283 research outputs found
The magnetic reorientation transition in thin ferromagnetic films treated by many-body Green's function theory
This contribution describes the reorientation of the magnetization of thin
ferromagnetic Heisenberg films as function of the temperature and/or an
external field. Working in a rotating frame allows an exact treatment of the
single-ion anisotropy when going to higher-order Green's functions. Terms due
to the exchange interaction are treated by a generalized Tyablikov (RPA)
decoupling.Comment: 9 pages, 2 figure
Single-ion versus exchange anisotropy in calculating anisotropic susceptibilities of thin ferromagnetic Heisenberg films within many-body Green's function theory
We compare transverse and parallel static susceptibilities of in-plane
uniaxial anisotropic ferromagnetic Heisenberg films calculated in the framework
of many-body Green's function theory using single-ion anisotropies with the
previously investigated case of exchange anisotropies. On the basis of the
calculated observables (easy and hard axes magnetizations and susceptibilities)
no significant differences are found, i. e. it is not possible to propose an
experiment that might decide which kind of anisotropy is acting in an actual
ferromagnetic film.Comment: 16 pages, 8 figure
The spectral theorem of many-body Green's function theory when there are zero eigenvalues of the matrix governing the equations of motion
In using the spectral theorem of many-body Green's function theory in order
to relate correlations to commutator Green's functions, it is necessary in the
standard procedure to consider the anti-commutator Green's functions as well
whenever the matrix governing the equations of motion for the commutator
Green's functions has zero eigenvalues. We show that a singular-value
decomposition of this matrix allows one to reformulate the problem in terms of
a smaller set of Green's functions with an associated matrix having no zero
eigenvalues, thus eliminating the need for the anti-commutator Green's
functions. The procedure is quite general and easy to apply. It is illustrated
for the field-induced reorientation of the magnetization of a ferromagnetic
Heisenberg monolayer and it is expected to work for more complicated cases as
well.Comment: 4 pages, 1 figure, accepted for publication in Physical Review B (16.
May 2003
The treatment of zero eigenvalues of the matrix governing the equations of motion in many-body Green's function theory
The spectral theorem of many-body Green's function theory relates
thermodynamic correlations to Green's functions. More often than not, the
matrix governing the equations of motion has zero eigenvalues. In this case,
the standard text-book approach requires both commutator and anti-commutator
Green's functions to obtain equations for that part of the correlation which
does not lie in the null space of the matrix. In this paper, we show that this
procedure fails if the projector onto the null space is dependent on the
momentum vector. We propose an alternative formulation of the theory in terms
of the non-null space alone and we show that a solution is possible if one can
find a momentum-independent projector onto some subspace of the non-null space.
To do this, we enlist the aid of the singular value decomposition (SVD) of the
equation of motion matrix in order to project out the null space, thus reducing
the size of the matrix and eliminating the need for the anti-commutator Green's
function. We extend our previous work, dealing with a ferromagnetic Heisenberg
monolayer and a momentum-independent projector onto the null space, where both
multilayer films and a momentum-dependent projector are considered. We develop
the numerical methods capable of handling these cases and offer a computational
algorithmus that should be applicable to any similar problem arising in Green's
function theory.Comment: 16 pages, 7 figure
Many-body Green's function theory for thin ferromagnetic anisotropic Heisenberg films: treatment of the exchange anisotropy
The many-body Green's function theory developed in our previous work for
treating the reorientation of the magnetization of thin ferromagnetic films is
extended to include the exchange anisotropy. This leads to additional momentum
dependencies which require some non-trivial changes in the formalism. The
theory is developed for arbitrary spin values S and for multilayers. The
effects of the exchange anisotropy and the single-ion anisotropy, which was
treated in our earlier work, on the magnetic properties of thin ferromagnetic
films are compared.Comment: 24 pages, 7 figures, accepted for publication in Eur. Phys. J.
Many-body Green's function theory of Heisenberg films
The treatment of Heisenberg films with many-body Green's function theory
(GFT) is reviewed. The basic equations of GFT are derived in sufficient detail
so that the rest of the paper can be understood without having to consult
further literature. The main part of the paper is concerned with applications
of the formalism to ferromagnetic, antiferromagnetic and coupled
ferromagnetic-antiferromagnetic Heisenberg films based on a generalized
Tyablikov (RPA) decoupling of the exchange interaction and exchange anisotropy
terms and an Anderson-Callen decoupling for a weak single-ion anisotropy. We
not only give a consistent description of our own work but also refer
extensively to related investigations. We discuss in particular the
reorientation of the magnetization as a function of the temperature and film
thickness. If the single-ion anisotropy is strong, it can be treated exactly by
going to higher-order Green's functions. We also discuss the extension of the
theory beyond RPA. Finally the limitations of GFT is pointed out.Comment: 124 pages, 32 figures, accepted by Physics Report
Anisotropic susceptibilities of thin ferromagnetic films within many-body Green's function theory
Transverse and parallel static susceptibilities of in-plane uniaxial
anisotropic ferromagnetic films are calculated within many-body Green's
function theory on the basis of an Heisenberg model. The importance of
collective magnetic excitations in particular in the paramagnetic regime are
stressed by comparing with mean field calculations. The paper generalizes the
work of Jensen et al. [1] to the multilayer case and to spins with S>1/2.Comment: 10 pages, 8 figures, submitted to Phys. Rev.
Coupled ferro-antiferromagnetic Heisenberg bilayers investigated by many-body Green's function theory
A theory of coupled ferro- and antiferromagnetic Heisenberg layers is
developed within the framework of many-body Green's function theory (GFT) that
allows non-collinear magnetic arrangements by introducing sublattice
structures. As an example, the coupled ferro- antiferromagnetic (FM-AFM)
bilayer is investigated. We compare the results with those of bilayers with
purely ferromagnetic or antiferromagnetic couplings. In each case we also show
the corresponding results of mean field theory (MFT), in which magnon
excitations are completely neglected. There are significant differences between
GFT and MFT. A remarkable finding is that for the coupled FM-AFM bilayer the
critical temperature decreases with increasing interlayer coupling strength for
a simple cubic lattice, whereas the opposite is true for an fcc lattice as well
as for MFT for both lattice types.Comment: 17 pages, 6 figures, accepted for publication in J. Phys. Condens.
Matter, missing fig.5 adde
Field-induced magnetic reorientation and effective anisotropy of a ferromagnetic monolayer within spin wave theory
The reorientation of the magnetization of a ferromagnetic monolayer is
calculated with the help of many-body Green's function theory. This allows, in
contrast to other spin wave theories, a satisfactory calculation of magnetic
properties over the entire temperature range of interest since interactions
between spin waves are taken into account. A Heisenberg Hamiltonian plus a
second-order uniaxial single-ion anisotropy and an external magnetic field is
treated by the Tyablikov (Random Phase Approximation: RPA) decoupling of the
exchange interaction term and the Anderson-Callen decoupling of the anisotropy
term. The orientation of the magnetization is determined by the spin components
\la S^\alpha\ra (), which are calculated with the help of the
spectral theorem. The knowledge of the orientation angle allows a
non-perturbative determination of the temperature dependence of the effective
second-order anisotropy coefficient. Results for the Green's function theory
are compared with those obtained with mean-field theory (MFT). We find
significant differences between these approaches.Comment: to appear in Europ.J.Phys.B, 13 pages, 9 figure
Evaporation residue cross-sections as a probe for nuclear dissipation in the fission channel of a hot rotating nucleus
Evaporation residue cross-sections are calculated in a dynamical description
of nuclear fission in the framework of the Langevin equation coupled with
statistical evporation of light particles. A theoretical model of one-body
nuclear friction which was developed earlier, namely the chaos-weighted wall
formula, is used in this calculation for the 224Th nucleus. The evaporation
residue cross-section is found to be very sensitive to the choice of nuclear
friction. The present results indicate that the chaotic nature of the
single-particle dynamics within the nuclear volume may provide an explanation
for the strong shape-dependence of nuclear friction which is usually required
to fit experimental data.Comment: 12 pages including 4 figure
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