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Relation between bandstructure and magnetocrystalline anisotropy : iron and nickelWang, Haiyan 14 February 2000 (has links)
A large amount of research has been done in which the magnetocrystalline
anisotropy energy for fcc Ni and bcc Fe was calculated based on the electronic
structure of these elements. Unfortunately; the results of these studies don't agree
with each other and also differ from the experimental observation. In a previous
thesis the effects of numerical errors in the Brillouin zone integrations were investigated.
The results of that work explain why different calculations give different
results, but do not explain the difference with experiment. The conclusion was
that the underlying bandstructure, which was calculated using standard approximations,
was not correct.
The bandstructure of these elements will be different when improved
prescriptions for the exchange-correlation energy are used. There is, however, no
clear indication along which lines this approximation should be improved. Here
we have taken a different approach to change the bandstructure. We suspected
that some important interactions between different atomic orbitals are either
ignored or miscounted. In this work, we examined the sensitivity of the energy on
the interaction between those orbitals and studied in detail the consequences of
changes in some interaction parameters which gave rise to a large energy change.
The main result of this work is a better understanding of the relation between
changes in the electronic structure in k-space and the resultant change in the
magnetocrystalline anisotropy energy. In addition, this work takes another step
in trying to find a better understanding how the magnetocrystalline anisotropy
energy relates to interactions between neighboring atoms. / Graduation date: 2000
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Calculation of magnetocrystalline anisotropySchneider, Gunter 20 January 1999 (has links)
The magnetocrystalline anisotropy energy (MAE) for fcc Ni and bcc Fe is calculated as the difference of single particle energy eigenvalue sums using a tight-binding model. For nickel we predict a MAE of -0.15 eV and the wrong easy axis, for iron we find a MAE of -0.7 eV with the easy axis in agreement with experiment. Our results compare favorably with previously reported first-principles calculations based on density functional theory and the local spin density approximation. The inclusion of an orbital polarization correction improves the magnitude of the MAE for iron, but fails to bring the result for nickel closer to the experimental value. The outstanding feature of our calculations is the careful handling of the necessary Brillouin zone integrals. Linear interpolation schemes and methods based on Fermi surface smearing were used and analyzed. An alternative method of calculating the MAE based on the torque on a magnetic moment centered on an atom is found to be equivalent to the calculation of the MAE in terms of energy differences. / Graduation date: 1999
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Methodology for determining the variance of the Taylor factor : application in Fe-3%Si /Przybyla, Craig P., January 2005 (has links) (PDF)
Thesis (M.S.)--Brigham Young University. Dept. of Mechanical Engineering, 2005. / Includes bibliographical references (p. 115-118).
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Anisotropy and the structural evolution of the oceanic upper mantle /Forsyth, Donald William. January 1900 (has links)
Thesis--Massachusetts Institute of Technology and Woods Hole Oceanographic Institution. / Includes bibliographical references (leaves 220-233).
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Spin valves and spin-torque oscillators with perpendicualr magnetic anisotropyMohseni Armaki, Seyed Majid January 2012 (has links)
Researches in spintronics, especially those remarkably classified in the current induced spin-transfer torque (STT) framework, circumvent challenges with different materials and geometries. Perpendicular magnetic anisotropy (PMA) materials are showing capability of holding promise to be employed in STT based spintronics elements, e.g. spin-torque oscillators (STOs), STT-magnetoresistive random access memories (STT-MRAMs) and current induced domain wall motion elements. This dissertation presents experimental investigations into developing sputter deposited Co/Ni multilayers (MLs) with PMA and employs these materials in nano-contact STOs (NC-STOs) based on giant magnetoresistance (GMR) effect and in pseudo-spin-valve (PSV) structures. The magnetostatic stray field coupling plays an important role in perpendicular PSVs. The temperature dependent coupling mechanism recommends that this coupling can be tailored, by i) the saturation magnetization and coercivity of the individual layers, ii) the coercivity difference in layers, and iii) the GMR spacer thickness, to get a well decoupled and distinguishable switching response. Moreover, this thesis focused on the implementation and detailed characterization of NC-STOs with strong PMA Co/Ni ML free layers and in-plane Co reference layers as orthogonal (Ortho) magnetic geometry in so-called Ortho-NC-STOs. The primary target of reaching record high STO frequencies, 12 GHz, at close to zero field, 0.02 Tesla, was achieved. However, in large external fields, >0.4 Tesla, an entirely new magnetodynamic object, a “magnetic droplet”, theoretically predicted in 1977, was discovered experimentally. Detailed experiments, combined with micromagnetic simulations, demonstrate the formation of a magnetic droplet with a partially reversed magnetization direction underneath the NC, and a zone of large amplitude precession in a region bounding the reversed magnetization. The magnetic droplet exhibits a very rich dynamics, including i) auto-modulation as a combine of droplet frequency with a slow time evolution (few GHz) of un-centering the droplet mode under the NC, ii) droplet breathing as reversible deformation of droplet mode with ½ droplet frequency. All observation of droplet opens a new mechanism of excitation for future fundamental studies as well as experiments especially for domain wall electronics and nano-scopic magnetism. / <p>QC 20121119</p>
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Wrinkling of sandwich panels for marine applicationsFagerberg, Linus January 2003 (has links)
The recent development in the marine industry with largerships built in sandwich construction and also the use of moreadvanced materials has enforced improvements of design criteriaregarding wrinkling. The commonly used Hoffs formula isnot suited for the highly anisotropic fibre reinforced sandwichface sheets of today. The work presented herein investigates the wrinklingphenomenon. A solution to wrinkling of anisotropic sandwichplates subjected to multi-axial loading is presented. Thesolution includes the possibility of skew wrinkling where thewrinkling waves are not perpendicular to the principal loaddirection. The wrinkling angle is obtained from the solutiontogether with the maximum wrinkling load. This method has beensupported with tests of anisotropic plates subjected touni-axial and bi-axial loading. The effect of the face sheet local bending stiffness showsthe importance of including the face sheet stacking sequence inthe wrinkling analysis. The work points out the influence ofthe face sheet local bending stiffness on wrinkling. Threedifferent means of improving the wrinkling load except changingcore material is evaluated. The effect of the differentapproaches is evaluated theoretically and also throughcomparative testing. The transition between wrinkling and pureface sheet compression failure is investigated. Theoreticaldiscussions are compared with compressive test results of twodifferent face sheet types on seven different core densities.The failure modes are investigated using fractography. Theresults clearly show how the actual sandwich compressionfailure mode is influenced by the choice of core material,changing from wrinkling failure to face sheet micro bucklingfailure as the modulus density increases. Finally, a new approach is presented where the wrinklingproblem is transferred from a pure stability problem to amaterial strength criterion. The developed theory providesmeans on how to decide which sandwich constituent will failfirst and at which load it will fail. The method give insightto and develop the overall understanding of the wrinklingphenomenon. A very good correlation is found when the developedtheory is compared with both finite element calculations and toexperimental tests. <b>Keywords:</b>wrinkling, local buckling, imperfection,stability, anisotropy, sandwich
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Quantum Wavepacket Dynamics in Molecular and Trapped Ion SystemsWang, Dong January 2008 (has links)
The motions of a wavepacket in the two coupled potentials studied in this thesis can be classified into either bistable or astable motion according to the wavepacket interference at the curve crossing. Bistable motion, in which the wavepacket performs a coupled oscillation but remains in the same adiabatic and diabatic state, can exist both in bound-bound systems and bound-unbound ones with long time stability. Astable motion, in which the wavepacket at the curve crossing switches between the adiabatic and diabatic states and thus alternates between the two possible turning points in the unforked part of the motion, can only exist in bound-bound systems on a limited time scale. The motion of a wavepacket under bistable interference conditions exhibits all of the features expected if the wavepacket moved in a single anharmonic potential. The revival time can be predicted from the revival times in the corresponding diabatic and adiabatic potentials. The phenomenon was observed not only in model molecular systems but also in the system of the harmonically trapped ion pumped by an external laser field with standing wave spatial profile. In order to study the bias effect of the detector on pump-probe rotational anisotropy measurements, in a specific direction the fluorescence polarization effect was removed by measuring the rovibrational wavepacket with the help of properly oriented polarizer placed in front of the detector. Our results show clearly the necessity to take polarization effects into account in ultrafast pump-probe rotational anisotropy measurements.
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Photoinduced dichroism in amorphous As2Se3 thin filmDeForrest, Dan 20 December 2005
The dichroism in amorphous As2Se3 induced by a polarized beam of near band-gap light (λ = 632.8 nm) was measured in films that ranged in thickness from 0.25 µm to 1.93 µm. Most noncrystalline materials are initially isotropic and homogeneous. When amorphous As2Se3 (a chalcogenide glass) absorbs an intense pump-beam of polarized light, the absorption coefficient for light polarized in the same direction as the pump-beam is less than for the perpendicular polarization, i.e. the sample becomes anisotropic (dichroic). The induced dichroism is reversible by rotating the polarization of the pump light by 90°. Induced dichroism is potentially useful in various photonic devices including optical switches, optical memory, and photowritable polarizers.<p>Several aspects of photoinduced anisotropy in a-As2Se3 have been analyzed:<p>(i) rate of photoinduced anisotropy buildup as a function of inducing beam intensity and sample thickness, <p>(ii) the kinetics of the photoinduced anisotropy buildup in terms of a stretched exponential curve, <p>(iii) the stretched exponent, β, as a function of inducing beam intensity and sample thickness, <p>(iv) reversibility of the photoinduced anisotropy, (v) saturation level of photoinduced anisotropy as a function of inducing beam intensity and sample thickness.<p>The anisotropy buildup kinetics has been found to follow a stretched exponential behavior and that there exists an inverse relationship between the pump intensity and the time constant, τ. The τ(I) vs intensity (I) relationship more closely follows a logistic dose response curve than a simple straight line or power law relationship. There exists a direct relationship between the time contact and the sample thickness, where a longer anisotropy buildup time is required as the sample thickness increases. The stretched exponent, β, was found to be approximately 0.6 and has a slight dependence on the inducing light intensity. The correlation of the stretching exponent, β, to sample thickness, L, was found to have a weak inverse relationship, that is β tends to decrease as the sample thickness increases.<p>The findings in this work demonstrate that the anisotropy orientation could be changed indefinitely since it was found that even after 100 orientation changes the anisotropy saturation had no measurable fatiguing. The anisotropy saturation level was found to be independent of the inducing beam intensity and linearly proportional to the sample thickness.
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Photoluminescence Specroscopy Of Cds And GaseSeyhan, Ayse 01 January 2003 (has links) (PDF)
With the use of photoluminescence (PL) spectroscopy one can able to get a
great deal of information about electronic structure and optical processes in
semiconductors by the aid of optical characterization. Among various compound
semiconductors, Cadmium Sulfide (CdS) and Gallium Selenide (GaSe) are
interesting materials for their PL emissions. Particularly, due to its strong anisotropy,
investigation of GaSe necessitates new experimental approaches to the PL technique.
We have designed, fabricated and used new experimental set-up for this purpose.
In this thesis, we have investigated the PL spectra of both CdS and GaSe as a
function of temperature. We observed interesting features in these samples. These
features were analyzed experimentally and described by taking the band structure of
the crystals into account. From the excitonic emissions, we determined the bandgap
energy of both materials. We studied various peaks that appear in the PL spectra and their origin in the material. We have found that donor acceptor transitions are
effective in CdS at low temperatures. A transition giving rise to a red emission was
observed and attributed to a donor level which is likely to result form an S vacancy
in CdS crystal. The PL peaks with energy close to the bandgap were observed in
GaSe. These peak were attributed to the bound excitons connected either to the direct
or indirect band edge of GaSe. The striking experimental finding in this work was the
PL spectra of GaSe measured in different angular position with respect to the crystal
axis. We observed that PL spectra exhibit substantial differences when the angular
position of the laser beam and the detector is changed. The optical anisotropy which
is responsible for these differences was measured experimentally and discussed by
considering the selection rules of the band states of GaSe.
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Influence of composition, grain size and manufacture process on the anisotropy of tube materialsGullberg, Daniel January 2010 (has links)
A problem with cold pilgered tubes for OCTG applications is that they can get anisotropic properties with regard to yield strength. One source of anisotropy is texture that is developed during the cold deformation. EBSD measurements have been made on several austenitic stainless steels with different deformations to see what influence the composition has on the texture formation. The same measurements were used to study the influence of grain size on texture formation. The conclusion was that the composition can have an impact on the texture and hence has potential to also affect the anisotropy. The differences in texture cannot be associated with a specific alloying element, but is rather a synergetic effect. It was also concluded that grain structure has no strong influence on texture formation. An evaluation of three different tool designs used for cold pilgering was made. The designs evaluated are referred to as design A, B and C. EBSD measurements showed large deviations in texture in the middle of the wall compared to close to the surface of pilgered OCTG. However, the measurements showed no large differences between the three designs and the texture could not be coupled to the anisotropy.
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