178 research outputs found
Conductance through a Magnetic Domain Wall in Double Exchange System
The conductance through a magnetic domain wall is calculated for the double
exchange system as a function of energy and the width of the domain wall. It is
shown that when the carrier density is low enough, the blockade is almost
complete even for the smoothly varying spin configuration, i.e., large width of
the domain wall. This result is applied to the manganese oxides.Comment: 3 pages, LaTex, 1 Postscript figur
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
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
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
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.
Critical Temperature of Ferromagnetic Transition in Three-Dimensional Double-Exchange Models
Ferromagnetic transition in three-dimensional double-exchange models is
studied by the Monte Carlo method. Critical temperature is
precisely determined by finite-size scaling analysis. Strong spin fluctuations
in this itinerant system significantly reduce from mean-field
estimates. By choosing appropriate parameters, obtained values of
quantitatively agree with experiments for the ferromagnetic metal regime of
(La,Sr)MnO, which is a typical perovskite manganite showing colossal
magnetoresistance. This indicates that the double-exchange mechanism alone is
sufficient to explain in this material. Critical exponents are also
discussed.Comment: 4 pages including 1 table and 4 figures, to be published in J. Phys.
Soc. Jp
Observation of anomalous single-magnon scattering in half-metallic ferromagnets by chemical pressure control
Temperature variation of resistivity and specific heat have been measured for
prototypical half-metallic ferromagnets,
R_0.6Sr_0.4MnO_3, with controlling the one-electron bandwidth W. We have
found variation of the temperature scalings in the resistivity from
T^2 (R = La, and Nd) to T^3 (R = Sm), and have interpreted the $T^3-law in
terms of the anomalous single-magnon scattering (AMS) process in the
half-metallic system.Comment: To appear in Phys. Rev. Lett., 3 pages + 4 EPS figure
Coulomb correlation and magnetic ordering in double-layered manganites: LaSrMnO
A detailed study of the electronic structure and magnetic configurations of
the 50 % hole-doped double layered manganite LaSrMnO is presented.
We demonstrate that the on-site Coulomb correlation (U) of Mn d electrons {\it
(i)} significantly modifies the electronic structure, magnetic ordering (from
FM to AFM), and interlayer exchange interactions, and {\it (ii)} promotes
strong anisotropy in electrical transport, reducing the effective hopping
parameter along the {\it c} axis for electrically active electrons. This
findng is consistent with observations of anisotropic transport -- a property
which sets this manganite apart from conventional 3D systems. A half-metallic
band structure is predicted with both the LSDA and LSDA+U methods. The
experimentally observed A-type AFM ordering in LaSrMnO is found to
be energetically more favorable with U 7 eV. A simple interpretation of
interlayer exchange coupling is given within double and super-exchange
mechanisms based on the dependencies on U of the effective exchange parameters
and state sub-band widths.Comment: 10 pages, 5 figure
Relation between crystal and magnetic structures of the layered manganites La2-2xSr1+2xMn2O7 (0.30 =< x =< 0.50)
Comprehensive neutron-powder diffraction and Rietveld analyses were carried
out to clarify the relation between the crystal and magnetic structures of
La2-2xSr1+2xMn2O7 (0.30 =< x =< 0.50). The Jahn-Teller (JT) distortion of Mn-O6
octahedra, i.e., the ratio of the averaged apical Mn-O bond length to the
equatorial Mn-O bond length, is Delta_JT=1.042(5) at x=0.30, where the magnetic
easy-axis at low temperature is parallel to the c axis. As the JT distortion
becomes suppressed with increasing x, a planar ferromagnetic structure appears
at x =< 0.32, which is followed by a canted antiferromagnetic (AFM) structure
at x =< 0.39. The canting angle between neighboring planes continuously
increases from 0 deg (planar ferromagnet: 0.32 =< x < 0.39) to 180 deg (A-type
AFM: x=0.48 where Delta_JT=1.013(5)). Dominance of the A-type AF structure with
decrease of JT distortion can be ascribed to the change in the eg orbital state
from d3z^2-r^2 to dx^2-y^2
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