Browsing by Author "Mewes, Claudia"
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Item Decoherence in Collective Quantum Memories for Photons(2005) Mewes, Claudia; Fleischhauer, Michael; University of Alabama TuscaloosaThe influence of decoherence on quantum memories for photons based on atomic ensembles is discussed. It is shown that despite the large entanglement of the collective storage states, corresponding to single photons or nonclassical states of light, the sensitivity to decoherence does not scale with the number of atoms. This is due to the existence of equivalence classes of storage states, which have the same projection onto the relevant quasiparticle mode (dark-state polariton.) Several decoherence processes resulting from uncorrelated individual reservoir couplings are analyzed in detail: single-atom spin flips and dephasing, atom loss, and motion of atoms. Furthermore, it is shown that the sensitivity to collective decoherence processes that affect all polariton modes with comparable strength does also not increase with the number of atoms.Item Enhanced Spin Pumping Near a Magnetic Ordering Transition(2017) Khodadadi, Behrouz; Mohammadi, Jamileh Beik; Mewes, Claudia; Mewes, Tim; Manno, M.; Leighton, C.; Miller, Casey W.; University of Alabama TuscaloosaItem FARCI: Fast and Robust Connectome Inference(MDPI, 2021) Meamardoost, Saber; Bhattacharya, Mahasweta; Hwang, Eun Jung; Komiyama, Takaki; Mewes, Claudia; Wang, Linbing; Zhang, Ying; Gunawan, Rudiyanto; State University of New York (SUNY) Buffalo; University of California San Diego; Rosalind Franklin University Medical & Science; University of Alabama Tuscaloosa; Virginia Polytechnic Institute & State University; University of Rhode IslandThe inference of neuronal connectome from large-scale neuronal activity recordings, such as two-photon Calcium imaging, represents an active area of research in computational neuroscience. In this work, we developed FARCI (Fast and Robust Connectome Inference), a MATLAB package for neuronal connectome inference from high-dimensional two-photon Calcium fluorescence data. We employed partial correlations as a measure of the functional association strength between pairs of neurons to reconstruct a neuronal connectome. We demonstrated using in silico datasets from the Neural Connectomics Challenge (NCC) and those generated using the state-of-the-art simulator of Neural Anatomy and Optimal Microscopy (NAOMi) that FARCI provides an accurate connectome and its performance is robust to network sizes, missing neurons, and noise levels. Moreover, FARCI is computationally efficient and highly scalable to large networks. In comparison with the best performing connectome inference algorithm in the NCC, Generalized Transfer Entropy (GTE), and Fluorescence Single Neuron and Network Analysis Package (FluoroSNNAP), FARCI produces more accurate networks over different network sizes, while providing significantly better computational speed and scaling.Item The growth temperature and measurement temperature dependences of soft magnetic properties and effective damping parameter of (FeCo)-Al alloy thin films(American Institute of Physics, 2018-01-05) Ariake, Yusuke; Wu, Shuang; Kanada, Isao; Mewes, Tim; Tanaka, Yoshitomo; Mankey, Gary; Mewes, Claudia; Suzuki, Takao; University of Alabama Tuscaloosa; TDK CorporationThe soft magnetic properties and effective damping parameters of Fe73Co25Al2 alloy thin films are discussed. The effective damping parameter alpha(eff) measured by ferromagnetic resonance for the 10 nm-thick sample is nearly constant (approximate to 0.004 +/- 0.0008) for a growth temperature T-s from ambient to 200 degrees C, and then tends to decrease for higher temperatures and alpha(eff) is 0.002 +/- 0.0004 at T-s = 300 degrees C. For the 80 nm-thick sample, the alpha(eff) seems to increase with T-s from alpha(eff) = 0.001 +/- 0.0002 at T-s = ambient to alpha(eff) = 0.002 +/- 0.0004. The alpha(eff) is found nearly constant (alpha(eff) = 0.004 +/- 0.0008) over a temperature range from 10 to 300 K for the 10 nm films with the different T-s (ambient, 100 and 200 degrees C). Together with an increasing non-linearity of the frequency dependence of the linewidth at low Ts, extrinsic contributions such as two-magnon scattering dominate the observed temperature dependence of effective damping and linewidth. (C) 2018 Author(s).Item The Growth Temperature and Measurement Temperature Dependences of Soft Magnetic Properties and Effective Damping Parameter of (FeCo)-Al Alloy Thin Films(2018) Ariake, Yusuke; Wu, Shuang; Kanada, Isao; Mewes, Tim; Tanaka, Yoshitomo; Mankey, Gary; Mewes, Claudia; Suzuki, Takao; University of Alabama TuscaloosaItem Interlayer Exchange Coupling in Asymmetric Co-Fe/Ru/Co-Fe Trilayers Investigated with Broadband Temperature-Dependent Ferromagnetic Resonance(2017) Khodadadi, Behrouz; Mohammadi, Jamileh Beik; Jones, Joshua Michael; Srivastava, Abhishek; Mewes, Claudia; Mewes, Tim; Kaiser, Christian; University of Alabama TuscaloosaWe report on a comprehensive study of the interlayer exchange coupling in Co-Feð5 nmÞ=RuðtÞ= Co-Feð8 nmÞ trilayers ðt ¼ 0.8;…; 2.8 nmÞ using broadband ferromagnetic resonance. A systematic frequency dependence of the field separation between the acoustic and optic modes is found, which is caused by different effective magnetizations of the two ferromagnetic layers. Hence, it is shown that the broadband measurements are vital for reducing the systematic error margins in the determination of interlayer exchange coupling using ferromagnetic resonance. We also investigate the temperature dependence of the interlayer exchange coupling and compare our results with existing theories. It is shown that models which take into account the temperature dependence due to thermal excitations of spin waves within the ferromagnetic layers have a considerably better agreement with the experiment than models solely based on spacer and interface contributions to the temperature dependence.Item Investigation of Magnetic Anisotropies and Magnetization Dynamics in Soft Magnetic Materials(University of Alabama Libraries, 2020) Srivastava, Abhishek; Mewes, Tim; University of Alabama TuscaloosaMagnetic anisotropy and damping are two main properties that determine the characteristics of many magnetic devices such as inductors, transformers, hard drives, GMR sensors, MRAM etc. The magnetic materials used in these devices can be in different forms. For example spintronic devices are made of thin films (single crystalline or polycrystalline), and thus in these systems the effect of the substrate such as lattice mismatch, strain etc plays an important role in determining and manipulating magnetic properties. In case of transformers and inductors magnetic materials are made of bulk ferrite or as thin (~ 20 micrometer) ribbons of nanocomposite alloys. This dissertation gives a basic introduction of the magnetization dynamics and the physics and instrumentation of FMR. The induced anisotropy and magnetization dynamics of Co74.6Fe2.7Mn2.7Nb4Si2B14 (at %) melt-spun, soft magnetic alloy ribbons after various secondary processing treatments was studied by broadband ferromagnetic resonance (FMR) technique. A new method of determining the relative permeability of these ribbons is discussed and compared to the established vibrating sample magnetometry (VSM) and the toroid method. This new method of determining the permeability does not require information about the volume or mass of the sample nor does it require any special sample preparation procedure. Another study presented in this thesis investigates the temperature dependence of the magnetic anisotropy of a single crystal magnetite (Fe3O4) thin film on MgGa2O4 substrate. The aim of this study is to characterize the magnetization dynamics and magnetic anisotropy of this magnetite thin film through the Verwey transition. The FMR study of this film suggest a continuous structural transition from cubic to monoclinic phase as the temperature is decreased. Finally the magnetic properties of polycrystalline semiconducting spinel CdCr2S4 films grown by low-pressure metal organic chemical vapor deposition are studied. This includes the investigation of the paramagnetic to ferromagnetic phase transition using broadband FMR. The effective magnetization vs temperature data shows a relatively sharp transition compared to magnetization vs temperature data obtained from VSM. The study shows that these differences can be traced to the different roles the applied magnetic field has when analyzing the data from these two techniques.Item Investigation of structural changes in vanadium containing transition metal oxides(University of Alabama Libraries, 2020-12) Davenport, Matthew Austin; Allred, Jared M.; University of Alabama TuscaloosaWe have completed an experiment to obtain diffuse scattering data for use in a comprehensive study of the local-structure changes as a function of molybdenum composition and have made large strides in interpreting some of the major changes found in this study. For these experiments, single crystals of molybdenum substituted VO2 with the formula V1-xMoxO2 were synthesized, with molybdenum compositions up to x = 0.60, using a novel, two-step chemical vapor transport synthesis. Using these large single crystals for total scattering experiments, we report the discovery of the sudden collapse of three-dimensional order in the low-temperature phase of V1-xMoxO2 at x = 0.17 and the emergence of a novel frustrated two-dimensional order at x = 0.19, with only a slight change in electronic properties. Single crystal diffuse x-ray scattering reveals that this transition from the 3D M1 phase to a 2D variant of the M2 phase results in long-range structural correlations along symmetry-equivalent (11L) planes of the tetragonal rutile structure, yet extremely short-range correlations transverse to these planes. Additionally, we report a combined study using single crystal X-ray diffraction, powder X-ray diffraction, and representational analysis to examine both the local and crystallographically averaged atomic structures simultaneously near x = 0.50. Between about x = 0.50 and 0.60, the average structure of V1-xMoxO2 is the parent rutile phase, but the local symmetry is broken by atomic displacements that are best described by an orthorhombic cell in the spacegroup Fmmm. This model is locally identical to the two-dimensionally ordered 2D-M2 phase except the correlation length is much shorter in the 2D plane, and longer in the frustrated one, making it more isotropic.Item Magnetic Anisotropies and Damping in Multilayered Thin Films(University of Alabama Libraries, 2021) Rai, Anish; Mewes, Tim; University of Alabama TuscaloosaMagnetic materials are ubiqutous in technology that we use on a daily basis. They are used as storage medium in hard drives, to read out the information with the hard drive read head, and to design sensors used in cars and cell phones to only name a few examples. The dynamic response of magnetic materials to an excitation is a crucial aspect of many of these applications and devices.In the following section a short introduction to the magnetization dynamics will be given with a special emphasis on ferromagnetic resonance phenomena. The concept of magnetic anisotropies and the importance of magnetic relaxation is also discussed in this section. This is followed by a detailed description of the experimental technique of broadband ferromagnetic resonance spectroscopy that is used throughout this dissertation. The last section of this chapter discusses and summarizes the magnetic anisotropy and relaxation studies carried out as part of this dissertation.Item Magnetization Dynamics in Ultrathin Films and Multilayers(University of Alabama Libraries, 2021) Sapkota, Arjun; Mewes, Claudia; Mewes, Tim; University of Alabama TuscaloosaRobust understanding of magnetization dynamics is very crucial to design and fabricate novelmaterials for next generation spintronic applications. A combination of experimental techniques and theoretical modeling facilitates fundamental research to establish new concepts. This dissertation discusses the magnetization dynamics of ultrathin films and multilayers using broadband ferromagnetic resonance (FMR) techniques and micromagnetic simulations. Chapter one includes the theoretical background on magnetization dynamics, the explanation of FMR experiments with schematic diagrams and data analysis, and insight in micromagnetic simulations using the in-house developed MATLAB-based code M^3. All simulations are performed with M^3. Chapter one also includes a summary of various publications which I co-authored during my time as a graduate student in the Mewes’ Magnetics Laboratory at UA. Chapter two discusses the limitations of the macrospin model of materials with inhomogeneous perpendicular anisotropy. This is a micromagnetic study motivated from experimental FMR measurements of [Co/Ni]_N multilayers. Chapter three is devoted to experimental FMR measurements carried out on FeGa thin films. These results show significantly lower Gilbert damping and linewidth in comparison to previous publications in similar structures, indicating the high quality of these thin films. This is also confirmed by X-ray diffraction (XRD) and X-ray reflectivity (XRR) measurements. These films are particularly interesting because of their well known superior magnetostrictive behavior. Motivated from previous FMR experiments in IrMn/CoFe and MnN/CoFeB exchange bias systems, we investigated the anisotropic damping in multilayer spintronic devices using micromagnetic simulations in chapter four. With the introduction of a damping tensor in the Landau-Lifshitz- Gilbert (LLG) equation, this study explains the observed unidirectional relaxation in IrMn/CoFe bilayers and co-existence of unidirectional and uniaxial relaxation in MnN/CoFeB bilayers .Item New soft magnetic materials for high frequency applications(University of Alabama Libraries, 2019) Wu, Shuang; Mewes, Tim; University of Alabama TuscaloosaSoft magnetic materials are widely used in devices such as inductors, transformers, antennas, magnetic hard drives, etc. Some of those devices will benefit greatly from operating at high frequencies. Thus fundamental study on finding the materials that have better soft magnetic properties is essential for improving the performance of those devices. Fe alloys have been proved to be promising candidates for high frequency applications. In this dissertation, an extensive study of magnetic properties of FeAl, (FeCo)-Al and (FeCo)-Si alloy thin films and their dependence on the film thickness and growth temperature has been presented. These films have body-centered cubic structure and columnar growth morphology. It is shown that the thickness of the film, which has an influence on the stress inside the film, may affect the coercivity through the magnetic-elastic coupling. The same mechanism is observed in the growth temperature dependence study, where reduced stress caused by increased growth temperature leads to a decrease in coercivity. The effective damping parameter shows a huge increase at small thickness due to the spin pumping effect. In-plane rotation ferromagnetic resonance measurements unveil the existence of four-fold anisotropy in (FeCo)-Si films. In addition, a four-fold symmetry is observed in the FMR linewidth vs. in-plane angle plot, which indicates anisotropic damping caused by the two-magnon scattering contribution. The film thickness dependence of FMR linewidth caused by the two-magnon scattering suggests that the origin of the two-magnon scattering is not pure interfacial.Item Soft magnetic properties and damping parameter of (FeCo)-Al alloy thin films(American Institute of Physics, 2017-02-06) Kanada, Isao; Cruce, Alex; Mewes, Tim; Wu, Shuang; Mewes, Claudia; Mankey, Gary; Suzuki, Takao; University of Alabama Tuscaloosa; TDK CorporationFor high frequency device applications, a systematic study of the soft magnetic properties and magnetization dynamics of (FeCo)-Al alloy thin films has been carried out. A low effective damping parameter alpha(eff) of 0.002 and a high saturation magnetization of about 1,800 emu/cc are obtained at y=0.2 similar to 0.3 for (Fe1-yCoy)(98)Al-2 alloy thin films deposited onto fused silica and MgO(100) at an ambient temperature during deposition. Those films are of the bcc structure with the <110> orientation normal to the film plane. They possess a columnar structure, grown along the film normal. The column width is found to be about 20 nm for y=0.25. It is concluded that the (FeCo)-Al thin films with a damping parameter as low as 0.002 and high saturation magnetization of about 1,800 emu/cc have been successfully fabricated, and that they are potential for future high frequency device applications. (C) 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).Item Soft Magnetic Properties and Damping Parameter of (FeCo)-Al Alloy Thin Films(2017) Kanada, Isao; Cruce, Alex; Mewes, Tim; Wu, Shuang; Mewes, Claudia; Mankey, Gary; Suzuki, Takao; University of Alabama TuscaloosaFor high frequency device applications, a systematic study of the soft magnetic properties and magnetization dynamics of (FeCo)-Al alloy thin films has been carried out. A low effective damping parameter αeff of 0.002 and a high saturation magnetization of about 1,800 emu/cc are obtained at y=0.2∼0.3 for (Fe1-yCoy)98Al2 alloy thin films deposited onto fused silica and MgO(100) at an ambient temperature during deposition. Those films are of the bcc structure with the <110> orientation normal to the film plane. They possess a columnar structure, grown along the film normal. The column width is found to be about 20 nm for y=0.25. It is concluded that the (FeCo)-Al thin films with a damping parameter as low as 0.002 and high saturation magnetization of about 1,800 emu/cc have been successfully fabricated, and that they are potential for future high frequency device applications.Item Two-photon Linewidth of Light “Stopping” via Electromagnetically Induced Transparency(2002) Mewes, Claudia; Fleischhauer, Michael; University of Alabama TuscaloosaWe analyze the two-photon linewidth of a recently proposed adiabatic transfer technique for ‘‘stopping’’ of light using electromagnetically induced transparency.