613 research outputs found
Successive Phase Transitions in a Metal-Ordered Manganite Perovskite YBaMn2O6
Structural, magnetic and electric properties of a metal-ordered perovskite
YBaMn2O6 have been studied by means of powder X-ray diffraction, DSC, magnetic
susceptibility, and electric resistivity. It is found that this material
undergoes a 1st order structural phase transition at Tc1=520 K, from a pseudo
orthorhombic (monoclinic) phase to a pseudo tetragonal (monoclinic) one.
Accompanied by this transition, the susceptibility exhibits a sharp drop, where
ferromagnetic exchange interaction becomes antiferromagnetic. Furthermore, two
more transitions, a metal-insulator transition at Tc2=480 K and an
antiferromagnetic ordering at Tc3=200 K, have been observed respectively.Comment: 4 pages, 5 figures, proceedings for ISSP
Charge, Orbital and Magnetic Order in Nd0.5Ca0.5MnO3
In the manganite Nd0.5Ca0.5MnO3, charge ordering occurs at much higher
temperature than the antiferromagnetic order (TCO=250K,TN=160K).
The magnetic behavior of the phase TN<T<TCO is puzzling: its magnetization
and susceptibility are typical of an antiferromagnet while no magnetic order is
detected by neutron diffraction.We have undertaken an extensive study of the
cristallographic, electric and magnetic properties of Nd0.5Ca0.5MnO3 and
established its phase diagram as a function of temperature and magnetic field.
The charge disordered, paramagnetic phase above TCO present ferromagnetic
correlations. An antiferromagnetic CE phase prevails below TN, with complete
charge and orbital ordering. In the intermediate temperature range, charge
ordering occurs while orbital ordering sets in progressively, with no magnetic
order. Strong magnetic fields destroy the charge ordered phases in a fisrt
order transition towards a ferromagnetic state.Comment: 17 pages, 11 Figures to appear in Phys. Rev.
Structures and Electromagnetic Properties of New Metal-Ordered Manganites; RBaMn_{2}O_{6} (R = Y and Rare Earth Elements)
New metal-ordered manganites RBaMn_{2}O_{6} have been synthesized and
investigated in the structures and electromagnetic properties. RBaMn_{2}O_{6}
can be classified into three groups from the structural and electromagnetic
properties. The first group (R = La, Pr and Nd) has a metallic ferromagnetic
transition, followed by an A-type antiferromagnetic transition in
PrBaMn_{2}O_{6}. The second group (R = Sm, Eu and Gd) exhibits a charge-order
transition, followed by an antiferromagnetic long range ordering. The third
group (R = Tb, Dy and Ho) shows successive three phase transitions, the
structural, charge/orbital-order and magnetic transitions, as observed in
YBaMn_{2}O_{6}. Comparing to the metal-disordered manganites
(R^{3+}_{0.5}A^{2+}_{0.5})MnO_{3}, two remarkable features can be recognized in
RBaMn_{2}O_{6}; (1) relatively high charge-order transition temperature and (2)
the presence of structural transition above the charge-order temperature in the
third group. We propose a possible orbital ordering at the structural
transition, that is a possible freezing of the orbital, charge and spin degrees
of freedom at the independent temperatures in the third group. These features
are closely related to the peculiar structure that the MnO_{2} square-lattice
is sandwiched by the rock-salt layers of two kinds, RO and BaO with extremely
different lattice-sizes.Comment: 5 pages, 4 figures, submitted to J. Phys. Soc. Jp
Manganite charge and orbitally ordered and disordered states probed by Fe substitution into Mn site in LnBaMn1.96Fe0.04O5, LnBaMn1.96Fe0.04O6 and LnBaMn1.96Fe0.04O5.5 (Ln=Y, Gd, Sm, Nd, Pr, La)
The layered manganese oxides LnBaMn1.96Fe0.04Oy (Ln=Y, Gd, Sm, Nd, Pr, La)
have been synthesized for y=5, 5.5 and 6. In the oxygen-saturated state (y=6)
they exhibit the charge and orbital order at ambient temperature for Ln=Y, Gd,
Sm, but unordered eg-electronic system for Ln=La,Pr,Nd. Fourfold increase of
quadrupole splitting was observed owing to the charge and orbital ordering.
This is in agreement with the jumplike increase in distortion of the reduced
perovskite-like cell for the charge and orbitally ordered manganites compared
to the unordered ones. Substitution of 2 percents of Mn by Fe suppresses the
temperatures of structural and magnetic transitions by 20 to 50 K. Parameters
of the crystal lattices and the room-temperature M\"{o}ssbauer spectra were
studied on forty samples whose structures were refined within five symmetry
groups: P4/mmm, P4/nmm, Pm-3m, Icma and P2/m. Overwhelming majority of the Fe
species are undifferentiated in the M\"{o}ssbauer spectra for most of the
samples. Such the single-component spectra in the two-site structures are
explained by the preference of Fe towards the site of Mn(III) and by the
segmentation of the charge and orbitally ordered domains.Comment: 8 figures; figures 2 and 3 were revise
Influence of Nd on the magnetic properties of Nd1-xCaxMnO3
The role played by the Nd ions in the magnetic properties of Nd0.5Ca0.5MnO3
and Nd0.7Ca0.3MnO3 is studied using static magnetization, neutron diffraction
and high frequency (9.4-475GHz) Electron Spin Resonance. We show that the Nd
ions are weakly coupled to the Mn ions via ferromagnetic exchange and are
responsible for the peculiar ferromagnetic resonance observed in the FM phase
of both compounds (ground state below 120K for x=0.3, high field state for
x=0.5). We then use ESR to look for magnetic phase separation in the low field,
CO phase of Nd0.5Ca0.5MnO3. We show that there is no trace of the FM phase
imbedded in the CO phase, contrary to what is observed in La0.5Ca0.5MnO3 or
Pr0.5Sr0.5MnO3.Comment: to be published in phys.Rev.B as a Rapid Com
Study of magnetic and specific heat measurements at low temperatures in Nd0.5Sr0.5MnO3 and Nd0.5Ca0.5MnO3
The magnetization at low temperatures for Nd0.5Sr0.5MnO3 and Nd0.5Ca0.5MnO3
samples showed a rapid increase with decreasing temperatures, contrary to a
La0.5Ca0.5MnO3 sample. Specific heat measurement at low temperatures showed a
Schottky-like anomaly for the first two samples. However, there is not a
straight forward correlation between the intrinsic magnetic moment of the Nd3+
ions and the Schottky-like anomaly.Comment: To be presented in the 1 Joint European Magnetic Symposia,
Grenoble,France (2001). Also submitted to Journal of Magnetism and Magnetic
Material
Van Hove Singularity and Superconductivity in Disordered Hubbard Model
We apply the Coherent Potential Approximation (CPA) to a simple extended
Hubbard model with a nearest and next nearest neighbour hopping for disordered
superconductors with s-, d- and p-wave pairing. We show how the Van Hove
singularities in the electron density of states enhance the transition
temperature Tc for exotic superconductors in a clean and weakly disordered
system. The Anderson theorem and pair-breaking effects in presence of Van Hove
singularity caused by non-magnetic disorder are also discussed.Comment: REVTEX 40 pages, 8 figures, (accepted by Physica Status Solidi b
Bis(μ-adamantane-1,3-dicarboxylÂato-κ4 O 1,O 1′:O 3,O 3′)bisÂ[aquaÂ(3-carboxyÂadamÂantane-1-carboxylÂato-κO 1)(1,10-phenÂanÂthroline-κ2 N,N′)erbium(III)] dihydrate
The asymmetric unit of the binuclear centrosymmetric title compound, [Er2(C12H14O4)2(C12H15O4)2(C12H8N2)2(H2O)2]·2H2O, contains one ErIII atom, one coordinated water molÂecule, one 1,10-phenanthroline (phen) ligand, two differently coordinated adamantane-1,3-dicarboxylÂate (H2L) ligands and one lattice water molÂecule. The ErIII ion is eight-coordinated by four O atoms from bridging L
2−, one O atom from HL
−, one O atom from the coordinated water and two N atoms from a phen ligand. Extensive O—H⋯O hydrogen-bonding interÂactions result in the formation of chains which are further linked into a layer-like network by π–π stacking interÂactions centroid–centroid distance = 3.611 (3) Å] between adjacent phen ligands belonging to neighbouring chains. The carboxy group of the HL
− ligand is equally disordered over two positions
Exchange of coordinated solvent during crystallisation of a metal-organic framework observed by in situ high energy X-ray diffraction
Using time-resolved monochromatic high energy X-ray diffraction, we present an in situ study of the solvothermal crystallisation of a new MOF [Yb2(BDC)3(DMF)2]â‹…H2O (BDC=benzene-1,4-dicarboxylate and DMF=N,N-dimethylformamide) under solvothermal conditions, from mixed water/DMF solvent. Analysis of high resolution powder patterns obtained reveals an evolution of lattice parameters and electron density during the crystallisation process and Rietveld analysis shows that this is due to a gradual topochemical replacement of coordinated solvent molecules. The water initially coordinated to Yb3+ is replaced by DMF as the reaction progresses
Electron- and Hole-Doping Effects on -site Ordered NdBaMnO
We have investigated electron- and hole-doping effects on -site ordered
perovskite manganite NdBaMnO, which has the -type (layered)
antiferromagnetic (AFM) ground state. Electrons (holes) are introduced by
partial substitution of Ba (Nd) with Nd (Ba).
Electron-doping generates ferromagnetic (FM) clusters in the -type AFM
matrix. With increasing the electron-doping level, the volume fraction of the
FM phase or the number of the FM clusters is abruptly increasing. In contrast,
the -type AFM phase is robust against the hole-doping, and no FM correlation
is observed in the hole-doped NdBaMnO.Comment: 8 pages, 5 figures, to be published in Journal of the Physical
Society of Japa
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