Spin Torque and Unidirectional Magnetoresistance in Light Metal-Ferromagnet heterostructures: Probing the origin of Orbital Hall Effect through interface modification and temperature variation

Mahapatra, Dhananjaya (2026) Spin Torque and Unidirectional Magnetoresistance in Light Metal-Ferromagnet heterostructures: Probing the origin of Orbital Hall Effect through interface modification and temperature variation. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Dhananjaya Mahapatra (20RS019)) 20RS019.pdf - Submitted Version Restricted to Repository staff only Download (20MB)

Abstract

The generation and control of angular momentum are key challenges in modern spintronic research. Conventionally, spin currents are generated using the spin Hall effect, which requires materials with strong spin–orbit coupling. Recently, alternative mechanisms related to orbital degrees of freedom have attracted interest, particularly in light metal–ferromagnet bilayer systems. This thesis presents an experimental study of current-induced transport phenomena using harmonic magnetotransport measurements in such heterostructures. A theoretical overview of angular momentum generation in solids is first discussed, followed by detailed descriptions of device fabrication and experimental techniques. Thin-film devices were fabricated using various physical vapor deposition methods, including DC and RF sputtering, thermal evaporation, and electron-beam evaporation. Harmonic measurements were carried out by applying an AC current and simultaneously detecting longitudinal and transverse first- and second-harmonic voltage signals using lock-in amplifiers. Angle-dependent and temperature-dependent measurements were performed with the help of a vector magnet and a helium cryostat. Experimental studies on light metal–ferromagnet bilayer devices reveal clear second-harmonic responses associated with orbital current–induced effects, including orbital torque and unidirectional orbital magnetoresistance. The angular dependence of the harmonic signals allows separation of different current-induced contributions. Temperature-dependent measurements show a systematic reduction in the harmonic signal magnitude with decreasing temperature. Furthermore, interface modification using Cu leads to an enhancement of the Slonczewski-like torque efficiency, highlighting the important role of orbital current transmission and interface engineering. In addition, harmonic magnetotransport measurements were carried out on a FePS₃/NiFe bilayer device, where exfoliated two-dimensional FePS₃ was transferred onto a ferromagnetic NiFe layer. In this system, transverse second-harmonic resistance measurements were used to extract the spin torque–related field-like torque efficiency. A clear enhancement of the fieldlike torque efficiency was observed with decreasing temperature; however, no direct evidence of orbital current generation was observed in the twodimensional material, and the microscopic origin of the enhancement remains an open question. Overall, this thesis provides a systematic experimental study of harmonic magnetotransport in light metal–ferromagnet and two-dimensional material– ferromagnet heterostructures. The results demonstrate the role of orbital current effects in light metal systems and spin torque–driven responses in two-dimensional heterostructures, emphasizing the importance of interface engineering and temperature-dependent studies for the development of lowpower magnetic devices. vii

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