27 research outputs found
Phase diagram for Ca_{1-x}Y_xMnO_3 type crystals
We present a simple model to study the electron doped manganese perovskites.
The model considers the competition between double exchange mechanism for
itinerant electrons and antiferromagnetic superexchange interaction for
localized electrons. It represents each Mn^{4+} ion by a spin 1/2, on which an
electron can be added to produce Mn^{3+}; we include a hopping energy t, a
strong intratomic interaction exchange J (in the limit J/t>>1), and an
interatomic antiferromagnetic interaction K between the local spins. Using the
Renormalized Perturbation Expansion and a Mean Field Approximation on the
hopping terms and on the superexchange interaction we calculate the free
energy. From it, the stability of the antiferromagnetic, canted, ferromagnetic,
and novel spin glass phases can be determined as functions of the parameters
characterizing the system. The model results can be expressed in terms of t and
K for each value of the doping x in phase diagrams. The magnetization m and
canting angle can also be calculated as fuctions of temperature for fixed
values of doping and model parameters.Comment: 4 figure
The effect of Coulomb interaction at ferromagnetic-paramagnetic metallic perovskite junctions
We study the effect of Coulomb interactions in transition metal oxides
junctions. In this paper we analyze charge transfer at the interface of a three
layer ferromagnetic-paramagnetic-ferromagnetic metallic oxide system. We choose
a charge model considering a few atomic planes within each layer and obtain
results for the magnetic coupling between the ferromagnetic layers. For large
number of planes in the paramagnetic spacer we find that the coupling
oscillates with the same period as in RKKY but the amplitude is sensitive to
the Coulomb energy. At small spacer thickness however, large differences may
appear as function of : the number of electrons per atom in the ferromagnetics
and paramagnetics materials, the dielectric constant at each component, and the
charge defects at the interface plane emphasizing the effects of charge
transfer.Comment: tex file and 7 figure
Effect of disorder on the magnetic and transport properties of La_{1-x}Sr_{x}MnO_{3}
We study a simplified model of the electronic structure of compounds of the
type of LaSrMnO. The model represents each Mn ion by a
spin S=1/2, on which an electron can be added to produce Mn. We include
two strong intratomic interactions in the Hamiltonian: exchange (% ) and
Coulomb (). Finally, to represent the effect of Sr substitution by La in a
simple way, we include a distribution of diagonal energies at the Mn sites.
Then we use Green function techniques to calculate a mobility edge and the
average density of states. We find that according to the amount of disorder and
to the concentration of electrons in the system, the Fermi level can cross the
mobility edge to produce a metal to insulator transition as the magnetization
decreases (increase of temperature). If the disorder is large, the system
remains insulating for all concentrations. Concentrations near zero or one
favor the insulating state while intermediate values of concentration favor the
metallic state.Comment: 11 pages, 4 figures available upon request, accepted for publication
in Solid State Communication
Electron-Doped Manganese Perovskites: The Polaronic State
Using the Lanczos method in linear chains we study the ground state of the
double exchange model including an antiferromagnetic super-exchange in the low
concentration limit. We find that this ground state is always inhomogeneous,
containig ferromagnetic polarons. The extention of the polaron spin distortion,
the dispersion relation and their trapping by impurities, are studied for
diferent values of the super exchange interaction and magnetic field. We also
find repulsive polaron polaron interaction.Comment: 4 pages, 6 embedded figure
Intermediate Valence Model for the Colossal Magnetoresistance in Tl_{2}Mn_{2}O_{7}
The colossal magnetoresistance exhibited by Tl_{2}Mn_{2}O_{7} is an
interesting phenomenon, as it is very similar to that found in perovskite
manganese oxides although the compound differs both in its crystalline
structure and electronic properties from the manganites. At the same time,
other pyrochlore compounds, though sharing the same structure with
Tl_{2}Mn_{2}O_{7}, do not exhibit the strong coupling between magnetism and
transport properties found in this material. Mostly due to the absence of
evidence for significant doping into the Mn-O sublattice, and the tendency of
Tl to form conduction bands, the traditional double exchange mechanism
mentioned in connection with manganites does not seem suitable to explain the
experimental results in this case. We propose a model for Tl_{2}Mn_{2}O_{7}
consisting of a lattice of intermediate valence ions fluctuating between two
magnetic configurations, representing Mn-3d orbitals, hybridized with a
conduction band, which we associate with Tl. This model had been proposed
originally for the analysis of intermediate valence Tm compounds. With a
simplified treatment of the model we obtain the electronic structure and
transport properties of Tl_{2}Mn_{2}O_{7}, with good qualitative agreement to
experiments. The presence of a hybridization gap in the density of states seems
important to understand the reported Hall data.Comment: 8 pages + 5 postscript fig
The periodic Anderson model from the atomic limit and FeSi
The exact Green's functions of the periodic Anderson model for
are formally expressed within the cumulant expansion in terms of an effective
cumulant. Here we resort to a calculation in which this quantity is
approximated by the value it takes for the exactly soluble atomic limit of the
same model. In the Kondo region a spectral density is obtained that shows near
the Fermi surface a structure with the properties of the Kondo peak.
Approximate expressions are obtained for the static conductivity
and magnetic susceptibility of the PAM, and they are employed to fit
the experimental values of FeSi, a compound that behaves like a Kondo insulator
with both quantities vanishing rapidly for . Assuming that the system
is in the intermediate valence region, it was possible to find good agreement
between theory and experiment for these two properties by employing the same
set of parameters. It is shown that in the present model the hybridization is
responsible for the relaxation mechanism of the conduction electrons.Comment: 26 pages and 8 figure
Ferromagnetic Polarons in Manganites
Using the Lanczos method in linear chains we study the double exchange model
in the low concentration limit, including an antiferromagnetic super-exchange
K. In the strong coupling limit we find that the ground state contains
ferromagnetic polarons whose length is very sensitive to the value of K/t. We
investigate the dispersion relation, the trapping by impurities, and the
interaction between these polarons. As the overlap between polarons increases,
by decreasing K/t, the effective interaction between them changes from
antiferromagnetic to ferromagnetic. The scaling to the thermodynamic limit
suggests an attractive interaction in the strong coupling regime (J_h > t) and
no binding in the weak limit (J_h \simeq t).Comment: 12 pages, accepted in PRB, to be published in Novembe