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Two-dimensional ferromagnetic correlations above Tc in the Naturally layered CMR manganite La{sub 2-2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7} (x = 0.3-0.4).

Description: Neutron diffuse scattering in the form of rod-like features are observed in single crystals of the layered CMR material La{sub 2{minus}2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7} (x=0.4,0.36), consistent with the presence of 2D ferromagnetic spin correlations. These diffuse features are observed over a wide temperature region, however, their coherence length does not appear to diverge at T{sub c}, although there is evidence of the development of three-dimensional correlations around ferromagnetic reflections of the 3D-ordered magnetic structure close to T{sub c}. Quasi-elastic neutron scattering on a ceramic sample of x = 0.3 shows that the lifetime of these ferromagnetic correlations increases at T {r_arrow} T{sub c}. They exhibit a spin-diffusion constant above T{sub c} of {approximately}5 meV {angstrom}{sup 2}, much lower than that reported for La{sub 2/3}Ca{sub 1/3}MnO{sub 3}. We discuss the relationship of these magnetic correlations to models of the ferromagnetic transition in CMR compounds.
Date: March 23, 1998
Creator: Argyriou, D. N.
Partner: UNT Libraries Government Documents Department

Charge delocalization and structural response in layered La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7}: Enhanced Jahn-Teller distortion in the metallic regime

Description: Temperature dependent neutron diffraction studies on melt-grown crystals of the layered compound La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} reveal a dramatic structural response to the onset of ferromagnetism and the coincident insulator-metal transition. Unlike the related manganite perovskites, whose Jahn-Teller distorted octahedra become more regular at temperatures below the metal-insulator transition, the MnO{sub 6} octahedra of this layered material are more severely distorted when the charge is itinerant than when it is localized.
Date: November 1996
Creator: Mitchell, J. F.; Argyriou, D. N. & Jorgensen, J. D.
Partner: UNT Libraries Government Documents Department

Magnetic phase diagram of layered manganites in the highly doped regime.

Description: The naturally layered colossal magnetoresistive (CMR) manganites La{sub 2{minus}2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7} exhibit an extremely varied range of magnetic and electronic behavior over a very narrow composition range between x = 0.3 and x = 0.5. The successful synthesis in our laboratories of compounds with nominally greater than 50 percent Mn{sup 4+} concentration has now allowed the study of this heretofore unexplored region of the phase diagram. Here we present detailed neutron diffraction measurements of these compounds with doping levels 0.5 < x <1.0. As predicted by simple theories, the type-A layered antiferromagnetic (AF) structure is found at x{approximately}0.5 and the type-G ''rocksalt'' AF structure at x = 1.0. Between these two extremes is found a C-type structure (ferromagnetic rods parallel to b coupled antiferromagnetically to all neighboring rods) stabilized by orbital ordering of y{sup 2} states. Also in this Mn{sup 4+}-rich regime is found a region in which no long-range magnetic order is observed. We discuss how semi-empirical models can explain the variety of magnetic structures and how structural trends as a function of doping corroborate the unifying notion of a shift from in-plane to axial orbital occupation as the Mn{sup 4+} concentration is decreased.
Date: November 2, 2000
Creator: Mitchell, J. F.; Ling, C. D.; Millburn, J. E.; Argyriou, D. N.; Berger, A. & Medarde, M.
Partner: UNT Libraries Government Documents Department

Structural and magnetic states in layered manganites: An expanding view of the phase diagram

Description: Colossal magnetoresistive (CMR) manganites display a spectacular range of structural, magnetic, and electronic phases as a function of hole concentration, temperature, magnetic field, etc. A1though the bulk of research has concentrated on the 3-D perovskite manganites, the ability to study anisotropic magnetic and electronic interactions made available in reduced dimensions has accelerated interest in the layered Ruddlesden-Popper (R-P) phases of the manganite class. The quest for understanding the coupling among lattice, spin, and electronic degrees of freedom (and dimensionality) is driven by the availability of high quality materials. In this talk, the authors will present recent results on synthesis and magnetic properties of layered manganites from the La{sub 2{minus}2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7} series in the Mn{sup 4+}-rich regime x >0.5. This region of the composition diagram is populated by antiferromagnetic structures that evolve from the A-type layered order to G-type ''rocksalt'' order as x increases. Between these two regimes is a wide region (0.7 < x < 0.9) where an incommensurate magnetic structure is observed. The IC structure joins spin canting and phase separation as a mode for mixed-valent manganites to accommodate FM/AF competition. Transport in these materials is dominated by highly insulating behavior, although a region close to x = 0.5 exhibits metal-nonmetal transitions and an extreme sensitivity to oxygen content. They suggest two possible explanations for this transport behavior at doping just above x = O.5: localization by oxygen defects or charge ordering of Mn{sup 3+}/Mn{sup 4+}sites.
Date: January 5, 2000
Creator: Mitchell, J. F.; Millburn, J. E.; Ling, C.; Argyriou, D. N. & Bordallo, H. N.
Partner: UNT Libraries Government Documents Department

Effects of dynamic Jahn-Teller distortions by Raman spectroscopy in the layered CMR manganite La{sub 2{minus}2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7}, x = 0.36

Description: The close interplay among charge, spin, and lattice degrees of freedom in the colossal magnetoresistive (CMR) manganite oxides is believed to play an important role in the transport mechanism in these itinerant ferromagnets. While the work on CMR materials has concentrated on the 3D perovskite manganites, the discovery of the layered compounds La{sub 2{minus}2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7} as another class of CMR oxides provides a rich opportunity to explore the relationship between structure and transport properties on varying length and time scales in reduced dimensions. The crystal structure of the layered CMR compounds La{sub 2{minus}2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7} is comprised of perovskite bilayers of corner-linked MnO{sub 6} octahedra forming infinite sheets. Doping of divalent cations such as Sr{sup 2+} gives rise to a mixed valent system were Jahn-Teller (JT) active Mn{sup 3+} and JT-inactive Mn{sup 4+} co-exist on the lattice. Among the current theoretical models of transport in the three-dimensional perovskite materials is the role of JT. This transport mechanism plays a fundamental role above the Curie temperature (T{sub c}). Indeed, localized lattice distortions have been observed experimentally above T{sub c} in the (La,Ca)MnO{sub 3} perovskite system. In this communication, the authors report resonant Raman spectroscopic measurements on a micro-crystalline sample of La{sub 1.28}Sr{sub 1.72}Mn{sub 2}O{sub 7} (x = 0.36). The measurements suggest discrete phonon modes for the Mn{sup 3+} and Mn{sup 4+} lattice sites, which may arise from either dynamic or static localization and the presence of Mn{sup 2+} defects in the lattice.
Date: December 1, 1998
Creator: Bordallo, H. N.; Argyriou, D. N.; Mitchell, J. F. & Strouse, G. F.
Partner: UNT Libraries Government Documents Department

Structural features that optimize high temperature superconductivity

Description: For example, various defects can be introduced into the blocking layer to provide the optimum carrier concentration, but defects that form in or adjacent to the CuO{sub 2} layers will lower T{sub c} and eventually destroy superconductivity. After these requirements are satisfied, the highest T{sub c}`s are observed for compounds (such as the HgBa{sub 2}Ca{sub n-1}CuO{sub 2n{plus}2{plus}x} family) that have flat and square CuO{sub 2} planes and long apical Cu-O bonds. This conclusion is confirmed by the study of materials in which the flatness of the CuO{sub 2} plane can be varied in a systematic way. In more recent work, attention has focused on how the structure can be modified, for example, by chemical substitution, to improve flux pinning properties. Two strategies are being investigated: (1) Increasing the coupling of pancake vortices to form vortex-lines by shortening or ``metallizing`` the blocking layer; and (2) the formation of defects that pin flux.
Date: January 1, 1996
Creator: Jorgensen, J.D.; Hinks, D.G. Chmaissem, O.; Argyriou, D.N.; Mitchell, J.F. & Dabrowski, B.
Partner: UNT Libraries Government Documents Department

Short range spin correlations in the CMR material La{sub 1.41}Sr{sub 1.6}Mn{sub 2}O{sub 7}

Description: The (La{sub 1{minus}x}Sr{sub x}){sub 2}Mn{sub 3}O{sub 7} compounds are layered materials that exhibit higher magneto-resistance than the corresponding 3D manganite perovskites. Quasi-elastic neutron scattering on a polycrystalline sample of La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7} shows that the spin fluctuation spectrum of these layered CMR materials is qualitatively similar to those found in the perovskite manganites (La,Ca)MnO{sub 3}; their concentration, lifetime, and coherence length increase as T decreases to {Tc}. Unlike the perovskites the authors found a lower spin-diffusion constant above {Tc} of {approximately} 5 meV {angstrom}{sup 2}.
Date: December 1, 1997
Creator: Kelley, T.M.; Argyriou, D.N.; Robinson, R.A.; Nakotte, H.; Mitchell, J.F.; Osborn, R. et al.
Partner: UNT Libraries Government Documents Department

High-temperature magnetic anomalies in Sr-doped La manganite structures

Description: The temperature dependence of the magnetization M, susceptibility {chi}, and magnetoresistance MR for 3 perovskite-variant manganite structures were studied: monoclinic (x=0.075) and orthorhombic (x=0. 125) La{sub 1-x}Sr{sub x}MnO{sub 3}, and tetragonal layered La{sub 2- 2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7} (x=0.4) with x also indicating the nominal fraction of Mn{sup 4+}. In each case, evidence is found for unusual magnetic states at temperatures T above their primary magnetic transitions. In the first case, the high-T {chi} deviates from Curie-Weiss expectations, in the second case the MR extends to high T, and in the last, M and {chi} exhibit short-range anomalies at high T. This suggests that a key feature of these systems is the existence of multiple magnetic energy scales, independent of structure, dimensionality, or doping levels.
Date: November 1996
Creator: Potter, C. D.; Swiatek, M.; Mitchell, J. F.; Hinks, D. G.; Jorgensen, J. D.; Bader, S. D. et al.
Partner: UNT Libraries Government Documents Department

Two-dimensional intrinsic and extrinsic ferromagnetic behavior of layered La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} single crystals

Description: The low-field magnetization M and susceptibility {chi} are reported for the two-layered Ruddleson-Popper phase SrO(La{sub 1{minus}x}Sr{sub x}MnO{sub 3}){sub 2} for x = 0.4 (denoted La{sub 1.2}Sr{sup 1.8}Mn{sub 2}O{sub 7}) with an in-plane magnetic easy axis. As T approaches the Curie temperature (T{sub C} = 116 K) on cooling, where the metal-insulator transition occurs in zero-field, {chi} diverges as H{sup {minus}1/{delta}{prime}}, with {delta}{prime} = 21 {+-} 5. Also, near an extrinsic Curie transition attributed to {approximately} 0.1% volume fraction of intergrowths at T* = 2.47T{sub C}, M scales as (1 {minus} T/T*){sup {beta}}, with {beta} = 0.25 {+-} 0.02. These results can be understood within the context of 2D XY models, and provide a new perspective of the layered manganites.
Date: June 6, 1997
Creator: Potter, C. D.; Swiatek, M.; Bader, S. D.; Argyriou, D.N.; Mitchell, J. F.; Miller, D. J. et al.
Partner: UNT Libraries Government Documents Department

Lattice effect in perovskite and pyrochlore CMR materials

Description: Colossal magnetoresistance (CMR) in doped La manganite thin films (La{sub 1-x}M{sub x}MnO{sub 3}, where M = divalent ion, either Ca or Pb) has been shown to result in a factor of 10{sup 6} suppression of the resistance. The driving force for the CMR transition is thought to be the double-exchange interaction. Many studies of both the crystal structure and the local structure of the La{sub 1-x}M{sub x}MnO{sub 3} (M = Ca, Sr, Ba, Pb) system have now been carried out. As expected, these systems all show a strong coupling of the lattice to the CMR transition. On the other hand, neutron diffraction and x-ray absorption for the Tl{sub 2}Mn{sub 2}O{sub 7} pyrochlore, which also exhibits CMR, shows no deviations from ideal stoichiometry, mixed valency, or Jahn-Teller distortions of the MnO{sub 6} octahedron. We present results of crystallographic and local structural studies of these two important classes of CMR materials, compare the differences in structural response, and discuss the implications of these findings to our understanding of these materials.
Date: September 1997
Creator: Kwei, G. H.; Argyriou, D. N.; Lawson, A. C.; Neumeier, J. J.; Thompson, J. D.; Billinge, S. J. L. et al.
Partner: UNT Libraries Government Documents Department

Sign reversal of the MN-O bond compressibility in La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} below T{sub C}: Exchange striction in the ferromagnetic state

Description: The crystal structure of the layered perovskite La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} has been studied under hydrostatic pressure up to {approximately} 6 kbar, in the paramagnetic and ferromagnetic states, with neutron powder diffraction. The compressibility of the Mn-O apical bonds in the double layer of MnO{sub 6} octahedra changes sign from the paramagnetic insulator (PI) to the ferromagnetic metal (FM) state; in the Fm state the Mn-O-Mn linkage between MnO{sub 2} planes expands under applied pressure, whereas they contract in the PI state. This counterintuative behavior is interpreted in terms of exchange striction, which reflect the competition between super- and double-exchange. An increase of the Mn-moment with applied pressure in the FM state is consistent with a positive dT{sub C}/dP, as well as a cant angle {theta}{sub 0} between the magnetizations of neighboring MnO{sub 2} sheets that decreases with pressure.
Date: March 1997
Creator: Argyriou, D. N.; Mitchell, J. F.; Chmaissem, O.; Short, S.; Jorgensen, J. D. & Goodenough, J. B.
Partner: UNT Libraries Government Documents Department

Layered CMR manganites: Structure, properties, and unconventional magnetism

Description: Neutron powder diffraction studies of the layered compounds R{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7}, (R = La, Pr, Nd), RSr{sub 2}Mn{sub 2}O{sub 7} (R = Pr, Nd), and La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7} show that the degree of distortion of the MnO{sub 6} octahedra do not correlate with the appearance of a metal-insulator (MI) transition in these compounds. Instead, the in-plane Mn-O bond length appears to be a better indicator of the electronic behavior. Detailed bulk magnetization studies on single crystal La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} show that there are three magnetic regimes as a function of temperature: paramagnetic insulator, short-range ordered (SRO) ferromagnet, and long-range ordered (LRO) ferromagnet. Scaling analysis indicates that a 2D finite-size XY model is an appropriate description of the magnetic state in the SRO regime.
Date: 1997
Creator: Mitchell, J. F.; Argyriou, D. N.; Potter, C. D.; Jorgensen, J. D.; Hinks, D. G. & Bader, S. D.
Partner: UNT Libraries Government Documents Department