3,658 research outputs found
Ferromagnetic insulating phase in Pr{1-x}Ca{x}MnO3
A ferromagnetic insulating (FM-I) state in Pr0.75Ca0.25MnO3 has been studied
by neutron scattering experiment and theoretical calculation. The insulating
behavior is robust against an external magnetic field, and is ascribed to
neither the phase separation between a ferromagnetic metallic (FM-M) phase and
a non-ferromagnetic insulating one, nor the charge ordering. We found that the
Jahn-Teller type lattice distortion is much weaker than PrMnO3 and the magnetic
interaction is almost isotropic. These features resembles the ferromagnetic
metallic state of manganites, but the spin exchange interaction J is much
reduced compared to the FM-M state. The theoretical calculation based on the
staggered type orbital order well reproduces several features of the spin and
orbital state in the FM-I phase.Comment: REVTeX4, 10 pages, 9 figure
Spin Dynamics of Double-Exchange Manganites with Magnetic Frustration
This work examines the effects of magnetic frustration due to competing
ferromagnetic and antiferromagnetic Heisenberg interactions on the spin
dynamics of the double-exchange model. When the local moments are non-colinear,
a charge-density wave forms because the electrons prefer to sit on lines of
sites that are coupled ferromagnetically. With increasing hopping energy, the
local spins become aligned and the average spin-wave stiffness increases. Phase
separation is found only within a narrow range of hopping energies. Results of
this work are applied to the field-induced jump in the spin-wave stiffness
observed in the manganite PrCaMnO with .Comment: 10 pages, 3 figure
Spin Dynamics of a Canted Antiferromagnet in a Magnetic Field
The spin dynamics of a canted antiferromagnet with a quadratic spin-wave
dispersion near \vq =0 is shown to possess a unique signature. When the
anisotropy gap is negligible, the spin-wave stiffness \dsw (\vq, B) =
(\omega_{\vq}-B)/q^2 depends on whether the limit of zero field or zero
wavevector is taken first. Consequently, \dsw is a strong function of
magnetic field at a fixed wavevector. Even in the presence of a sizeable
anisotropy gap, the field dependence of both \dsw and the gap energy
distinguishes a canted antiferromagnet from a phase-separated mixture
containing both ferromagnetic and antiferromagnetic regions.Comment: 10 pages, 3 figure
Property of Large-sized Cylindrical Superconductor Composed with Ni Meshes for a Current Lead
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.
Spin dynamical properties and orbital states of the layered perovskite La_2-2x_Sr_1+2x_Mn_2_O_7 (0.3 <= x < 0.5)
Low-temperature spin dynamics of the double-layered perovskite
La_2-2x_Sr_1+2x_Mn_2_O_7 (LSMO327) was systematically studied in a wide hole
concentration range (0.3 <= x < 0.5). The spin-wave dispersion, which is almost
perfectly 2D, has two branches due to a coupling between layers within a
double-layer. Each branch exhibits a characteristic intensity oscillation along
the out-of-plane direction. We found that the in-plane spin stiffness constant
and the gap between the two branches strongly depend on x. By fitting to
calculated dispersion relations and cross sections assuming Heisenberg models,
we have obtained the in-plane (J_para), intra-bilayer (J_perp) and
inter-bilayer (J') exchange interactions at each x. At x=0.30, J_para=-4meV and
J_perp=-5meV, namely almost isotropic and ferromagnetic. Upon increasing x,
J_perp rapidly approaches zero while |J_para| increases slightly, indicating an
enhancement of the planar magnetic anisotropy. At x=0.48, J_para reaches -9meV,
while J_perp turns to +1meV indicating an antiferromagnetic interaction. Such a
drastic change of the exchange interactions can be ascribed to the change of
the relative stability of the d_x^2-y^2 and d_3z^2-r^2 orbital states upon
doping. However, a simple linear combination of the two states results in an
orbital state with an orthorhombic symmetry, which is inconsistent with the
tetragonal symmetry of the crystal structure. We thus propose that an ``orbital
liquid'' state realizes in LSMO327, where the charge distribution symmetry is
kept tetragonal around each Mn site.Comment: 10 pages including 7 figure
Double Exchange in a Magnetically Frustrated System
This work examines the magnetic order and spin dynamics of a double-exchange
model with competing ferromagnetic and antiferromagnetic Heisenberg
interactions between the local moments. The Heisenberg interactions are
periodically arranged in a Villain configuration in two dimensions with
nearest-neighbor, ferromagnetic coupling and antiferromagnetic coupling
. This model is solved at zero temperature by performing a
expansion in the rotated reference frame of each local moment.
When exceeds a critical value, the ground state is a magnetically
frustrated, canted antiferromagnet. With increasing hopping energy or
magnetic field , the local moments become aligned and the ferromagnetic
phase is stabilized above critical values of or . In the canted phase, a
charge-density wave forms because the electrons prefer to sit on lines of sites
that are coupled ferromagnetically. Due to a change in the topology of the
Fermi surface from closed to open, phase separation occurs in a narrow range of
parameters in the canted phase. In zero field, the long-wavelength spin waves
are isotropic in the region of phase separation. Whereas the average spin-wave
stiffness in the canted phase increases with or , it exhibits a more
complicated dependence on field. This work strongly suggests that the jump in
the spin-wave stiffness observed in PrCaMnO with at a field of 3 T is caused by the delocalization of the electrons rather
than by the alignment of the antiferromagnetic regions.Comment: 28 pages, 12 figure
A geometric Newton method for Oja's vector field
Newton's method for solving the matrix equation runs
up against the fact that its zeros are not isolated. This is due to a symmetry
of by the action of the orthogonal group. We show how
differential-geometric techniques can be exploited to remove this symmetry and
obtain a ``geometric'' Newton algorithm that finds the zeros of . The
geometric Newton method does not suffer from the degeneracy issue that stands
in the way of the original Newton method
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