magnetic moment measurements show both superconducting and ferromagnetic state below a superconducting transition. We have found that superlattice structure has profound influences on the superconducting transition and anisotropy of manganite. Such observation can indicate on a possibility of ferromagnetic coupling through a superconductor in such heterostructures.

4PO8-63 Low-Temperature MFM Studies of CMR Manganites

Guangming Xiao, Joseph H. Ross, Jr., Anastasios Parasiris, K.D.D. Rathnayaka, and D.G. Naugle, Department of Physics, Texas A&M University, College Station, TX 77843-4242

Presenting Author: G. Xiao

Colossal magneto-resistive (CMR) (La_1-xCa_x)MnO₃ films have been studied using low-temperature Magnetic Force Microscopy (MFM). CMR films were prepared by both physical vapor codeposition (PVCD) and pulsed laser deposition (PLD), on LaAlO₃ substrates. Film compositions were x = 0.37 for the PVCD film and x = 0.33 for the PLD film, with thicknesses in the range 100-300 nm. Both samples exhibit isulator-to-metal transitions around 265K (T_c).

MFM studies were performed down to T = 77 K, in fields to 100 G. Irregularly-shaped magnetic features appeared at temperatures below T_c (with T_c deduced from resistivity measurements). These features showed significant changes in the temperature range 210-265 K, the temperature range in which the resistance also changes rapidly. Characteristic magnetic feature sizes were 1 to 2 µm, with the size decreasing with lowering temperature. There was also some reduction in amplitude of these features, which we discuss in relation to magnetization inhomogeneities in the films.

The average grain size for the PVCD film we found to be 0.4 µm, from AFM topography measurents. Magnetic domains in this film are observed to encompass multiple grains, indicating magnetic coherence between neighboring domains. These results are compared with previous MFM studies of manganite films.

4PO8-64 Scaling Property of Electron Localization and Transport in Manganites

H.Y. Teng, L. Sheng and C.S. Ting, Texas Center for Superconductivity, University of Houston, Houston, TX 77204, USA

Presenting Author: H.Y. Teng

The scaling behavior of localization length x as a function of magnetization M is studied numericallly based on double exchange and nonmagnetic disorder. The localization length is found proportional to ½ M-M_c½ ^-1/2 near the metal-insulator transition, where M_c is the critical value of magnetization at the Anderson metal-insulator transition. By including this scaling rule, the magnetization and temperature dependence of resistivity and thermal electric power is also investigated. Comparison between our results and experimental data is presented.

4PO8-65 Systematics in the transport and the structural properties of electron-doped layered-manganites

B. Fisher, L. Patlagan, G.M. Reisner, and A. Knizhnik, Physics Department and Crown Center of Superconductivity, Technion, Haifa, Israel 32000

Presenting Author: B. Fisher

We report on a comparative study of transport properties of lightly La-doped layered CaO(CaMnO₃)_m (m=1, 2 and ¥ and similar Mn³⁺/Mn⁴⁺ ratios). These materials undergo a paramagnetic (PM) to antiferromagnetic (AF) transition at TN » 114 K (for m=1) - 123 K (for m= ¥ ). The absolute thermopower (S) of the samples is negative. In the PM state, S varies linearly with temperature and its extrapolation to T=0 has a finite (negative) intercept. For samples with the same doping and various m, the slopes and intercepts vary systematically with m. In particular, within the experimental error, (dS/dT)^-1 varies linearly with 1/m, which in its turn varies linearly with the average number of Mn-Mn near neighbours. At T_N for m=¥ and m=2 or slightly above T_N for m=1, the sign of dS/dT changes from negative in the PM state to positive at lower temperatures, ½ S½ reaches a maximum and then drops towards zero. The implications of the simple systematics found in the PM state, are discussed using a model developed by Cutler and Mott (Phys. Rev. 181, 1336 (1968)) for transport by activated hopping of carriers obeying non-degenerate statistics. This model leads also to a simple, qualitative interpretation of S(T) of these materials on crossing T_N from the PM state and below T_N.

Systematics is observed also in the dependence of the (room-temperature) lattice parameters of these materials on m. The ratio V_m/m (V_m – the unit-cell volume) increases linearly with 1/m. The slope of this plot is slightly larger than the unit-cell volume of CaO.

4PO8-66 Evidence for Adiabatic Polaron Hopping in La_0.65Ca_0.35MnO₃

B. Lorenz 1, A.K. Heilman 1, Y.Y. Xue 1, C.W. Chu 1, W. Chen 2, and J.P. Franck 2. 1 Texas Center for Superconductivity and Department of Physics, University of Houston, Houston, TX 77204-5932. 2 Department of Physics, University of Alberta, Edmonton, Alberta, Canada TOG 2J1.

Presenting Author: B. Lorenz

We have measured the oxygen isotope effect of La_0.65Ca_0.35MnO₃ under hydrostatic pressure up to 1.7 GPa. The conductivity above the metal-insulator transition temperature, T_c, is thermally activated. The activation energy, E_a, varies with both the oxygen mass and pressure, but appears to be a universal linear function of T_c. The observation suggests that the pressure and oxygen mass affect the electrical conduction in La_0.65Ca_0.35MnO₃ through a common mechanism. The linear relation between E_a and T_c indicates an adiabatic polaron hopping process above T_c.

4PO8-67 High pressure study on manganites-the double exchange and polaron effects

Y.S. Wang 1, A.K. Heilman 1, B. Lorenz 1, Y.Y. Xue 1, C.W. Chu 1, J.P. Franck 2, and W.M. Chen 2. 1 TCSUH, University of Houston, Houston, TX 77004, USA. 2 Department of Physics, University of Alberta, Edmonton, Alberta, Canada TOG 2J1.

Presenting Author: Y.S. Wang

We have examined the influence of oxygen-isotope on the pressure effects on the metal-insulator (M-I) transition in the optimally doped La_0.65Ca_0.35MnO₃ (A) and on both the M-I and charge ordering (CO) transition in La_0.57Ca_0.43MnO₃ (B) under hydrostatic pressures up to 1.9 GPa, in an attempt to understand the roles of double exchange interaction and polarons. The pressure was found to enhance the M-I transition temperature (T_c) and suppress the resistivity (R) in A-samples, but at different rates, depending on the O-isotopic mass of the sample. The greater dlnT_c/dP observed in the O-18 sample in comparison with the O-16 one can be understood qualitatively in terms of the double exchange between the Mn-ions and the band reduction effect of polarons. However, the dlnT_c/dP-difference between the two isotopic samples is too large to be explained by the model. In the B-samples, the situation is much more complex. Pressure enhances T_c linearly at rates independent of the O-isotopic mass but varies T_c0 nonlinearly in a complex way. The thermal hysteresis of the CO transition diminishes with increase in pressure. The implication of these observations on the above model will be presented and discussed.

4PO8-68 Finite-Temperature Tunneling in Mn Oxides Junctions

H.Y. Teng, L. Sheng, and C.S. Ting, Texas Center for Superconductivity and Physics Department, University of Houston, Houston, TX 77204

Presenting Author: L. Sheng

In this work, we study the tunneling of electrons in Manganite/Insulator/Manganite junctions. The magnetic configuration near the Manganites/Insulator interface is calculated using Monte Carlo simulation. At finite temperatures but below the Curie temperature T_c, the local magnetization is found to drop by several tens of percents near the interfaces from its bulk value. By including this effect, the tunneling resistivity is calculated. The temperature dependence of the magnetoresistance amplitude is in agreement with experimental data.

4PO8-69 Spin reversal in (Gd,Ca)MnO₃ perovskites

Octavio Peña 1, Carlos Moure 2, J.F. Fernandez 2, M. Villegas 2, and P. Duran 2. 1 LCSIM -UMR 6511 CNRS, Université de Rennes 1, 35042 Rennes, France. 2 ICV - CSIC, 28500 Arganda del Rey, Madrid, Espagne.

Presenting Author: O. Peña

Magnetic properties of perovskite-structured manganite Gd_1-xCa_xMnO₃ solid solution (0 £ x £ 0.4) were studied by ac and dc magnetization techniques, such as ZFC/FC cycles under different applied fields, isothermal magnetization loops M(H) or the temperature dependence of the remanent magnetization M(T). It was seen in particular that the experimental magnetization reverses its sign when samples are cooled under small or intermediate fields (H < 0.5 T), reaching similar values as those obtained upon reversal of the magnetic field. Also interesting, the remanent magnetization M(T) changes its sign twice when warming the sample from an initial state characterized by an ordered Mn-sublattice and saturated Gd spins (that is, a state obtained after cooling the system under 5 T and subsequently reducing the applied field to zero). Results will be interpreted in terms of two interacting networks : a Mn-based ferromagnetic sublattice and a negatively-polarized gadolinium sublattice. Local fields at a given site depend on the exchange interactions between these two sublattices, producing a spin reversal when the absolute value of the gadolinium sublattice is larger than that of the ferromagnetic network. A full comparison of these and other results will be made with those obtained in other (RE,Ca)MnO₃ solid solutions.