Development of a system for low-temperature ultrafast optical study of three-dimensional magnon and spin orbital torque dynamics.

An ultrafast vector magneto-optical Kerr effect (MOKE) microscope with integrated time-synchronized electrical pulses, two-dimensional magnetic fields, and low-temperature capabilities is reported. The broad range of capabilities of this instrument allows the comprehensive study of spin-orbital interaction-driven magnetization dynamics in a variety of novel magnetic materials or heterostructures: (1) electrical-pump and optical-probe spectroscopy allows the study of current-driven magnetization dynamics in the time domain, (2) two-dimensional magnetic fields along with the vector MOKE microscope allow the thorough study of the spin-orbital-interaction induced magnetization re-orientation in arbitrary directions, and (3) the low-temperature capability allows us to explore novel materials/devices where emergent phenomena appear at low temperature. We discuss the details and challenges of this instrument development and integration and present two datasets that demonstrate and benchmark the capabilities of this instrument: (a) a room-temperature time-domain study of current-induced magnetization dynamics in a ferromagnet/heavy metal bilayer and (b) a low-temperature quasi-static polar MOKE study of the magnetization of a novel compensated ferrimagnet.

Exchange-Driven Spin Relaxation in Ferromagnet-Oxide-Semiconductor Heterostructures.

We demonstrate that electron spin relaxation in GaAs in the proximity of a Fe/MgO layer is dominated by interaction with an exchange-driven hyperfine field at temperatures below 60 K. Temperature-dependent spin-resolved optical pump-probe spectroscopy reveals a strong correlation of the electron spin relaxation with carrier freeze-out, in quantitative agreement with a theoretical interpretation that at low temperatures the free-carrier spin lifetime is dominated by inhomogeneity in the local hyperfine field due to carrier localization. As the regime of large nuclear inhomogeneity is accessible in these heterostructures for magnetic fields <3 kG, inferences from this result resolve a long-standing and contentious dispute concerning the origin of spin relaxation in GaAs at low temperature when a magnetic field is present. Further, this improved fundamental understanding clarifies the importance of future experiments probing the time-dependent exchange interaction at a ferromagnet-semiconductor interface and its consequences for spin dissipation and transport during spin pumping.