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Title: Competing Ferromagnetic and Charge ordered states in models for manganites: The Origin of the CMR effect

Abstract

The one-orbital model for manganites with cooperative phonons and superexchange coupling J{sub AF} is investigated via large-scale Monte Carlo simulations. The results for two orbitals are also briefly discussed. Focusing on the electron density n=0.75, a regime of competition between ferromagnetic metallic and charge-ordered (CO) insulating states is identified. In the vicinity of the associated bicritical point, colossal magnetoresistance (CMR) effects are observed. The CMR is associated with the development of short-distance correlations among polarons, above the spin ordering temperatures, resembling the charge arrangement of the low-temperature CO state.

Authors:
 [1];  [2];  [2]
  1. Florida State University
  2. ORNL
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); Center for Nanophase Materials Sciences; Center for Computational Sciences
Sponsoring Org.:
USDOE Laboratory Directed Research and Development (LDRD) Program; USDOE Office of Science (SC)
OSTI Identifier:
931476
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review Letters; Journal Volume: 98
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; ELECTRON DENSITY; FOCUSING; MAGNETORESISTANCE; ORIGIN; PHONONS; POLARONS; SPIN

Citation Formats

Sen, Cengiz, Alvarez, Gonzalo, and Dagotto, Elbio R. Competing Ferromagnetic and Charge ordered states in models for manganites: The Origin of the CMR effect. United States: N. p., 2007. Web. doi:10.1103/PhysRevLett.98.127202.
Sen, Cengiz, Alvarez, Gonzalo, & Dagotto, Elbio R. Competing Ferromagnetic and Charge ordered states in models for manganites: The Origin of the CMR effect. United States. doi:10.1103/PhysRevLett.98.127202.
Sen, Cengiz, Alvarez, Gonzalo, and Dagotto, Elbio R. Mon . "Competing Ferromagnetic and Charge ordered states in models for manganites: The Origin of the CMR effect". United States. doi:10.1103/PhysRevLett.98.127202.
@article{osti_931476,
title = {Competing Ferromagnetic and Charge ordered states in models for manganites: The Origin of the CMR effect},
author = {Sen, Cengiz and Alvarez, Gonzalo and Dagotto, Elbio R},
abstractNote = {The one-orbital model for manganites with cooperative phonons and superexchange coupling J{sub AF} is investigated via large-scale Monte Carlo simulations. The results for two orbitals are also briefly discussed. Focusing on the electron density n=0.75, a regime of competition between ferromagnetic metallic and charge-ordered (CO) insulating states is identified. In the vicinity of the associated bicritical point, colossal magnetoresistance (CMR) effects are observed. The CMR is associated with the development of short-distance correlations among polarons, above the spin ordering temperatures, resembling the charge arrangement of the low-temperature CO state.},
doi = {10.1103/PhysRevLett.98.127202},
journal = {Physical Review Letters},
number = ,
volume = 98,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • The metal-insulator transition, and the associated magnetic transition, in the colossal magnetoresistance (CMR) regime of the one-orbital model for manganites is studied here using Monte Carlo (MC) techniques in two-dimensional clusters. Both cooperative oxygen lattice distortions and a finite superexchange coupling among the t{sub 2g} spins are included in our investigations. Charge and spin correlations are studied. In the CMR regime, a strong competition between the ferromagnetic metallic and the antiferromagnetic charge-ordered insulating states is observed. This competition is shown to be important to understand the resistivity peak that appears near the critical temperature. Moreover, it is argued that themore » system is dynamically inhomogeneous with short-range charge and spin correlations that slowly evolve with MC time, producing the glassy characteristics of the CMR state. The local density of states (LDOS) is also investigated and a pseudogap (PG), identified as a dip in the LDOS at the Fermi energy, is found to exist in the CMR temperature range. The width of the PG in the LDOS is calculated and directly compared to recent scanning-tunneling-spectroscopy (STS) experimental results. The observed agreement between our calculation and the experiment suggests that the depletion of the conductance at low bias observed experimentally is a reflection on the existence of a PG in the LDOS spectra. The apparent homogeneity observed via STS techniques could be caused by the slow time characteristics of this probe. Faster experimental methods should unveil a rather inhomogeneous state in the CMR regime, as already observed in neutron-scattering experiments.« less
  • Most previous investigations have shown that the surface of a ferromagnetic material may have antiferromagnetic tendencies. However, experimentally, the opposite effect has been recently observed ferromagnetism appears in some nanosized manganites with a composition such that the antiferromagnetic charge-ordered CE state is observed in the bulk. A possible origin is the development of ferromagnetic correlations at the surface of these small systems. To clarify these puzzling experimental observations, we have studied the two-orbital double-exchange model near half doping, n=0.5, using open boundary conditions to simulate the surface of either bulk or nanosized manganites. Considering the enhancement of surface charge densitymore » due to a possible AO termination A=trivalent/ divalent ion composite, O=oxygen, an unexpected surface phase-separated state emerges when the model is studied using Monte Carlo techniques on small clusters. This tendency suppresses the CE charge ordering and produces a weak ferromagnetic signal that could explain the experimental observations.« less
  • Disorder broadening due to randomly distributed La{sup 3+} and A{sup 2+} (A=Ca,Sr,Ba) cations is combined with a virtual-crystal treatment of the average system to evaluate the effects on both majority and minority transport in the ferromagnetic La{sub 2/3}A{sub 1/3}MnO{sub 3} system. The low-density minority carriers which lie in the band tail are localized by disorder, while the majority carriers retain long mean free paths reflected in the observed strongly metallic conductivity. In addition to obtaining transport parameters, we provide evidence that local distortions are due to nearby ionic charges rather than to ion size considerations. {copyright} {ital 1997} {ital Themore » American Physical Society}« less
  • No abstract prepared.