Energy-efficient magnetization control by picosecond spin-orbit torques

The increasing demand for high-speed, low-power memory solutions calls for the development of alternatives to the increasingly costly CMOS-based memory technologies. Spintronic devices, harnessing the electron’s spin in addition to its charge, appear as a promising alternative. In such devices, data is stored in magnetic bits that don’t require a constant power supply to retain its information, providing a path towards energy-efficient memory.

Significant progress has been made over the past 40 years in controlling spin and magnetic materials at high speeds. Among the various methods explored for this purpose, the spin-orbit torque mechanism is a leading candidate for the next generation of magnetic memories. In 2020, the fastest magnetization switching induced by this mechanism was achieved using a 6 ps pulse in a cobalt film. Furthermore, subsequent studies revealed that switching dynamics are well described by a macrospin model, suggesting coherent magnetization dynamics across the entire film at ultrafast timescales. Since switching by spin-orbit torques has typically been understood through the nucleation and propagation of magnetic domains, the above-described findings raise important questions about the present mechanisms and the implications in moving towards shorter pulses.

This doctoral thesis aims to characterize the critical currents and energy costs of the spin-orbit torque switching mechanism in the picosecond regime, providing insights into the physics at play and new pathways towards ultrafast switching by electrical means. The key achievements of this work can be summarized as:

• The successful on-chip calibration of picosecond pulses in a waveguide in real units of amplitude, current, and power delivery, enabling full electrical signal characterization. The calibration system works in practice as an on-chip high-frequency oscilloscope, a versatile technique that can be applied to research in the terahertz band.
• The first-ever characterization of the critical currents and energy costs of spin-orbit torque in the picosecond regime. Contrary to prior predictions, energy costs were found to decrease significantly towards the picosecond scale, revealing an emerging energy efficiency due to out-of-equilibrium physics induced by ultrafast current injection.