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Direct and inverse spin orbit torques

Subject Area Experimental Condensed Matter Physics
Term from 2017 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 387161541
 
Final Report Year 2023

Final Report Abstract

The field of spintronics aims to exploit the interaction of electronic spin and charge degrees of freedom for future information technology. One key spintronic challenge is the energy-efficient control of magnetization direction. Such a control can be used to, e.g., encode data bits in magnetization orientation. While control with external magnetic fields is straightforward, it is not very energy efficient because the magnetic field generation requires substantial electrical currents with ensuing dissipation. If the electrical currents are applied directly to nanostructures that contain the magnetic material, so-called spin-orbit torques can act on the magnetization and control its orientation without the need for external magnetic fields. This spin-orbit torque process can be more efficient than magnetic-field control and is more suitable for applications in nanostructures. Understanding and optimizing spin-orbit torques is thus highly relevant for efficient spintronic devices such as magnetic random-access memory (MRAM) or spin-wave logic (magnonics). This project obtained insight into the physical concepts of spin-orbit torques and identified promising material combinations and new concepts for spintronics. To this end, we experimentally determined the spin-orbit torques in magnetic heterostructures. We studied spin-orbit torques in heterostructures consisting of thin-film ferromagnets and heavy metals and thin-film ferromagnets and superconductors. In our experimental studies, we employed a novel spin-orbit torque spectroscopy technique that has been developed by us in collaboration with NIST researchers. The technique is based on quantitative and self-calibrated vectornetwork-analysis. It uniquely allows to study spin-orbit torques in unpatterned thin film heterostructures and thus enables high-throughput spin-orbit torque measurements. The technique takes advantages of the reciprocity of direct and inverse spin-orbit torques in the linear response regime. In addition to determining the spin-orbit torques in an integral fashion, spatially resolved measurements of magnetization dynamics are key to their microscopic understanding. To this end, we established an optical microscopy technique that can measure phase-resolved GHz-frequency magnetization dynamics with sub-µm spatial resolution. Using our experimental techniques, we revealed the material composition dependence of spinorbit torques in heavy metal / ferromagnet based heterostructures as well as the temperature dependence of spin-orbit torques in superconductor / ferromagnet based heterostructures. We furthermore demonstrated excitation and detection of spin-waves with phase-resolution in ultralow-damping metallic magnets that have high relevance for magnon logic devices. The obtained results have direct consequences for the practical realization of spintronic devices, which require small structure size and GHz-frequency operation.

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