254 research outputs found
On the origin of temperature dependence of interlayer exchange coupling in metallic trilayers
We study the influence of collective magnetic excitations on the interlayer
exchange coupling (IEC) in metallic multilayers. The results are compared to
other models that explain the temperature dependence of the IEC by mechanisms
within the spacer or at the interfaces of the multilayers. As a main result we
find that the reduction of the IEC with temperature shows practically the same
functional dendence in all models. On the other hand the influence of the
spacer thickness, the magnetic material, and an external field are quite
different. Based on these considerations we propose experiments, that are able
to determine the dominating mechanism that reduces the IEC at finite
temperatures.Comment: 8 pages, 7 figures, accepted for PR
Mechanism of temperature dependence of the magnetic anisotropy energy in ultrathin Cobalt and Nickel films
Temperature dependent FMR-measurements of Ni and Co films are analysed using
a microscopic theory for ultrathin metallic systems. The mechanism governing
the temperature dependence of the magnetic anisotropy energy is identified and
discussed. It is reduced with increasing temperature. This behavior is found to
be solely caused by magnon excitations.Comment: 3 pages, 4 figures III Joint European Magnetic Symposia, San
Sebastian, Spai
A new type of temperature driven reorientation transition in magnetic thin films
We present a new type of temperature driven spin reorientation transition
(SRT) in thin films. It can occur when the lattice and the shape anisotropy
favor different easy directions of the magnetization. Due to different
temperature dependencies of the two contributions the effective anisotropy may
change its sign and thus the direction of the magnetization as a function of
temperature may change. Contrary to the well-known reorientation transition
caused by competing surface and bulk anisotropy contributions the reorientation
that we discuss is also found in film systems with a uniform lattice
anisotropy. The results of our theoretical model study may have experimental
relevance for film systems with positive lattice anisotropy, as e.g. thin iron
films grown on copper.Comment: 7 pages, 4 figures, accepted for publication in EPJ
Spin wave excitations: The main source of the temperature dependence of Interlayer exchange coupling in nanostructures
Quantum mechanical calculations based on an extended Heisenberg model are
compared with ferromagnetic resonance (FMR) experiments on prototype trilayer
systems Ni_7/Cu_n/Co_2/Cu(001) in order to determine and separate for the first
time quantitatively the sources of the temperature dependence of interlayer
exchange coupling. Magnon excitations are responsible for about 75% of the
reduction of the coupling strength from zero to room temperature. The remaining
25% are due to temperature effects in the effective quantum well and the
spacer/magnet interfaces.Comment: accepted for publication in PR
Hierarchical ZSM‐5 catalysts: The effect of different intracrystalline pore dimensions on catalyst deactivation behaviour in the MTO reaction
We present the effect of different combinations of intracrystalline pore systems in hierarchical ZSM‐5 zeolites on their performance as MTO catalysts. We prepared ZSM‐5 zeolites with additional intracrystalline mesoporous, intracrystalline macropores and a novel ZSM‐5 type zeolite with intracrystalline meso and macropores. The catalytic results showed that both used catalysts with mesopores and macropores exhibited three times longer catalyst lifetime compared to a conventional catalyst. However, TGA analysis of the deactivated catalysts showed much larger coke content in the mesoporous catalyst than in the macroporous catalyst. Consequently, macropores predominantly led to reduced coke formation rate while additional mesopores predominantly enhanced the resistance against deactivation by coke. Combining both intracrystalline meso and macropores in one catalyst lead to a tenfold increase in catalyst lifetime. Besides the effect on the catalyst lifetime there was also a strong effect of the additional pore systems on the selectivity of the catalysts. The catalysts containing mesopores showed reduced selectivity to short chain olefins and increased selectivity to larger hydrocarbons in comparison to the catalysts without a mesopores system
Ferromagnetism in the Periodic Anderson Model - a Modified Alloy Analogy
We introduce a new aproximation scheme for the periodic Anderson model (PAM).
The modified alloy approximation represents an optimum alloy approximation for
the strong coupling limit, which can be solved within the CPA-formalism.
Zero-temperature and finite-temperature phase diagrams are presented for the
PAM in the intermediate-valence regime. The diversity of magnetic properties
accessible by variation of the system parameters can be studied by means of
quasiparticle densities of states: The conduction band couples either ferro- or
antiferromagneticaly to the f-levels. A finite hybridization is a necessary
precondition for ferromagnetism. However, too strong hybridization generally
suppresses ferromagnetism, but can for certain system parameters also lead to a
semi-metallic state with unusual magnetic properties. By comparing with the
spectral density approximation, the influence of quasiparticle damping can be
examined.Comment: 20 pages, 13 figure
Correlation and surface effects in Vanadium Oxides
Recent photoemission experiments have shown strong surface modifications in
the spectra from vanadium oxides as (V,Cr)_2O_3 or
(Sr,Ca)VO_3. The effective mass is enhanced at the surface and the coherent
part of the surface spectrum is narrowed as compared to the bulk. The
quasiparticle weight is more sensitive at the surface than in the bulk against
bandwidth variations. We investigate these effects theoretically considering
the single-band Hubbard model for a film geometry. A simplified dynamical
mean-field scheme is used to calculate the main features of the interacting
layer-dependent spectral function. It turns out that the experimentally
confirmed effects are inherent properties of a system of strongly correlated
electrons. The reduction of the weight and the variance of the coherent part of
the surface spectrum can be traced back to the reduced surface coordination
number. Surface correlation effects can be strongly amplified by changes of the
hopping integrals at the surface.Comment: to appear in PRB; 8 pages, 6 figure
Comparison of Predicted and Measured Attenuation of Turbine Noise from a Static Engine Test
Aircraft noise has become an increasing concern for commercial airlines. Worldwide demand for quieter aircraft is increasing, making the prediction of engine noise suppression one of the most important fields of research. The Low-Pressure Turbine (LPT) can be an important noise source during the approach condition for commercial aircraft. The National Aeronautics and Space Administration (NASA), Pratt & Whitney (P&W), and Goodrich Aerostructures (Goodrich) conducted a joint program to validate a method for predicting turbine noise attenuation. The method includes noise-source estimation, acoustic treatment impedance prediction, and in-duct noise propagation analysis. Two noise propagation prediction codes, Eversman Finite Element Method (FEM) code [1] and the CDUCT-LaRC [2] code, were used in this study to compare the predicted and the measured turbine noise attenuation from a static engine test. In this paper, the test setup, test configurations and test results are detailed in Section II. A description of the input parameters, including estimated noise modal content (in terms of acoustic potential), and acoustic treatment impedance values are provided in Section III. The prediction-to-test correlation study results are illustrated and discussed in Section IV and V for the FEM and the CDUCT-LaRC codes, respectively, and a summary of the results is presented in Section VI
Nematic suspension of a microporous layered silicate obtained by forceless spontaneous delamination via repulsive osmotic swelling for casting high-barrier all-inorganic films
Exploiting the full potential of layered materials for a broad range of applications requires delamination into functional nanosheets. Delamination via repulsive osmotic swelling is driven by thermodynamics and represents the most gentle route to obtain nematic liquid crystals consisting exclusively of single-layer nanosheets. This mechanism was, however, long limited to very few compounds, including 2:1-type clay minerals, layered titanates, or niobates. Despite the great potential of zeolites and their microporous layered counterparts, nanosheet production is challenging and troublesome, and published procedures implied the use of some shearing forces. Here, we present a scalable, eco-friendly, and utter delamination of the microporous layered silicate ilerite into single-layer nanosheets that extends repulsive delamination to the class of layered zeolites. As the sheet diameter is preserved, nematic suspensions with cofacial nanosheets of ≈9000 aspect ratio are obtained that can be cast into oriented films, e.g., for barrier applications
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