2,121 research outputs found
Anisotropic strains and magnetoresistance of La_{0.7}Ca_{0.3}MnO_{3}
Thin films of perovskite manganite La_{0.7}Ca_{0.3}MnO_{3} were grown
epitaxially on SrTiO_3(100), MgO(100) and LaAlO_3(100) substrates by the pulsed
laser deposition method. Microscopic structures of these thin film samples as
well as a bulk sample were fully determined by x-ray diffraction measurements.
The unit cells of the three films have different shapes, i.e., contracted
tetragonal, cubic, and elongated tetragonal for SrTiO_3, MgO, and LaAlO_3
cases, respectively, while the unit cell of the bulk is cubic. It is found that
the samples with cubic unit cell show smaller peak magnetoresistance than the
noncubic ones do. The present result demonstrates that the magnetoresistance of
La_{0.7}Ca_{0.3}MnO_{3} can be controlled by lattice distortion via externally
imposed strains.Comment: Revtex, 10 pages, 2 figure
Impact of Charge Ordering on Magnetic Correlations in Perovskite (Bi,Ca)MnO_3
Single crystalline (Bi,Ca)MnO3 (74< %Ca <82) were studied with neutron
scattering, electron diffraction and bulk magnetic measurement. We discovered
dynamic ferromagnetic spin correlations at high temperatures, which are
replaced by antiferromagnetic spin fluctuations at a concomitant charge
ordering and structural transition. Our results indicate that thermal-activated
hopping of the Jahn-Teller active e_g electrons in these insulating materials,
nevertheless, induce ferromagnetic interaction through double-exchange
mechanism. It is the ordering of these charges competing with the
double-exchange ferromagnetic metallic state.Comment: 11 pages, 3 figures, Revte
Phase Competition in Ln0.5a0.5mno3 Perovskites
Single crystals of the systems Pr0.5(Ca1-xSrx)0.5MnO3,
(Pr1-yYy)0.5(Ca1-xSrx)0.5MnO3, and Sm0.5Sr0.5MnO3 were grown to provide a
series of samples with fixed ratio Mn(III)/Mn(IV)=1 having geometric tolerance
factors that span the transition from localized to itinerant electronic
behavior of the MnO3 array. A unique ferromagnetic phase appears at the
critical tolerance factor tc= 0.975 that separates charge ordering and
localized-electron behavior for t<tc from itinerant or molecular-orbital
behavior for t>tc. This ferromagnetic phase, which has to be distinguished from
the ferromagnetic metallic phase stabilized at tolerance factors t>tc,
separates two distinguishable Type-CE antiferromagnetic phases that are
metamagnetic. Measurements of the transport properties under hydrostatic
pressure were carried out on a compositions t a little below tc in order to
compare the effects of chemical vs. hydrostatic pressure on the phases that
compete with one another near t=tc.Comment: 10 pages. To be publised in Phys. Rev.
Double Degeneracy and Jahn-Teller Effects in CMR Perovskites
Jahn-Teller (JT) electron-phonon coupling effects in the colossal
magnetoresistance perovskite compounds are investigated.
Electron-electron correlations between two degenerate Mn orbitals are
studied in the Gutzwiller approximation. The static JT distortion and
antiadiabatic polaron effects are studied in a modified Lang-Firsov
approximation. We find that (i) the electron or hole character of the charge
carrier depends on the static JT distortion, and (ii) due to the two-component
nature of the JT coupling, fluctuations in the JT distortion direction
contribute to the charge transport in similar fashion as the local spins.Comment: 11 RevTeX pages. 3 Figures available upon request. submitted to Phys.
rev. B (Rapid Communications
Charge and orbital ordering in underdoped La1-xSrxMnO3
We have explored spin, charge and orbitally ordered states in La1-xSrxMnO3 (0
< x < 1/2) using model Hartree-Fock calculations on d-p-type lattice models. At
x=1/8, several charge and orbitally modulated states are found to be stable and
almost degenerate in energy with a homogeneous ferromagnetic state. The present
calculation indicates that a ferromagnetic state with a charge modulation along
the c-axis which is consistent with the experiment by Yamada et al. might be
responsible for the anomalous behavior around x = 1/8.Comment: 5 pages, 5 figure
Spin blockade, orbital occupation and charge ordering in La_(1.5)Sr_(0.5)CoO4
Using Co-L_(2,3) and O-K x-ray absorption spectroscopy, we reveal that the
charge ordering in La_(1.5)Sr_(0.5)CoO4 involves high spin (S=3/2) Co^2+ and
low spin (S=0) Co^3+ ions. This provides evidence for the spin blockade
phenomenon as a source for the extremely insulating nature of the
La_(2-x)Sr_(x)CoO4 series. The associated e_g^2 and e_g^0 orbital occupation
accounts for the large contrast in the Co-O bond lengths, and in turn, the high
charge ordering temperature. Yet, the low magnetic ordering temperature is
naturally explained by the presence of the non-magnetic (S=0) Co^3+ ions. From
the identification of the bands we infer that La_(1.5)Sr_(0.5)CoO4 is a narrow
band material.Comment: 5 pages, 3 figure
d0 Perovskite-Semiconductor Electronic Structure
We address the low-energy effective Hamiltonian of electron doped d0
perovskite semiconductors in cubic and tetragonal phases using the k*p method.
The Hamiltonian depends on the spin-orbit interaction strength, on the
temperature-dependent tetragonal distortion, and on a set of effective-mass
parameters whose number is determined by the symmetry of the crystal. We
explain how these parameters can be extracted from angle resolved
photo-emission, Raman spectroscopy, and magneto-transport measurements and
estimate their values in SrTiO3
Structural, magnetic and transport properties in the Cu-doped manganites La0.85Te0.15Mn1-xCuxO3 (0 \leq x \leq 0.20)
The effect of Cu-doping at Mn-site on structural, magnetic and transport
properties in electron-doped manganites La0.85Te0.15Mn1-xCuxO3 has been
investigated. Based on the analysis of structural parameter variations, the
valence state of the Cu ion in Cu-doped manganites is suggested to be +2. All
samples undergo the paramagnetic-ferromagnetic (PM-FM) phase transition. The
Curie temperature decreases and the transition becomes broader with increasing
Cu-doping level, in contrast, the magnetization magnitude of Cu-doping samples
at low temperatures increase as x \leq 0.15. The insulator-metal (I-M)
transition moves to lower temperatures with increasing Cu-doping content and
disappears as x > 0.1. In addition, the higher temperature resistivity r peak
in double-peak-like r(T) curves observed in no Cu-doping sample is completely
suppressed as Cu-doping level x = 0.1 and r(T) curve only shows single I-M
transition at the low temperature well below . The results are discussed
according to the change of magnetic exchange interaction caused by Cu-doping.Comment: 17 pages, 5 figures. Phys.Rev.B (to be published
Electronic structure and magnetic properties of the spin-1/2 Heisenberg system CuSe2O5
A microscopic magnetic model for the spin-1/2 Heisenberg chain compound
CuSe2O5 is developed based on the results of a joint experimental and
theoretical study. Magnetic susceptibility and specific heat data give evidence
for quasi-1D magnetism with leading antiferromagnetic (AFM) couplings and an
AFM ordering temperature of 17 K. For microscopic insight, full-potential DFT
calculations within the local density approximation (LDA) were performed. Using
the resulting band structure, a consistent set of transfer integrals for an
effective one-band tight-binding model was obtained. Electronic correlations
were treated on a mean-field level starting from LDA (LSDA+U method) and on a
model level (Hubbard model). In excellent agreement of experiment and theory,
we find that only two couplings in CuSe2O5 are relevant: the nearest-neighbour
intra-chain interaction of 165 K and a non-frustrated inter-chain coupling of
20 K. From a comparison with structurally related systems (Sr2Cu(PO4)2,
Bi2CuO4), general implications for a magnetic ordering in presence of
inter-chain frustration are made.Comment: 20 pages, 8 figures, 3 table
Valence, spin, and orbital state of the Co ions in the one-dimensional Ca3Co2O6: an x-ray absorption and magnetic circular dichroism study
We have investigated the valence, spin, and orbital state of the Co ions in
the one-dimensional cobaltate Ca3Co2O6 using x-ray absorption and x-ray
magnetic circular dichroism at the Co-L2,3 edges. The Co ions at both the
octahedral Co_oct and trigonal Co_trig sites are found to be in a 3+ state.
From the analysis of the dichroism we established a low-spin state for the
Co_oct and a high-spin state with an anomalously large orbital moment of 1.7
muB at the Co3+ trig ions. This large orbital moment along the c-axis chain and
the unusually large magnetocrystalline anisotropy can be traced back to the
double occupancy of the d2 orbital in trigonal crystal field.Comment: 5 pages, 4 figure
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