40 research outputs found
'Theory for the enhanced induced magnetization in coupled magnetic trilayers in the presence of spin fluctuations'
Motivated by recent experiments, the effect of the interlayer exchange
interaction on the magnetic properties of coupled Co/Cu/Ni
trilayers is studied theoretically. Here the Ni film has a lower Curie
temperature than the Co film in case of decoupled layers. We
show that by taking into account magnetic fluctuations the interlayer coupling
induces a strong magnetization for T\gtsim T_{C,\rm Ni} in the Ni film. For
an increasing the resonance-like peak of the longitudinal Ni
susceptibility is shifted to larger temperatures, whereas its maximum value
decreases strongly. A decreasing Ni film thickness enhances the induced Ni
magnetization for T\gtsim T_{C,\rm Ni}. The measurements cannot be explained
properly by a mean field estimate, which yields a ten times smaller effect.
Thus, the observed magnetic properties indicate the strong effect of 2D
magnetic fluctuations in these layered magnetic systems. The calculations are
performed with the help of a Heisenberg Hamiltonian and a Green's function
approach.Comment: 4 pages, 3 figure
Order of Two-Dimensional Isotropic Dipolar Antiferromagnets
The question of the existence of order in two-dimensional isotropic dipolar
Heisenberg antiferromagnets is studied. It is shown that the dipolar
interaction leads to a gap in the spin-wave energy and a nonvanishing order
parameter. The resulting finite N\'eel-temperature is calculated for a square
lattice by means of linear spin-wave theory.Comment: 10 pages, REVTEX, 1 figure available upon request, TUM-CP-93-0
Macroscopic anisotropy in superconductors with anisotropic gaps
It is shown within the weak-coupling model that the macroscopic
superconducting anisotropy for materials with the gap varying on the Fermi
surface cannot be characterized by a single number, unlike the case of clean
materials with isotropic gaps. For clean uniaxial materials, the anisotropy
parameter defined as the ratio of London penetration depths,
, is evaluated for all 's. Within the two-gap model
of MgB, is an increasing function of .Comment: 4 pages, 2 figure
In-plane dipole coupling anisotropy of a square ferromagnetic Heisenberg monolayer
In this study we calculate the dipole-coupling-induced quartic in-plane
anisotropy of a square ferromagnetic Heisenberg monolayer. This anisotropy
increases with an increasing temperature, reaching its maximum value close to
the Curie temperature of the system. At T=0 the system is isotropic, besides a
small remaining anisotropy due to the zero-point motion of quantum mechanical
spins. The reason for the dipole-coupling-induced anisotropy is the disturbance
of the square spin lattice due to thermal fluctuations ('order-by-disorder'
effect). For usual ferromagnets its strength is small as compared to other
anisotropic contributions, and decreases by application of an external magnetic
field. The results are obtained from a Heisenberg Hamiltonian by application of
a mean field approach for a spin cluster, as well as from a many-body Green's
function theory within the Tyablikov-decoupling (RPA).Comment: 6 pages, 2 figures, accepted for publication in RP
Fluctuations of an Atomic Ledge Bordering a Crystalline Facet
When a high symmetry facet joins the rounded part of a crystal, the step line
density vanishes as sqrt(r) with r denoting the distance from the facet edge.
This means that the ledge bordering the facet has a lot of space to meander as
caused by thermal activation. We investigate the statistical properties of the
border ledge fluctuations. In the scaling regime they turn out to be
non-Gaussian and related to the edge statistics of GUE multi-matrix models.Comment: Version with major revisions -- RevTeX, 4 pages, 2 figure
Critical Dynamics of Magnets
We review our current understanding of the critical dynamics of magnets above
and below the transition temperature with focus on the effects due to the
dipole--dipole interaction present in all real magnets. Significant progress in
our understanding of real ferromagnets in the vicinity of the critical point
has been made in the last decade through improved experimental techniques and
theoretical advances in taking into account realistic spin-spin interactions.
We start our review with a discussion of the theoretical results for the
critical dynamics based on recent renormalization group, mode coupling and spin
wave theories. A detailed comparison is made of the theory with experimental
results obtained by different measuring techniques, such as neutron scattering,
hyperfine interaction, muon--spin--resonance, electron--spin--resonance, and
magnetic relaxation, in various materials. Furthermore we discuss the effects
of dipolar interaction on the critical dynamics of three--dimensional isotropic
antiferromagnets and uniaxial ferromagnets. Special attention is also paid to a
discussion of the consequences of dipolar anisotropies on the existence of
magnetic order and the spin--wave spectrum in two--dimensional ferromagnets and
antiferromagnets. We close our review with a formulation of critical dynamics
in terms of nonlinear Langevin equations.Comment: Review article (154 pages, figures included
Specific Heat Discontinuity in Impure Two-Band Superconductors
The Ginzburg-Landau coefficients, and the jump of the specific heat are
calculated for a disordered two-band superconductor. We start with the analysis
of a more general case arbitrary anisotropy. While the specific heat
discontinuity at the critical temperature T_c decreases with increasing
disorder, its ratio to the normal state specific heat at T_c increases and
slowly converges to the isotropic value. For a strong disorder the deviation
from the isotropic value is proportional to the elastic electron scattering
time. In the case of a two-band superconductor we apply a simplified model of
the interaction independent on momentum within a band. In the framework of this
model all thermodynamic values can be found explicitly at any value of the
scattering rate. This solution explains the sample dependence of the specific
heat discontinuity in MgB_2 and the influence of the disorder on the critical
temperature.Comment: New results relate to two-band superconductors, 9 pages, 2 figure
Schwinger boson theory of anisotropic ferromagnetic ultrathin films
Ferromagnetic thin films with magnetic single-ion anisotropies are studied
within the framework of Schwinger bosonization of a quantum Heisenberg model.
Two alternative bosonizations are discussed. We show that qualitatively correct
results are obtained even at the mean-field level of the theory, similar to
Schwinger boson results for other magnetic systems. In particular, the
Mermin-Wagner theorem is satisfied: a spontaneous magnetization at finite
temperatures is not found if the ground state of the anisotropic system
exhibits a continuous degeneracy. We calculate the magnetization and effective
anisotropies as functions of exchange interaction, magnetic anisotropies,
external magnetic field, and temperature for arbitrary values of the spin
quantum number. Magnetic reorientation transitions and effective anisotropies
are discussed. The results obtained by Schwinger boson mean-field theory are
compared with the many-body Green's function technique.Comment: 14 pages, including 7 EPS figures, minor changes, final version as
publishe
Enhancement of the upper critical field by nonmagnetic impurities in dirty two-gap superconductors
Quasiclassic Uzadel equations for two-band superconductors in the dirty limit
with the account of both intraband and interband scattering by nonmagnetic
impurities are derived for any anisotropic Fermi surface. From these equations
the Ginzburg-Landau equations, and the critical temperature are obtained.
An equation for the upper critical field, which determines both the temperature
dependence of and the orientational dependence of
as a function of the angle between and the c-axis is
obtained. It is shown that the shape of the curve essentially
depends on the ratio of the intraband electron diffusivities and ,
and can be very different from the standard one-gap dirty limit theory. In
particular, the value can considerably exceed ,
which can have important consequences for applications of . A scaling
relation is proposed which enables one to obtain the angular dependence of
from the equation for at . It is shown
that, depending on the relation between and , the ratio of the upper
critical field for and can both increase and decrease as the temperature decreases. Implications
of the obtained results for are discussed
Complete breeding failures in ivory gull following unusual rainy storms in North Greenland
Natural catastrophic events such as heavy rainfall and windstorms may induce drastic decreases in breeding success of animal populations. We report the impacts of summer rainfalls on the reproductive success of ivory gull (Pagophila eburnea) in north-east Greenland. On two occasions, at Amdrup Land in July 2009 and at Station Nord in July 2011, we observed massive ivory gull breeding failures following violent rainfall and windstorms that hit the colonies. In each colony, all of the breeding birds abandoned their eggs or chicks during the storm. Juvenile mortality was close to 100% at Amdrup Land in 2009 and 100% at Station Nord in 2011. Our results show that strong winds associated with heavy rain directly affected the reproductive success of some Arctic bird species. Such extreme weather events may become more common with climate change and represent a new potential factor affecting ivory gull breeding success in the High Arctic