UC Irvine

Antiferromagnetic (AF) order is robust against magnetic-field perturbations and can exhibit dynamics at terahertz (THz) frequencies. This is because the opposing magnetic moments in antiferromagnets largely compensate, resulting in a suppressed net magnetization that makes their order robust but difficult to manipulate and detect. Furthermore, exchange interactions generate large restoring torques that drive spin dynamics into the sub-THz regime, beyond the reach of many conventional measurement techniques. This combination of robustness and speed makes antiferromagnetism promising for ultrafast spintronics, yet experimentally challenging to study. Recent advances have demonstrated electrical control and readout of Néel order and the excitation of antiferromagnetic resonance (AFMR), establishing the feasibility of antiferromagnetic spintronics. Further progress requires reliable methods to excite and detect antiferromagnetic dynamics, particularly the large-angle Néel-vector motion associated with switching. This dissertation presents a new class of highly efficient spintronic terahertz emitters (STEs) based on a Pt–Au alloy with high spin Hall efficiency. Through a systematic study of alloy composition, heterostructure geometry, and annealing conditions, we showed that Pt75Au25 increases the emitted THz power by approximately 30% compared with an optimized CoFeB/Pt bilayer and by 10% compared with a W/CoFeB/Pt trilayer. Annealing revealed the critical role of interfaces: interfacial alloying strongly suppresses THz emission, showing that selecting materials with a low tendency toward interfacial alloying provides a route to more efficient STEs. Pt75Au25 therefore provides a simple, readily implemented materials route toward higher-efficiency STEs by alloying Au into the Pt layers of commonly used Pt/CoFeB- and W/CoFeB/Pt-based emitters. This advance addresses the need for high-power sources for pulsed THz spectroscopy across scientific disciplines, particularly for probing ultrafast antiferromagnetic dynamics. Next, this dissertation presents the first demonstration of a sub-100-nm electrically wired AF nanoparticle and develops a method for fabricating such devices. The stochastic switching of individual AF nanoparticles has not been directly probed, despite theoretical predictions of exchange-enhanced attempt rates, unusual damping-dependent relaxation, and switching rates up to three orders of magnitude faster than in comparable ferromagnets. These devices enable direct tests of these predictions while providing the electrical access required to realize antiferromagnetic probabilistic bits (p-bits). Preliminary measurements show a reproducible magnetoresistive response, demonstrating the feasibility of the electrical readout geometry. This achievement provides a path toward ultrafast antiferromagnetic p-bits, in which stochastic Néel-vector dynamics could be used directly for probabilistic computing. Finally, this dissertation presents a new method for measuring AFMR in canted antiferromagnets using widely available tabletop THz systems, termed field-modulated continuous-wave terahertz (CW-THz) spectroscopy. The method determines the AFMR frequency and linewidth and identifies when the observed line shape is distorted by the optical environment. I provide explicit guidance on the limitations and operating conditions required for reliable and reproducible extraction of antiferromagnetic material properties. This advance provides a practical and accessible route to sensitive sub-THz spectroscopy of antiferromagnetic dynamics using tabletop CW-THz systems, making these measurements accessible to a broader scientific community.