Ultrafast Spin-Charge Conversion Schemes in Magnets: THz Time Domain Spectroscopy Study
Conventional electronics, which rely on using electron charge to transport and store information, are reaching a developmental plateau due to fundamental limits. The most prominent candidate for the next generation technology is spintronics. The use of the spin degree of freedom as the information carrier can not only complement the existing charge-based functionalities but also opens a wide scope of new solutions. Most importantly, spintronics offers energy-efficient computing and memory devices operating at unprecedented ultrafast speeds, corresponding to Terahertz frequencies. The crucial requirement for implementing spins into electronic systems is the efficient spin-charge conversion at ultrafast speeds. Therefore, the research requires experimental tools operating at the desired timescales. An exceptional method for studies on spintronic material systems and conversion mechanisms is Terahertz time domain spectroscopy (THz TDS). In this thesis, we use THz TDS to investigate the routes by which the spin ordering, spin dynamics, and spin currents can lead to the generation of ultrafast charge currents. We study a variety of magnetic systems to experimentally demonstrate distinct spin-charge conversion schemes and understand the associated effects. First, we propose a novel approach of estimating spin diffusion lengths in magnetic materials using THz TDS. We test the method in a conventional thin film bilayer consisting of a ferromagnet, CoFeB, and a nonmagnetic metal with a strong spin-orbit coupling, Pt. Next, we use THz TDS to study FeRh/Pt heterostructures over a wide range of temperatures and magnetic fields. Most importantly, we demonstrate a previously unreported spin current enhancement in the antiferromagnet-ferromagnet mixed phase state at low temperatures and propose potential mechanisms responsible for it. Finally, we report the generation of helicity-dependent ultrafast photocurrents in a noncollinear antiferromagnet Mn3Sn. We combine the experimental data with a theoretical approach to relate this discovery to the nontrivial spin ordering of the material.