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Layered manganites : magnetic structure at extreme doping levels.

Description: We report powder neutron diffraction results on the crystal and magnetic structures of the bilayer Ruddlesden-Popper phase Sr{sub 3}Mn{sub 2}O{sub 7{minus}{delta}} ({delta} = 0.0, 0.45) and correlate these structures with their magnetic and transport properties. The {delta} = 0.45 compound contains a large number of oxygen vacancies that are disordered in the MnO{sub 2} planes. As a result of this disordered vacancy structure, Sr{sub 3}Mn{sub 2}O{sub 6.55} is a nonmagnetic insulator. Sr{sub 3}Mn{sub 2}O{sub 7.0} ({delta} = 0) is an antiferromagnetic insulator whose magnetic structure is related to that of the SrMnO{sub 3} perovskite. Comparison of this end-member compound to its doped congeners in the La{sub 2{minus}2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7} series highlights the extreme sensitivity of magnetic structure to dopant concentration in these layered materials.
Date: September 11, 1998
Creator: Mitchell, J. F.
Partner: UNT Libraries Government Documents Department

Time dependent effects and transport evidence for phase separation in La{sub 0.5}Ca{sub 0.5}MnO{sub 3}

Description: The ground state of La{sub 1{minus}x}Ca{sub x}MnO{sub 3} changes from a ferromagnetic metallic to an antiferromagnetic charge-ordered state as a function of Ca concentration at x {approximately} 0.50. The authors present evidence from transport measurements on a sample with x = 0.50 that the two phases can coexist, in agreement with other observations of phase separation in these materials. They also observe that, by applying and then removing a magnetic field to the mainly charge-ordered state at some temperatures, they can ``magnetically anneal'' the charge order, resulting in a higher zero-field resistivity. They also observe logarithmic time dependence in both resistivity and magnetization after a field sweep at low temperatures.
Date: February 17, 2000
Creator: Roy, M.; Mitchell, J. F. & Schiffer, P.
Partner: UNT Libraries Government Documents Department

Evidence for competing order parameters in the paramagnetic phase of layered manganites.

Description: The magnetic field and temperature dependence of the magnetic susceptibility and magnetization is studied for the ferromagnetic layered manganites SrO(La{sub t{minus}x}Sr{sub x}MnO{sub 3}){sub 2} in the composition range x = 0.32-0.40. In the paramagnetic phase, the susceptibility exhibits an anomalous maximum at an intermediate magnetic field value. The size of this field-induced susceptibility enhancement increases dramatically with x from 10% for x = 0.32 to 160% for x = 0.40. The temperature dependence of the effect shows a maximum at T {approx} 1.1 T{sub c} for all x. Quantitative analysis in terms of the Landau theory of phase transitions enables us to identify a distortion of the free energy F in the paramagnetic phase that is associated with the susceptibility anomaly. This free energy distortion corresponds to a magnetic system that approaches a first order magnetic phase transition as the temperature is lowered towards T{sub c}. Such a behavior is indicative of a second, competing order parameter, which is identified as the recently observed charge density wave. In the immediate vicinity of T{sub c}, the anomaly disappears and the system seems to undergo a more conventional second order paramagnetic-ferromagnetic phase transition.
Date: November 8, 2000
Creator: Berger, A.; Mitchell, J. F.; Miller, D. J. & Bader, S. D.
Partner: UNT Libraries Government Documents Department

Twinning microstructure and charge ordering in the colossal magnetoresistive manganite Nd{sub 1/2}Sr{sub 1/2}MnO{sub 3}

Description: Charge ordering (C.O.) in the colossal magnetoresistive (CMR) manganites gives rise to an insulating, high-resistance state. This charge ordered state can be melted into a low-resistance metallic-like state by the application of magnetic field. Thus, the potential to attain high values of magnetoresistance with the application of small magnetic fields may be aided by a better understanding of the charge-ordering phenomenon. This study focused on microstructural characterization in Nd{sub 1/2}Sr{sub 1/2}MnO{sub 3}. In Nd{sub 1/2}Sr{sub 1/2}MnO{sub 3}, the nominal valence of Mn is 3.5+. On cooling, charge can localize and lead to a charge ordering between Mn 3+ and Mn 4+. The ordering of charge results in a superlattice structure and a reduction in symmetry. Thin foil specimens were prepared from bulk samples by conventional thinning and ion milling (at LiqN{sub 2} temperature) methods. The room temperature TEM observation of Nd{sub 1/2}Sr{sub 1/2}MnO{sub 3} reveals that it contains a highly twinned microstructure, together with a small number of stacking faults (SFS). A figure shows the same area of the specimen at different zone axes obtained by tilting around two perpendicular directions as indicated. Three grains A, B and C are labeled for each of the zone axes. The room temperature EDPs from the matrix and twins shows an approximate 90{degree} rotation suggesting a 90{degree} twin orientation. These results are further confirmed by C.O. at low temperatures. The twinning planes can be determined by tilting with large angles.
Date: May 10, 2000
Creator: Luo, Z. P.; Miller, D. J. & Mitchell, J. F.
Partner: UNT Libraries Government Documents Department

Raman study of phonons in Sr{sub 3}Mn{sub 2}O{sub 6.54} and Sr{sub 3}Mn{sub 2}O{sub 7} layered manganites

Description: Polycrystalline Sr{sub 3}Mn{sub 2}O{sub 6.54} and Sr{sub 3}Mn{sub 2}O{sub 7} layered manganites were studied at room temperature using Raman spectroscopy. While the phonon spectrum of the stoichiometric Sr{sub 3}Mn{sub 2}O{sub 7} sample is consistent with the group theoretical analysis for the 14/mm structure, three additional modes are observed in the phonon spectrum of the oxygen deficient Sr{sub 3}Mn{sub 2}O{sub 6.54} sample. Based on an analysis of the temperature dependence of the Raman spectrum of Sr{sub 3}Mn{sub 2}O{sub 6.54} and a comparison with the Raman spectrum of LaSr{sub 2}Mn{sub 2}O{sub 7}, the authors conclude that the extra modes should be viewed as disorder induced Raman scattering.
Date: April 27, 2000
Creator: Guedes, I.; Mitchell, J. F.; Argyriou, D. & Grimsditch, M.
Partner: UNT Libraries Government Documents Department

Spin excitations in [La{sub 1-x}Ca{sub x}MnO{sub 3}] in the mixed-phase region.

Description: The magnetic excitations and the ferromagnetic order parameter have been studied by neutron scattering in a series of the manganese-based CMR perovskites [La{sub 1-x}Ca{sub x}MnO{sub 3}] (x=0.46, 0.48, 0.50) near the metallic ferromagnetic to insulating antiferromagnetic phase. Well-defined ferromagnetic spin waves were detected for the x=0.46 and x=0.48 compositions. From the measurements of the order parameter, only the x=0.48 sample showed conclusive evidence of a coexistence of an antiferromagnetic phase with the ferromagnetic phase. For this composition, hysteresis was observed in the spin wave intensity but not in the spin stiffness parameter. This effect indicates that the ferromagnetic exchange is not perturbed by the antiferromagnetic ordering. No measurable ferromagnetic magnetization was found in the x=0.50 sample; thus no spin waves could be detected. The results indicate that the onset of the antiferromagnetism upon hole doping for the series occurs in a narrow region of x below the x=0.50 phase boundary.
Date: December 18, 2001
Creator: Stumpe, L.; Kirby, B.; Kaiser, H.; Rhyne, J. J. & Mitchell, J. F.
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

Low energy spin-wave excitations in the bilayer manganite La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7}.

Description: Inelastic neutron scattering experiments were performed on a single crystal of the bilayer manganite La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7}. Low energy spin-wave excitations were observed along the c direction with a maximum energy of {approx} 0.5 meV at the zone boundary. The dispersion of these acoustic spin wave modes is modeled by a nearest-neighbor Heisenberg model with an inter-bilayer exchange interaction between neighboring spins in different bilayers of 0.048(1) meV and an anisotropy gap of {Delta} = 0.077(3) meV. These results confirm the two-dimensional nature of the spin-correlations in the bilayer manganites, with a ratio of the in-plane to inter-bilayer interaction of {approx}200. The temperature dependence of the energies and intensities of the spin wave excitations are in agreement with our earlier conclusion that the ferromagnetic transition is second-order.
Date: September 21, 1999
Creator: Rosenkranz, S.; Osborn, R.; Mitchell, J. F.; Vasiliu-Doloc, L.; Lynn, J. W. & Sinha, S. K.
Partner: UNT Libraries Government Documents Department

Intrinsic and extrinsic magnetic properties of the naturally layered manganites

Description: Structural and magnetic properties of the two-layered Ruddlesden-Popper phase SrO(La{sub 1{minus}x}Sr{sub x}MnO{sub 3}){sub 2} with x = 0.3--0.5 are highlighted. Intrinsic properties of these naturally layered manganites include a colossal magnetoresistance, a composition-dependent magnetic anisotropy, and almost no remanence. Above the Curie temperature there is a non-vanishing extrinsic magnetization attributed to intergrowths (stacking faults in the layered structure). These lattice imperfections consist of additional or missing manganite layers, as observed in transmission electron microscopy. Their role in influencing the properties of the host material is highlighted.
Date: November 30, 1999
Creator: Berger, A.; Mitchell, J. F.; Miller, D. J.; Jiang, J. S. & Bader, S. D.
Partner: UNT Libraries Government Documents Department

Local Structure of La1-xSrxCoO3 determined from EXAFS and neutron PDF studies

Description: The combined local structure techniques, extended x-ray absorption fine structure (EXAFS) and neutron pair distribution function analysis, have been used for temperatures 4<= T<= 330 K to rule out a large Jahn-Teller (JT) distortion of the Co-O bond in La1?xSrxCoO3 for a significant fraction of Co sites (x<= 0.35), indicating few, if any, JT-active, singly occupied eg Co sites exist.
Date: January 26, 2009
Creator: Sundaram, N.; Jiang, Y.; Anderson, I. E.; Belanger, D. P.; Booth, C. H.; Bridges, F. et al.
Partner: UNT Libraries Government Documents Department

Local structure of La1-xSrxCoO3 determined from EXAFS and neutron PDF studies

Description: The combined local structure techniques, extended X-ray absorption fine struture (EXAFS) and neutron pair distribution function (PDF) analysis, have been used for temperatures 4 {<=} T {<=} 330 K to rule out a large Jahn-Teller (J-T) distortion of the Co-O bond in La{sub 1-x}Sr{sub x}CoO{sub 3} for a significant fraction of Co sites (x {<=} 0.35), indicating few, if any, J-T active, single-occupied e{sub g} Co sites exist.
Date: January 1, 2008
Creator: Proffen, Thomas; Sundaram, Nalini; Jiang, Y; Anderson, Ingrid; Belanger, D P; Booth, C H et al.
Partner: UNT Libraries Government Documents Department

Changes of the spin dynamics in perovskite and bilayer manganite.

Description: The authors report a series of {micro}SR measurements performed on single crystals of La{sub 1-x}Sr{sub x}MnO{sub 3} (0 {le} x {le} 0.125) and on a polycrystalline bilayer manganite La{sub 2-2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7} (x = 0.52). They find that the temperature dependence of the spin-lattice relaxation rate is strongly dependent on the hole doping and the associated changes in the magnetic structure. The systems have competing, anisotropic interactions, leading to a complex interplay of charge, spin and orbital order. This has a significant influence on the spin dynamics, both for perovskite and layered manganites.
Date: July 2, 2002
Creator: Coldea, A. I.; Blundell, S. J.; Steer, C. A.; Pratt, F. L.; Prabhakaran, D. & Mitchell, J. F.
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

Role of intergrowths in the properties of the naturally layered manganites.

Description: The structural and magnetic properties of the two-layered Ruddlesden-Popper phase SrO(La{sub 1{minus}x}Sr{sub x}MnO{sub 3}){sub 2} with x = 0.3 and x = 0.4 are investigated. These naturally layered manganites exhibit a colossal magnetoresistance, a magnetic anisotropy which is strongly composition-dependent, almost no remanence, and a non-vanishing magnetization in an extended temperature range above the Curie temperature (T{sub c}). The magnetization in this temperature range is not intrinsic to the crystal, but is attributed to intergrowths. These two-dimensional lattice imperfections consist of additional or missing SrO layers and have been observed in transmission electron micrographs. The magnetic properties of the intergrowths differ from the bulk crystal which results in unusual magnetic behavior.
Date: October 27, 1998
Creator: Bader, S.D.; Berger, A.; Jiang, J.S.; Miller, D.J.; Mitchell, J.F. & Osgood, R.M. III
Partner: UNT Libraries Government Documents Department

Direct imaging of the first order spin flop transition in the layered manganite La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7}.

Description: The spin-flop transition in the antiferromagnetic layered manganite La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7} was studied using magnetization measurements and a high-resolution magneto-optical imaging technique. We report the direct observation of the formation of ferromagnetic domains appearing at the first order spin-flop transition. The magnetization process proceeds through nucleation of polarized domains at crystal defect sites and not through the expansion of polarized domains due to domain wall motion. A small magnetic hysteresis is caused by the difference between the mechanisms of nucleation and annihilation of domains in the mixed state. These results establish a direct link between the magnetic structure on the atomic scale as seen in neutron scattering and the macroscopic properties of the sample as seen in magnetization and conductivity measurements.
Date: August 31, 1999
Creator: Berger, A.; Gray, K. E.; Miller, D. J.; Mitchell, J. F.; Vlasko-Vlasov, V. K. & Welp, U.
Partner: UNT Libraries Government Documents Department

Polaron Coherence Condensation As the Mechanism for Colossal Magnetoresistance in Layered Manganites

Description: Angle-resolved photoemission spectroscopy data for the bilayer manganite La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} show that, upon lowering the temperature below the Curie point, a coherent polaronic metallic groundstate emerges very rapidly with well defined quasiparticles which track remarkably well the electrical conductivity, consistent with macroscopic transport properties. Our data suggest that the mechanism leading to the insulator-to-metal transition in La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} can be regarded as a polaron coherence condensation process acting in concert with the Double Exchange interaction.
Date: November 20, 2007
Creator: Mannella, N.; Yang, W. L.; Tanaka, K.; Zhou, X. J.; Zheng, H.; Mitchell, J. F. et al.
Partner: UNT Libraries Government Documents Department

Structural study of Sr{sub 2}CuO{sub 3+delta} by neutron powder diffraction

Description: Average crystal structures of superconducting Sr{sub 2}CuO{sub 3+{delta}} synthesized at ambient pressure from a hydroxometallate precursor were refined from neutron powder diffraction data. A simplified model was used to fit the modulated superstructures. Both compounds have an oxygen deficient La{sub 2}CuO{sub 4}-type tetragonal T structure with O vacancies located in the CuO{sub 2} planes, not in the Sr{sub 2}O{sub 2} layers. This raises important questions about the superconductivity in Sr{sub 2}CuO{sub 3+{delta}} reported to be a 70 K superconductor.
Date: November 1, 1996
Creator: Shimakawa, Y.; Jorgensen, J.D.; Mitchell, J.F.; Hunter, B.A.; Shaked, S.; Hinks, D.G. et al.
Partner: UNT Libraries Government Documents Department

Strength and flexibility of bulk high-{Tc} superconductors

Description: Strength, fracture toughness, and elastic modulus data have been gathered for bulk high-temperature superconductors, commercial 99.9% Ag, and a 1.2 at.% Mg/Ag alloy. These data have been used to calculate fracture strains for bulk conductors. The calculations indicate that the superconducting cores of clad tapes should begin to fracture at strains below 0.2%. In addition, residual strains in Ag-clad (Bi,Pb){sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} tapes have been measured by neutron diffraction. An explanation is offered for why many tapes appear to be able to tolerate large strains before exhibiting a reduction in current transport.
Date: August 1, 1996
Creator: Goretta, K.C.; Jiang, M.; Kupperman, D.S.; Lanagan, M.T.; Singh, J.P.; Vasanthamohan, N. 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