Orbital order and a canted phase in the paramagnetic and ferromagnetic states of 50% hole-doped colossal magnetoresistance manganites (original) (raw)
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Spin disorder in Fe-doped manganites
Journal of Non-Crystalline Solids, 2007
Ln 0.7 M 0.3 MnO 3 compounds are well-known ferromagnets mediated by a double exchange mechanism. As Mn atoms are substituted by Fe in the ratio Mn 1Àx Fe x the magnetic structure dramatically changes, because the ferromagnetic double exchange chain is broken. At low Fe concentrations all compounds are magnetically ordered. For intermediate values ferro (FM), antiferro (AF) and paramagnetic (PM) phases co-exist in a large temperature range, and at x % 0.2 spin or cluster-glass behavior is found. Magnetization, Mö ssbauer, polarized and low angle neutron scattering as well as muon spin relaxation experiments have been performed on 0 6 x 6 0.30 compounds showing the transit from long range ferromagnetism to spin glass. Coexistence of FM and AF clusters of different size has been found for all doped compounds.
Physical Review B, 2011
We report inelastic neutron scattering experiments on single crystals of Nd1−xSrxMnO3 with x=0.5 and x=0.49. The spin-wave dispersion in the charge, orbital, and spin ordered state in Nd0.5Sr0.5MnO3 exhibits a strongly anisotropic stiffness. The sign of the anisotropy is characteristic for the site-centered model for charge and orbital ordering in half-doped manganites. Within this model, linear spin-wave theory yields a perfect description of the experimental dispersion. In the ferromagnetic metallic state of Nd1−xSrxMnO3 with x=0.49 and x=0.50 magnetic excitations exhibit nearly the same magnon dispersion. High intense signals near the zone-boundary over a wide energy level overlap with a sharp spin-wave dispersion which can be described with a Heisenberg model including nearest-neighbor interactions.
Microscopic spin interactions in colossal magnetoresistance manganites
Physical Review B, 2002
Using inelastic neutron scattering we measured the microscopic magnetic coupling associated with the ferromagnetic clusters of the ''colossal magnetoresistance'' compound Pr 0.70 Ca 0.30 MnO 3. When the insulatingto-metal ͑I-M͒ transition is induced by an external magnetic field there is a discontinuous change in the spin-wave stiffness constant. This result implies that the I-M transition is not achieved by the simple percolation of micron-sized metallic clusters as currently believed, but involves a first-order transformation.
Evolution of Spin-Wave Excitations in Ferromagnetic Metallic Manganites
Physical Review Letters, 2006
Neutron scattering results are presented for spin-wave excitations of three ferromagnetic metallic A1−xA ′ x MnO3 manganites (where A and A ′ are rare-and alkaline-earth ions), which when combined with previous work elucidate the systematics of the interactions as a function of carrier concentration x, on-site disorder, and strength of the lattice distortion. The long wavelength spin dynamics show only a very weak dependence across the series. The ratio of fourth to first neighbor exchange (J4/J1) that controls the zone boundary magnon softening changes systematically with x, but does not depend on the other parameters. None of the prevailing models can account for these behaviors.
Magnetic and neutron diffraction studies of long-range ferromagnetic order in monolayered manganites
Physical Review B, 2003
The structural, magnetic, and electrical properties of single crystals of Ru-doped monolayered manganites La0.5Sr1.5Mn1-xRuxO4 (0⩽x⩽0.5) were investigated by neutron-diffraction, magnetization, and transport measurements. With increasing Ru content, the magnetic ground state changed from the parent antiferromagnetic (AFM) to the spin-glass to the ferromagnetic (FM) phase. The temperature-dependent neutron-diffraction measurements at x=0.35 clearly revealed long-range FM order in the monolayered manganite, which was confirmed by bulk magnetometry. The uniaxial magnetic anisotropy, in which the Ru spins are aligned along the c axis, was also observed in the compounds. This unique magnetic behavior might be ascribed to the magnetic anisotropy of the Ru ions stabilized under the structural distortion, as well as the FM ordering of Mn spins induced by AFM coupling among Mn/Ru pairs. The magnetoresistance of the Ru-doped system, which exhibits conduction confinement within a layer, diminishes with increasing Ru doping because of an increase in the proportion of FM domains at the expense of the magnetic glassy state.
Nanoscale ferromagnetism in phase-separated manganites
Journal of Magnetism and Magnetic Materials, 2007
Magnetic domain structures in phase-separated manganites were investigated by low-temperature Lorentz electron microscopy, in order to understand some unusual physical properties such as a colossal magnetoresistance (CMR) effect and a metal-to-insulator transition. In particular, we examined a spatial distribution of the charge/orbital-ordered (CO/OO) insulator state and the ferromagnetic (FM) metallic one in phase-separated manganites; Cr-doped Nd 0:5 Ca 0:5 MnO 3 and (La 1Àx Pr x Þ 5=8 Ca 3=8 MnO 3 with x ¼ 0:375, by obtaining both the dark-field images and Lorentz electron microscopic ones. It is found that an unusual coexistence of the CO/OO and FM metallic states below a FM transition temperature in the two compounds. The present experimental results clearly demonstrated the coexisting state of the two distinct ground states in manganites.
First Order Colossal Magnetoresistance Transitions in the Two-Orbital Model for Manganites
Physical Review Letters, 2010
Large scale Monte Carlo simulation results for the two-orbital model for manganites, including Jahn-Teller lattice distortions, are here presented. At hole density x = 1/4 and in the vicinity of the region of competition between the ferromagnetic metallic and spin-charge-orbital ordered insulating phases, the colossal magnetoresistance (CMR) phenomenon is observed with a magnetoresistance ratio ∼ 10, 000%. Our main result is that this CMR transition is found to be of first order in some portions of the phase diagram, in agreement with early results from neutron scattering, specific heat, and magnetization, thus solving a notorious discrepancy between experiments and previous theoretical studies. The first-order characteristics of the transition survive, and are actually enhanced, when weak quenched disorder is introduced.
Journal of Magnetism and Magnetic Materials, 2004
Lacunar-controlled Nd0.93MnO2.96 orthorhombic perovskite prepared at 1200°C has been studied at low temperature. Magnetization measurements suggest the coexistence below 80 K of ferro- and antiferromagnetic interactions between Mn atoms as in the stoichiometric parent compound NdMnO3. Nevertheless, spontaneous magnetization curves obtained after sample cooling under a magnetic field of 500 Oe (FC) and without magnetic field (ZFC) reveal a much original behavior. Magnetization changes sign twice on ZFC curve; strong negative magnetization is observed below 19 K on FC curve. On the basis of neutron diffraction data, this behavior is interpreted by considering the coexistence of two perovskite phases having chemical compositions and magnetic ordering temperatures slightly different; ferromagnetic components in the clusters of both phases are antiferromagnetically coupled in the range 20–80 K. Ferromagnetic ordering at 19 K of Nd atoms is accompanied by a rearrangement of the Mn ferromagnetic components which become parallel in all the clusters.
Orbital Nature of Ferromagnetic Magnons in Manganites
Physical Review Letters, 2005
Magnon excitation in a ferromagnetic state of Sm 0:55 Sr 0:45 MnO 3 located on the verge of the metalinsulator transition has been studied in terms of the neutron scattering experiment. The anomalous magnon dispersion with the zone-boundary softening is well described by the Heisenberg model with extended exchange coupling constants Js. In particular the fourth neighbor coupling J 4 is as large as 0.6 times the nearest neighbor one J 1. Theoretical analysis based on the local density approximation Hubbard U band calculation reveals that this one-dimensional exchange path is due to the 3z 2 ÿ r 2-type orbital correlation, in sharp contrast to previous proposals.