Investigations of magnetic structures in one dimensional geometry for spintronics applications

• Main Author: Bhatti, Sabpreet
• Other Authors: S.N. Piramanayagam
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Science::Physics::Electricity and magnetism; Science::Chemistry::Physical chemistry::Solid state chemistry
• Online Access: https://hdl.handle.net/10356/154533
• Institution: Nanyang Technological University

Spintronics is a branch of magnetism, which exploits the individual spin of electrons. The devices based on spintronics are propitious for memory, computing, and sensing applications. Such devices utilize the spin degree of freedom of electrons. For spintronics devices, one-dimensional geometry has been explored extensively as it facilitates a guided motion of magnetic domain textures. This thesis emphasizes the physical and magnetic properties of some of these one-dimensional spintronics devices that can be used to harvest energy, generate microwaves, and racetrack memory applications. The work reported in this thesis has been divided into three parts. The first part explores the energy harvesting devices for spintronics-based internet of things (IoT) sensors. In this work, an FeCo based alloy, which depicts magnetostrictive properties, has been used. The stress-induced magnetic behavior of the FeCo thin films and microwires has been studied. The application of external mechanical stress induces a change in the direction of the easy axis. The studied behavior assists to fathom the nuance of material properties, which were further used to demonstrate energy harvesting. We have fabricated a one-dimensional device with wrapped pick-up coils, which was introduced under external stress mimicking the ambient vibrations. The harvested power from the fabricated devices was in the range of µW, which can be used as a power source for spintronics IoT sensors. The second part of this thesis explored new ideas for further exploration. The studies involved exploring the confined motion of domains walls and skyrmions for microwave generation and logic/computing applications, respectively. The demonstrations have been carried out by employing micromagnetic simulations. Microwave generation in GHz range was demonstrated using back and forth motion of domain walls in a nanotrack with synthetic pinning sites. The latter portion explores the skyrmion motion confined in a Dzyaloshinskii-Moriya interaction (DMI) energy well. We have demonstrated that the skyrmion Hall effect can be suppressed after introducing energy barriers, which proved to be beneficial for high-density skyrmion devices. The last part of the thesis explores how to achieve a high-density skyrmion state in thin films and demonstrates the DMI modification for confined motion of skyrmions experimentally. Intermixing of atoms at interface of Pt/CoFeB/MgO stack was utilized to vary the interfacial properties, viz. K_eff and DMI, via He+ ion irradiation. The effect of irradiation dose was studied on the magnetic and skyrmion physical properties. The undertaken approach effectively enhanced the skyrmion density by five folds (reaching up to 50 skyrmion/µm2). Moreover, the skyrmion size was reduced by ~70 nm. The variation in the physical property of skyrmions can be construed due to a reduction in the domain wall energy. This technique can be used for generating high-density skyrmions at nucleation sites of racetrack memories. This technique also can be used to achieve the tracks with low DMI for confined motion of skyrmions.