Hysteretic behavior of angular dependence of exchange bias in FeNi/FeMn bilayers

For FeNi/FeMn bilayers, the angular dependence of exchange bias shows hysteresis between clockwise and counterclockwise rotations, as a new signature. The hysteresis decreases for thick antiferromagnet layers. Calculations have clearly shown that the orientation of antiferromagnet spins also exhibits hysteresis between clockwise and counterclockwise rotations. This furnishes an interpretation of the macroscopic behavior of the ferromagnetic layer in terms of the thermally driven evolution of the magnetic state of the antiferromagnet layer

Predicting the failure of ultrasonic spot welds by pull-out from sheet metal

AbstractA methodology for determining the cohesive fracture parameters associated with pull-out of spot welds is presented. Since failure of a spot weld by pull-out occurs by mixed-mode fracture of the base metal, the cohesive parameters for ductile fracture of an aluminum alloy were determined and then used to predict the failure of two very different spot-welded geometries. The fracture parameters (characteristic strength and toughness) associated with the shear and normal modes of ductile fracture in thin aluminum alloy coupons were determined by comparing experimental observations to numerical simulations in which a cohesive-fracture zone was embedded within a continuum representation of the sheet metal. These parameters were then used to predict the load–displacement curves for ultrasonically spot-welded joints in T-peel and lap-shear configurations. The predictions were in excellent agreement with the experimental data. The results of the present work indicate that cohesive-zone models may be very useful for design purposes, since both the strength and the energy absorbed by plastic deformation during weld pull-out can be predicted quite accurately

Computational experiments for local search algorithms for binary and mixed integer optimization

Thesis (S.M.)--Massachusetts Institute of Technology, Computation for Design and Optimization Program, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 53).In this thesis, we implement and test two algorithms for binary optimization and mixed integer optimization, respectively. We fine tune the parameters of these two algorithms and achieve satisfactory performance. We also compare our algorithms with CPLEX on large amount of fairly large-size instances. Based on the experimental results, our binary optimization algorithm delivers performance that is strictly better than CPLEX on instances with moderately dense constraint matrices, while for sparse instances, our algorithm delivers performance that is comparable to CPLEX. Our mixed integer optimization algorithm outperforms CPLEX most of the time when the constraint matrices are moderately dense, while for sparse instances, it yields results that are close to CPLEX, and the largest gap relative to the result given by CPLEX is around 5%. Our findings show that these two algorithms, especially the binary optimization algorithm, have practical promise in solving large, dense instances of both set covering and set packing problems.by Jingting Zhou.S.M

Rotation of the pinning direction in the exchange bias training effect in polycrystalline NiFe/FeMn bilayers

For polycrystalline NiFe/FeMn bilayers, we have observed and quantified the rotation of the pinning direction in the exchange bias training and recovery effects. During consecutive hysteresis loops, the rotation of the pinning direction strongly depends on the magnetization reversal mechanism of the ferromagnet layer. The interfacial uncompensated magnetic moment of antiferromagnetic grains may be irreversibly switched and rotated when the magnetization reversal process of the ferromagnet layer is accompanied by domain wall motion and domain rotation, respectively

Singular operators on boson fields as forms on spaces of entire functions on Hilbert space

AbstractInvariant scales of entire analytic functions on Hilbert space are introduced and applied. Singular operators represented by sesquilinear forms on spaces of regular vectors are given explicit integral representations via kernels that are entire functions on the direct sum of the Hilbert space with its dual. The Weyl (or, exponentiated boson field) operators act smoothly and irreducibly on corresponding spaces of entire functions. Arbitrary symplectic operators on a single-particle Hilbert space are shown to be implementable on the corresponding boson field by appropriate generalized operators

Analysis on Sealing Performance for a New Type of Rubber Saddle-shaped Sealing Ring Based on AQAQUS

AbstractAt present, rubber sealing C-ring has been widely used in the quick opening end closures for natural gas transmission filters. In this paper, a new saddle-shaped sealing ring was proposed for substituting of traditional C-ring. It has the feature of floatability that makes the assembling and disassembling easier than before. In addition, its sealing and load-bearing performance is superior than C-ring based on the analysis by using FEM software ABAQUS. At the same time, the reasonable range and values for the key parameters that affecting the sealing performance of saddle-shaped sealing ring were discussed and recommended

Approximate relativistic bound state solutions of the Tietz-Hua rotating oscillator for any -state

Approximate analytic solutions of the Dirac equation with Tietz-Hua (TH) potential are obtained for arbitrary spin-orbit quantum number using the Pekeris approximation scheme to deal with the spin-orbit coupling terms In the presence of exact spin and pseudo-spin (pspin) symmetric limitation, the bound state energy eigenvalues and associated two-component wave functions of the Dirac particle moving in the field of attractive and repulsive TH potential are obtained using the parametric generalization of the Nikiforov-Uvarov (NU) method. The cases of the Morse potential, the generalized Morse potential and non-relativistic limits are studied.Comment: 19 pages; 7 figures; Few-Body Systems (2012) (at press

Second-Order Belief Hidden Markov Models

Hidden Markov Models (HMMs) are learning methods for pattern recognition. The probabilistic HMMs have been one of the most used techniques based on the Bayesian model. First-order probabilistic HMMs were adapted to the theory of belief functions such that Bayesian probabilities were replaced with mass functions. In this paper, we present a second-order Hidden Markov Model using belief functions. Previous works in belief HMMs have been focused on the first-order HMMs. We extend them to the second-order model

Enterprise information technology project portfolio selection through system dynamics simulations

Thesis (S.M.)--Massachusetts Institute of Technology, System Design and Management Program, 2007.Includes bibliographical references (p. 166-169).As companies are increasingly relying on information technologies (IT) to help maintain their existing and develop new competitive advantages, investing effectively in IT is becoming more and more important. One of the biggest challenges facing an enterprise IT organization is how to select a project portfolio that is best aligned with the business strategies and to deliver highest value using limited IT resources. In this research paper, I examined in detail a recently proposed IT governance framework, designed a System Dynamics model based on this framework, and developed a simulation application to investigate constructs, relationships and scenarios suggested by the framework. My research identified and examined several levers through which IT managers can achieve better alignment with business goals and more efficient use of IT resources. I examined alternative IT governance regimes (combinations of rules and policies for selecting among opportunities and retaining existing systems) in terms of their effects on efficiency, feature satisfaction, and cost of the resulting legacy asset base. By choosing the right combination of relatively straightforward selection and retention policies, IT managers can steer their legacy assets toward a desired efficiency or satisfaction goal in concert with company strategies.by Bin Zhou.S.M

Quantum interference in the fluorescence of a molecular system

It has been observed experimentally [H.R. Xia, C.Y. Ye, and S.Y. Zhu, Phys. Rev. Lett. {\bf 77}, 1032 (1996)] that quantum interference between two molecular transitions can lead to a suppression or enhancement of spontaneous emission. This is manifested in the fluorescent intensity as a function of the detuning of the driving field from the two-photon resonance condition. Here we present a theory which explains the observed variation of the number of peaks with the mutual polarization of the molecular transition dipole moments. Using master equation techniques we calculate analytically as well as numerically the steady-state fluorescence, and find that the number of peaks depends on the excitation process. If the molecule is driven to the upper levels by a two-photon process, the fluorescent intensity consists of two peaks regardless of the mutual polarization of the transition dipole moments. If the excitation process is composed of both a two-step one-photon process and a one-step, two-photon process, then there are two peaks on transitions with parallel dipole moments and three peaks on transitions with antiparallel dipole moments. This latter case is in excellent agreement with the experiment.Comment: 11 pages, including 8 figure