Hall measurement-based study of ferromagnetic thin films stimulated by the idea of spin-orbit torque




Aryal, Mukesh

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This study is motivated by the idea of spin-orbit torque (SOT). The research attempts to study SOT effects in ferromagnetic systems through numerical and experimental approaches. The numerical simulation segment employs an object-oriented framework tool (OOMMF) to model magnetic systems with a focus on understanding critical parameters. The first part of the simulation incorporates thermal effects into a nanometer-sized CoFeB Hall structure, revealing a significant reduction in critical parameters for magnetization switching due to the inclusion of thermal effects. Subsequently, a micrometer-sized CoFeB Hall structure is simulated, although with compromises in accuracy due to discretization changes and the exclusion of thermal effects. The simulations predict current amplitude requirements for magnetization reversal and highlight the transient nature of magnetization changes under a short current pulse. The experimental facet involves the fabrication and assessment of tungsten films for their viability in SOT applications. Employing sheet resistance measurements via Van der Pauw's 4-point method, tungsten films exhibit varying resistivity based on thickness, suggesting the presence of distinct phases. The investigation then shifts to the study of magnetization in fabricated Hall structures through current-based Hall measurement techniques. Initial attempts utilizing cobalt films uncover in-plane anisotropy, motivating a shift to the use of Co20Fe60B20 films, a recognized ferromagnetic material in spintronics. Careful fabrication and annealing of CoFeB films appeared to improve film quality, yielding promising Hall signals; however, the expected out-of-plane anisotropy required for the targeted SOT experiment could not be achieved. Furthermore, attempts to measure SOT-induced effects encountered challenges, highlighting the need for superior film quality and a more sensitive measurement setup. Despite facing setbacks in detecting SOT-induced effects, the research includes a comparative study of Hall signals across different thicknesses in ferromagnetic devices. The study collectively contributes valuable insights into the complex interplay of spin-orbit torque effects, film quality, and magnetization characteristics within the realm of ferromagnetic systems. This work can serve as a reference for future endeavors in the exploration of spin-orbit torque-driven phenomena and their potential applications.



Spin-orbit torque (SOT), Anomalous Hall effect (AHE), Ferromagnetic thin films, Micromagnetics, OOMMF