284 research outputs found
Magnetic Transition Temperature of (La,Sr)MnO
Using the Kondo lattice model with classical spins in infinite dimension,
magnetic phase transition in the perovskite-type transition-metal oxide
(La,Sr)MnO is theoretically studied. On the Bethe lattice, the
self-consistency equations are solved exactly. Curie temperatures at the region
of double-exchange ferromagnetism as well as the Neel
temperature at are well reproduced quantitatively. Pressure effect on the
Curie temperature is also discussed.Comment: 7 pages, 1 PS file with 3 figures appended at the end, LaTe
Phase Diagram of Half Doped Manganites
An analysis of the properties of half-doped manganites is presented. We build
up the phase diagram of the system combining a realistic calculation of the
electronic properties and a mean field treatment of the temperature effects.
The electronic structure of the manganites are described with a double exchange
model with cooperative Jahn-Teller phonons and antiferromagnetic coupling
between the core spins. At zero temperature a variety of electronic phases
as ferromagnetic (FM) charge ordered (CO) orbital ordered (OO), CE-CO-OO and FM
metallic, are obtained. By raising the temperature the CE-CO-OO phase becomes
paramagnetic (PM), but depending on the electron-phonon coupling and the
exchange coupling the transition can be direct or trough intermediate states: a
FM disorder metallic, a PM-CO-OO or a FM-CO-OO. We also discus the nature of
the high temperature PM phase in the regime of finite electron phonon coupling.
In this regime half of the oxygen octahedra surrounding the ions are
distorted. In the weak coupling regime the octahedra are slightly deformed and
only trap a small amount of electronic charge, rendering the system metallic
consequentially. However in the strong coupling regime the octahedra are
strongly distorted, the charge is fully localized in polarons and the system is
insulator.Comment: 10 pagses, 9 figures include
Superconducting and normal-state interlayer-exchange-coupling in LaSrMnO-YBaCuO_{0.67}_{0.33}{3}$ epitaxial trilayers
The issue of interlayer exchange coupling in magnetic multilayers with
superconducting (SC) spacer is addressed in LaSrMnO
(LSMO) - YBaCuO (YBCO) - LaSrMnO
(LSMO) epitaxial trilayers through resistivity, ac-susceptibility and
magnetization measurements. The ferromagnetic (FM) LSMO layers possessing
in-plane magnetization suppress the critical temperature (T of the
c-axis oriented YBCO thin film spacer. The superconducting order, however,
survives even in very thin layers (thickness d 50 {\AA}, 4
unit cells) at T 25 K. A predominantly antiferromagnetic (AF) exchange
coupling between the moments of the LSMO layers at fields 200 Oe is seen in
the normal as well as the superconducting states of the YBCO spacer. The
exchange energy J ( 0.08 erg/cm at 150 K for d = 75
{\AA}) grows on cooling down to T, followed by truncation of this growth
on entering the superconducting state. The coupling energy J at a fixed
temperature drops exponentially with the thickness of the YBCO layer. The
temperature and d dependencies of this primarily non-oscillatory J
are consistent with the coupling theories for systems in which transport is
controlled by tunneling. The truncation of the monotonic T dependence of
J below T suggests inhibition of single electron tunneling across
the CuO planes as the in-plane gap parameter acquires a non-zero value.Comment: Accepted for publication in Phys. Rev.
Resonant inelastic x-ray scattering study of hole-doped manganites La1-xSrxMnO3 (x=0.2 and 0.4)
Electronic excitations near the Fermi energy in the hole doped manganese
oxides (La1-xSrxMnO3, x=0.2 and 0.4) have been elucidated by using the resonant
inelastic x-ray scattering (RIXS) method. A doping effect in the strongly
correlated electron systems has been observed for the first time. The
scattering spectra show that a salient peak appears in low energies indicating
the persistence of the Mott gap. At the same time, the energy gap is partly
filled by doping holes and the energy of the spectral weight shifts toward
lower energies. The excitation spectra show little change in the momentum space
as is in undoped LaMnO3, but the scattering intensities in the low energy
excitations of x=0.2 are anisotropic as well as temperature dependent, which
indicates a reminiscence of the orbital nature
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
An X-Ray Induced Structural Transition in La_0.875Sr_0.125MnO_3
We report a synchrotron x-ray scattering study of the magnetoresistive
manganite La_0.875Sr_0.125MnO_3. At low temperatures, this material undergoes
an x-ray induced structural transition at which charge ordering of Mn^3+ and
Mn^4+ ions characteristic to the low-temperature state of this compound is
destroyed. The transition is persistent but the charge-ordered state can be
restored by heating above the charge-ordering transition temperature and
subsequently cooling. The charge-ordering diffraction peaks, which are
broadened at all temperatures, broaden more upon x-ray irradiation, indicating
the finite correlation length of the charge-ordered state. Together with the
recent reports on x-ray induced transitions in Pr_(1-x)Ca_xMnO_3, our results
demonstrate that the photoinduced structural change is a common property of the
charge-ordered perovskite manganites.Comment: 5 pages, 4 embedded EPS figures; significant changes in the data
analysis mad
Existence of orbital polarons in ferromagnetic insulating LaSrMnO (0.110.14) evidenced by giant phonon softening
We present an inelastic light scattering study of single crystalline
(LaPr)SrMnO (, and
,). A giant softening up to 20 - 30 cm of the
Mn-O breathing mode has been observed only for the ferromagnetic insulating
(FMI) samples () upon cooling below the Curie
temperature. With increasing Pr-doping the giant softening is gradually
suppressed. This is attributed to a coupling of the breathing mode to orbital
polarons which are present in the FMI phase.Comment: 4 pages, 5 figure
Interplay of the CE-type charge ordering and the A-type spin ordering in a half-doped bilayer manganite La{1}Sr{2}Mn{2}O{7}
We demonstrate that the half-doped bilayer manganite La_{1}Sr_{2}Mn_{2}O_{7}
exhibits CE-type charge-ordered and spin-ordered states below K and below K, respectively. However, the volume
fraction of the CE-type ordering is relatively small, and the system is
dominated by the A-type spin ordering. The coexistence of the two types of
ordering is essential to understand its transport properties, and we argue that
it can be viewed as an effective phase separation between the metallic
orbital ordering and the charge-localized
orbital ordering.Comment: 5 pages, 4 figures, submitted to Phys. Rev.
The Density of States of hole-doped Manganites: A Scanning Tunneling Microscopy/Spectroscopy study
Variable temperature scanning tunneling microscopy/spectroscopy studies on
single crystals and epitaxial thin films of hole-doped manganites, which show
colossal magnetoresistance, have been done. We have investigated the variation
of the density of states, at and near the Fermi energy (), as a function
of temperature. Simple calculations have been carried out, to find out the
effect of temperature on the tunneling spectra and extract the variation of
density of states with temperature, from the observed data. We also report
here, atomic resolution images, on the single crystals and larger range images
showing the growth patterns on thin films. Our investigation shows
unambiguously that there is a rapid variation in density of states for
temperatures near the Curie temperature (). While for temperatures below
, a finite DOS is observed at , for temperatures near a hard
gap opens up in the density of states near . For temperatures much higher
than , this gap most likely gives way to a soft gap. The observed hard gap
for temperatures near , is somewhat higher than the transport gap for all
the materials. For different materials, we find that the magnitude of the hard
gap decreases as the of the material increases and eventually, for
materials with a close to 400 K, the value of the gap approaches zero.Comment: 9 pages RevTeX, 12 postscript figures, 1 table included in text,
submitted to Physical Review
Spin Excitation Spectrum of LaMnO
As an effective model to describe perovskite-type manganates (La,)MnO,
the double-exchange model on a cubic lattice is investigated. Spin excitation
spectrum of the model in the ground state is studied using the spin wave
approximation. Spin wave dispersion relation observed in the inelastic neutron
scattering experiment of LaPbMnO is reproduced. Effective
values for the electron bandwidth as well as Hund's coupling is estimated from
the data.Comment: 10 pages LaTeX including 4 PS figure
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