118 research outputs found
An Origin of CMR: Competing Phases and Disorder-Induced Insulator-to-Metal Transition in Manganites
We theoretically explore the mechanism of the colossal magnetoresistance in
manganese oxides by explicitly taking into account the phase competition
between the double-exchange ferromagnetism and the charge-ordered insulator. We
find that quenched disorder causes a drastic change of the multicritical phase
diagram by destroying the charge-ordered state selectively. As a result, there
appears a nontrivial phenomenon of the disorder-induced insulator-to-metal
transition in the multicritical regime. On the contrary, the disorder induces a
highly-insulating state above the transition temperature where charge-ordering
fluctuations are much enhanced. The contrasting effects provide an
understanding of the mechanism of the colossal magnetoresistance. The obtained
scenario is discussed in comparison with other theoretical proposals such as
the polaron theory, the Anderson localization, the multicritical-fluctuation
scenario, and the percolation scenario.Comment: 16 pages, 7 figures, submitted to Wandlitz Days on Magnetism:
Local-Moment Ferromagnets: Unique Properties for Modern Application
Global versus Local Ferromagnetism in a Model for Diluted Magnetic Semiconductors Studied with Monte Carlo Techniques
A model recently introduced for diluted magnetic semiconductors by Berciu and
Bhatt (PRL 87, 107203 (2001)) is studied with a Monte Carlo technique, and the
results are compared to Hartree-Fock calculations. For doping rates close to
the experimentally observed metal-insulator transition, a picture dominated by
ferromagnetic droplets formed below a T* scale emerges. The moments of these
droplets align as the temperature is lowered below a critical value Tc<T*. Our
Monte Carlo investigations provide critical temperatures considerably smaller
than Hartree-Fock predictions. Disorder does not seem to enhance ferromagnetism
substantially. The inhomogeneous droplet state should be strongly susceptible
to changes in doping and external fields.Comment: 4 pages, 4 figure
Study of Percolative Transitions with First-Order Characteristics in the Context of CMR Manganites
The unusual magneto-transport properties of manganites are widely believed to
be caused by mixed-phase tendencies and concomitant percolative processes.
However, dramatic deviations from "standard" percolation have been unveiled
experimentally. Here, a semi-phenomenological description of Mn oxides is
proposed based on coexisting clusters with smooth surfaces, as suggested by
Monte Carlo simulations of realistic models for manganites, also briefly
discussed here. The present approach produces fairly abrupt percolative
transitions and even first-order discontinuities, in agreement with
experiments. These transitions may describe the percolation that occurs after
magnetic fields align the randomly oriented ferromagnetic clusters believed to
exist above the Curie temperature in Mn oxides. In this respect, part of the
manganite phenomenology could belong to a new class of percolative processes
triggered by phase competition and correlations.Comment: 4 pages, 4 eps figure
Pressure induced transition from a spin glass to an itinerant ferromagnet in half doped manganite Ln0.5Ba0.5MnO3 (Ln=Sm and Nd) with quenched disorder
The effect of quenched disorder on the multiphase competition has been
investigated by examining the pressure phase diagram of half doped manganite
Ln0.5B0.5MnO3 (Ln = Sm and Nd) with A-site disorders. Sm0.5Ba0.5MnO3, a spin
glass insulator at ambient pressure, switches to a ferromagnetic metal with
increasing pressure, followed by a rapid increase of the ferromagnetic
transition temperature Tc. The rapid increase of Tc was confirmed also for
Nd0.5Ba0.5MnO3. These observations indicate that the unusual suppression of the
multicritical phase boundary in the A-site disordered system, previously
observed as a function of the averaged A-site ionic radius, is essentially
controlled by the pressure and hence the band width. The effect of quenched
disorder is therefore much enhanced with approaching the multicritical region.Comment: 4 pages including 3 figure
Dopant-dependent impact of Mn-site doping on the critical-state manganites: R0.6Sr0.4MnO3 (R=La, Nd, Sm, and Gd)
Versatile features of impurity doping effects on perovskite manganites,
SrMnO, have been investigated with varying the doing
species as well as the -dependent one-electron bandwidth. In
ferromagnetic-metallic manganites (=La, Nd, and Sm), a few percent of Fe
substitution dramatically decreases the ferromagnetic transition temperature,
leading to a spin glass insulating state with short-range charge-orbital
correlation. For each species, the phase diagram as a function of Fe
concentration is closely similar to that for SrMnO
obtained by decreasing the ionic radius of site, indicating that Fe doping
in the phase-competing region weakens the ferromagnetic double-exchange
interaction, relatively to the charge-orbital ordering instability. We have
also found a contrastive impact of Cr (or Ru) doping on a spin-glass insulating
manganite (=Gd). There, the impurity-induced ferromagnetic magnetization is
observed at low temperatures as a consequence of the collapse of the inherent
short-range charge-orbital ordering, while Fe doping plays only a minor role.
The observed opposite nature of impurity doping may be attributed to the
difference in magnitude of the antiferromagnetic interaction between the doped
ions.Comment: 7 pages, 6 figure
Charge-ordered ferromagnetic phase in manganites
A mechanism for charge-ordered ferromagnetic phase in manganites is proposed.
The mechanism is based on the double exchange in the presence of diagonal
disorder. It is modeled by a combination of the Ising double-exchange and the
Falicov-Kimball model. Within the dynamical mean-field theory the charge and
spin correlation function are explicitely calculated. It is shown that the
system exhibits two successive phase transitions. The first one is the
ferromagnetic phase transition, and the second one is a charge ordering. As a
result a charge-ordered ferromagnetic phase is stabilized at low temperature.Comment: To appear in Phys. Rev.
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.
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