Browsing by Author "Mohammadi, Jamileh Beik"

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    Broadband Ferromagnetic Resonance Characterization of Anisotropies and Relaxation in Exchange-biased IrMn/CoFe Bilayers
    (2017) Mohammadi, Jamileh Beik; Jones, Joshua Michael; Paul, Soumalya; Khodadadi, Behrouz; Mewes, Claudia K. A.; Mewes, Tim; Kaiser, Christian; University of Alabama Tuscaloosa
    The magnetization dynamics of exchange-biased IrMn/CoFe bilayers have been investigated using broadband and in-plane angle-dependent ferromagnetic resonance spectroscopy. The interface energy of the exchange bias effect in these bilayers exceeds values previously reported for metallic antiferromagnets. A strong perpendicular magnetic anisotropy and a small in-plane uniaxial anisotropy are also observed in these films. The magnetization relaxation of the bilayers has a strong unidirectional contribution, which is in part caused by two-magnon scattering. However, a detailed analysis of in-plane angle– and thickness-dependent linewidth data strongly suggests the presence of a previously undescribed unidirectional relaxation mechanism.
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    Bulk Single Crystal-Like Structural and Magnetic Characteristics of Epitaxial Spinel Ferrite Thin Films with Elimination of Antiphase Boundaries
    (Wiley-VCH, 2017) Singh, Amit V.; Khodadadi, Behrouz; Mohammadi, Jamileh Beik; Keshavarz, Sahar; Mewes, Tim; Negi, Devendra Singh; Datta, Ranjan; Galazka, Zbigniew; Uecker, Reinhard; Gupta, Arunava; University of Alabama Tuscaloosa; Department of Science & Technology (India); Jawaharlal Nehru Center for Advanced Scientific Research (JNCASR); Leibniz Institut fur Kristallzuchtung (IKZ)
    Spinel ferrite NiFe2O4 thin films have been grown on three isostructural substrates, MgAl2O4, MgGa2O4, and CoGa2O4 using pulsed laser deposition. These substrates have lattice mismatches of 3.1%, 0.8%, and 0.2%, respectively, with NiFe2O4. As expected, the films grown on MgAl2O4 substrate show the presence of the antiphase boundary defects. However, no antiphase boundaries (APBs) are observed for films grown on near-lattice-matched substrates MgGa2O4 and CoGa2O4. This demonstrates that by using isostructural and lattice-matched substrates, the formation of APBs can be avoided in NiFe2O4 thin films. Consequently, static and dynamic magnetic properties comparable with the bulk can be realized. Initial results indicate similar improvements in film quality and magnetic properties due to the elimination of APBs in other members of the spinel ferrite family, such as Fe3O4 and CoFe2O4, which have similar crystallographic structure and lattice constants as NiFe2O4.
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    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 Tuscaloosa
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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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    Quasistatic and Dynamic Characterization of Magnetic Thin Films
    (University of Alabama Libraries, 2024) Karki, Upama; Mohammadi, Jamileh Beik
    This thesis explores damping processes and dynamic magnetic properties in layered ferromagnetic thin films, crucial for optimizing microwave and magnetic recording devices. It employs broadband ferromagnetic resonance (FMR) and Vibrating Sample Magnetometer (VSM) techniques.Chapter one introduces thin films, the fundamental principles of FMR, experimental setup, and data analysis methodologies. Chapter two investigates bilinear and biquadratic exchange coupling dependence on spacer layer thickness in CoFe(10 nm)/Ru(x=0.05-1.00 nm)/CoFe(5 nm) trilayers. Previous studies have noted the presence of a biquadratic effect in multilayer systems, but the effect of spacer thickness on both bilinear and biquadratic coupling has not been reported yet. Hysteresis loops from VSM reveal non-collinear magnetization alignments and multi-step switching. The modified Stoner-Wohlfarth model is used to derive energy densities for these couplings, showing that biquadratic coupling is more pronounced at smaller spacer thicknesses. From FMR, the effective exchange coupling (J$_{eff}$) is determined, while X-ray Reflectivity (XRR) suggests that interface roughness correlates with the observed biquadratic effect.Chapter three extends this investigation to the impact of thickness and annealing temperature on interfacial perpendicular magnetic anisotropy (PMA) and damping parameters in CoFeB/W bilayers on Al$_2$O$_3$, SiO$_2$, and MgO substrates. Effective magnetization decreases linearly with inverse CoFeB thickness, indicating interfacial PMA, with CoFeB on MgO showing larger PMA. Effective magnetization increases with temperature on MgO but decreases on Al$_2$O$_3$, while damping parameters decrease with higher annealing temperatures.Chapter four examines ferromagnetic relaxation and effective magnetization in CoCrFeAl films of varying thicknesses (50, 20, and 10 nm). In-plane FMR measurements reveal four-fold anisotropy, and VSM shows a linear relationship between saturation magnetization and thickness. SEM analysis indicates a homogeneous island-like structure with aluminum-rich and aluminum-deficient phases.Chapter five summarizes the findings, emphasizing the importance of understanding static and dynamic magnetic properties in CoFe/Ru/CoFe, CoFeB/W, and CoCrFeAl thin films for advancing magnetic device technologies.