Browsing by Author "Srivastava, Abhishek"

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    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 Tuscaloosa
    We 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.
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    Investigation of Magnetic Anisotropies and Magnetization Dynamics in Soft Magnetic Materials
    (University of Alabama Libraries, 2020) Srivastava, Abhishek; Mewes, Tim; University of Alabama Tuscaloosa
    Magnetic 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.