57 research outputs found
Volume contraction at the Jahn-Teller transition of LaMnO
We have studied the volume collapse of LaMnO at the Jahn- Teller (JT)
transition temperature T=750 K which has recently been found in high
temperature powder x- ray and neutron diffraction experiments. We construct a
model Hamiltonian involving the pseudospin of Mn e states, the
staggered JT distortion and the volume strain coordinate. We show that the
anharmonic coupling between these primary and secondary order parameters leads
to the first order JT phase transition associated with a comparatively large
reduction of the unit cell volume of V/V 10. We explain
the temperature dependence of JT distortions and volume strain and discuss the
volume change as function of the anharmonic coupling constant. A continuous
change to a second order transition as function of model parameters is
obtained. This behaviour is also observed under Ba doping.Comment: 5 pages, 4 figure
Commensurate-Incommensurate transition in the melting process of the orbital ordering in Pr0.5Ca0.5MnO3: neutron diffraction study
The melting process of the orbital order in
Pr0.5Ca0.5MnO3 single crystal has been studied in detail as a function of
temperature by neutron diffraction. It is demonstrated that a
commensurate-incommensurate (C-IC) transition of the orbital ordering takes
place in a bulk sample, being consistent with the electron diffraction studies.
The lattice structure and the transport properties go through drastic changes
in the IC orbital ordering phase below the charge/orbital ordering temperature
Tco/oo, indicating that the anomalies are intimately related to the partial
disordering of the orbital order, unlike the consensus that it is related to
the charge disordering process. For the same T range, partial disorder of the
orbital ordering turns on the ferromagnetic spin fluctuations which were
observed in a previous neutron scattering study.Comment: 5 pages, 2 figures, REVTeX, to be published in Phys. Rev.
Ferromagnetic Polarons in La0.5Ca0.5MnO3 and La0.33Ca0.67MnO3
Unrestricted Hartree-Fock calculations on La0.5Ca0.5MnO3 and La0.33Ca0.67MnO3
in the full magnetic unit cell show that the magnetic ground states of these
compounds consist of 'ferromagnetic molecules' or polarons ordered in
herring-bone patterns. Each polaron consists of either three or five Mn ions
separated by O- ions with a magnetic moment opposed to those of the Mn ions.
Ferromagnetic coupling within the polarons is strong while coupling between
them is relatively weak. Magnetic moments on the Mn ions range between 3.8 and
3.9 Bohr magnetons in La0.5Ca0.5MnO3 and moments on the O- ions are -0.7 Bohr
magnetons. Each polaron has a net magnetic moment of 7.0 Bohr magnetons, in
good agreement with recently reported magnetisation measurements from electron
microscopy. The polaronic nature of the electronic structure reported here is
obviously related to the Zener polaron model recently proposed for
Pr0.6Ca0.4MnO3 on the basis of neutron scattering data.Comment: 4 pages 5 figure
Thickness dependence of the stability of the charge-ordered state in PrCaMnO thin films
Thin films of the charge-ordered (CO) compound PrCaMnO
have been deposited onto (100)-oriented SrTiO substrates using the Pulsed
Laser Deposition technique. Magnetization and transport properties are measured
when the thickness of the film is varied. While the thinner films do not
exhibit any temperature induced insulator-metal transition under an applied
magnetic field up to 9T, for thickness larger than 1100\UNICODE{0xc5} a 5T
magnetic field is sufficient to melt the CO state. For this latest film, we
have measured the temperature-field phase diagram. Compared to the bulk
material, it indicates that the robustness of the CO state in thin films is
strongly depending on the strains and the thickness. We proposed an explanation
based on the distortion of the cell of the film.Comment: 9 pages, 6 figures, submitted to Phys. Rev.
Orbital Ordering in Paramagnetic LaMnO3 and KCuF3
{\it Ab-initio} studies of the stability of orbital ordering, its coupling to
magnetic structure and its possible origins (electron-phonon and/or
electron-electron interactions) are reported for two perovskite systems,
LaMnO and KCuF. We present a new Average Spin State (ASS) calculational
scheme that allowed us to treat a paramagnetic state. Using this scheme, we
succesfully described the experimental magnetic/orbital phase diagram of both
LaMnO and KCuF in crystal structures when the Jahn-Teller distortions
are neglected. Hence, we conclude that the orbital ordering in both compounds
is purely electronic in origin.Comment: 10 pages, 5 figure
Magnetic Phases of Electron-Doped Manganites
We study the anisotropic magnetic structures exhibited by electron-doped
manganites using a model which incorporates the double-exchange between orbital
ly degenerate electrons and the super-exchange between
electrons with realistic values of the Hund's coupling(), the
super-exchange coupling(), and the bandwidth(). We look at the
relative stabilities of the G, C and A type antiferromagnetic ph ases. In
particular we find that the G-phase is stable for low electron doping as seen
in experiments. We find good agreement with the experimentally observed
magnetic phase diagrams of electron-doped manganites
() such as NdSrMnO, PrSrMnO,
and SmCaMnO. We can also explain the experimentally
observed orbital structures of the C a nd A phases.
We also extend our calculation for electron-doped bilayer manganites of the
form RAMnO and predict that the C-phase will be
absent in t hese systems due to their reduced dimensionality.Comment: 7 .ps files included. To appear in Phys. Rev. B (Feb 2001
Optical Investigations of Charge Gap in Orbital Ordered La1/2Sr3/2MnO4
Temperature and polarization dependent electronic structure of La1/2Sr3/2MnO4
were investigated by optical conductivity analyses. With decreasing
temperature, for E//ab, a broad mid-infrared (MIR) peak of La1/2Sr3/2MnO4
becomes narrower and moves to the higher frequency, while that of
Nd1/2Sr3/2MnO4 nearly temperature independent. We showed that the MIR peak in
La1/2Sr3/2MnO4 originates from orbital ordering associated with CE-type
magnetic ordering and that the Jahn-Teller distortion has a significant
influence on the width and the position of the MIR peak.Comment: 10 pages, 4 figure
Field-induced segregation of ferromagnetic nano-domains in PrSrMnO, detected by Mn NMR
The antiferromagnetic manganite PrSrMnO was investigated
at low temperature by means of magnetometry and Mn NMR. A field-induced
transition to a ferromagnetic state is detected by magnetization measurements
at a threshold field of a few tesla. NMR shows that the ferromagnetic phase
develops from zero field by the nucleation of microscopic ferromagnetic
domains, consisting of an inhomogeneous mixture of tilted and fully aligned
parts. At the threshold the NMR spectrum changes discontinuously into that of a
homogeneous, fully aligned, ferromagnetic state, suggesting a percolative
origin for the ferromagnetic transition.Comment: Latex 2.09 language. 4 pages, 3 figures, 23 references. Submitted to
physical Review
Polaronic Signatures in Mid-Infrared Spectra: Prediction for LaMnO3 and CaMnO3
Hole-doped LaMnO3 and electron-doped CaMnO3 form self-trapped electronic
states. The spectra of these states have been calculated using a two orbital
(Mn eg Jahn-Teller) model, from which the non-adiabatic optical conductivity
spectra are obtained. In both cases the optical spectrum contains weight in the
gap region, whose observation will indicate the self-trapped nature of the
carrier states. The predicted spectra are proportional to the concentration of
the doped carriers in the dilute regime, with coefficients calculated with no
further model parameters.Comment: 6 pages with 3 figures imbedde
Theory of spin wave excitations of metallic A-type antiferromagnetic manganites
The spin dynamic of the metallic A-type antiferromagnetic manganites is
studied. An effective nearest-neighbour Heisenberg spin wave dispersion is
derived from the double exchange model taking into account the superexchange
interaction between the core spins. The result of inelastic neutron scattering
experiment on is qualitatively reproduced.
Comparing theory with experimental data two main parameters of the model:
nearest-neighbour electron transfer amplitude and superexchange coupling
between the core spins are estimated.Comment: to appear in Phys. Rev.
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