Strain Engineering of Perpendicular Magnetic Insulators for Magnetoelectronics
Vu, Nguyen
2021
Abstract
Modern electronics industry has been focusing on the development of semiconductor-based hardware, with tremendous effort being invested in transistor scaling technology. While worldwide energy consumption is predicted to increase up to 21% by 2030 with ever-growing demands in computing power and speed, the scaling in power consumption in semiconductor-based devices has come to a halt. Spintronic technology, which relies on the use of electron spin in magnetic materials, has the potential to alleviate this problem. Magnetization switching offers the benefit of nonvolatility, low operation energy, and fast performance. Notwithstanding, the miniaturization capabilities in these magnetic systems are often overlooked. Despite the recession in voltage scaling, the semiconductor industry has set a standard for desired transistor size, which currently reaches 3 nanometers in dimensions. At the same time, materials performance in room temperature magnetic systems are primarily focused on hundreds of nanometers scale. This thesis explores the size scaling capacity of two classes of thin-film magnetic oxides with perpendicular magnetic anisotropy, a configuration that is promising for high-density spintronics-based electronics. This work addresses prior challenges resulting from lattice strain and epitaxy for improved performance at scale. Spin current switching of magnetism in insulating magnets has the advantage of simple device geometry and isolation of read and write operations. At nanoscale limits, however, insulating ferro/ferrimagnetic oxides experience instability caused by thermal fluctuation. To overcome this problem, the anisotropy energy, the energy needed to switch its magnetization direction and scales with the volume of the magnet, is required to stay above 42k_B T value, which is the industry standard for ten years of stability of recorded information. At the same time, minimizing anisotropy energy would reduce the switching current threshold. The relation between anisotropy energy and lattice distortion in a crystal opens a means to tune materials property to meet both requirements. The effect of strain modulation is studied on Tm3Fe5O12, an insulating ferrimagnet with perpendicular magnetic anisotropy. Through deposition control and systematic variation of in-plane strain, the magnetic anisotropy field is tuned by 14 times. I have demonstrated the tuning window for anisotropy energy that pushes the size limit closer to the desired value. Electric field switching of magnetization is the most desirable form of operation due to the elimination of electric current. Among potential candidates that exhibit the ability to manipulate magnetism with an electric field, Cr2O3 is single-phase, possesses perpendicular anisotropy, and functions at room temperature. Despite being one of the most studied materials, the technological challenge in thickness scaling has made Cr2O3 an unattractive solution in the semiconductor industry. Considerably large leakage current and low breakdown voltage in crystallographically twinned Cr2O3 grown on metallic electrodes hamper further development. By taking advantage of isostructural growth with an oxide electrode and minimal lattice strain, I have achieved a high-quality single crystal thin film at sub-100 nm thickness. The films exhibit not only bulk-like resistivity and significantly improved breakdown voltage but also magnetoelectric symmetry at room temperature for thicknesses as low as 30 nm. For the first time, without elemental doping, I have shown an increase in operating temperature for thin-film Cr2O3 compared to the room temperature limit of bulk crystal. This result has transformed Cr2O3 from being a mere interesting study subject to an auspicious system for future practical applications.Deep Blue DOI
Subjects
Perpendicular magnetic anisotropy Magnetoelectrics Spintronics
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