Spin and orbital effects in antiferromagnetic CoO thin films
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Abstract
Antiferromagnetic materials have emerged as key candidates in the field of spintronics due to their intrinsic advantages: zero net magnetic moment, robustness against external magnetic fields, and ultrafast magnetization dynamics in the THz range. These properties highlight their potential to revolutionize information technologies by enabling stable, high-speed, and densely integrated devices. However, the development of efficient mechanisms for writing and reading magnetic information in antiferromagnets remains a significant challenge. Although electrical switching of the Néel vector has been widely studied, it is often dominated by
thermal effects that obscure the ultrafast dynamics responsible for the materials’ unique appeal. Similarly, conventional readout schemes based on spin currents tend to yield weak signals, limiting their practical applicability. Although readout schemes based on orbital currents exist, the orbital currents themselves cannot
be directly utilized because they couple inefficiently to the spin magnetization in magnetic materials. Consequently, they must first be converted into spin currents to be utilized.
This thesis investigates thin films of CoO, a prototypical antiferromagnet with a large, orbital moment, and explores switching and readout mechanisms from a novel perspective. By studying CoO/Pt and CoO/Cu* heterostructures, where Cu* refers to a naturally oxidized Cu layer, this work addresses the central challenges
related to the manipulation and detection of antiferromagnetic order in insulating systems.
First, we demonstrate the coexistence of spin-orbit torque-driven switching and thermal switching in ultrathin CoO/Pt films. By correlating the electrical readout with magnetic domain imaging, we demonstrate that the final magnetic state cannot be accounted for solely by thermomagnetoelastic effects, indicating an additional torque-driven contribution. This finding points to the potential for ultrafast, current-induced manipulation and switching in ultrathin antiferromagnetic layers.
Second, we present the first (to our knowledge) experimental evidence of a purely orbital interaction mechanism in antiferromagnets using CoO/Cu* bilayers. By combining orbital currents with the orbital-dominated magnetization of CoO, we achieve a two-order-of-magnitude enhancement in a magnetoresistance signal. Notably, the absence of a heavy metal layer, and thus the absence of a conventional spin current source, excludes spin-based mechanisms as the dominating origin of the observed signal. Instead, Cu* is known to generate significant orbital currents, and we demonstrate that the enhanced readout signal arises from
the coupling of these currents to the large orbital moment in CoO. This finding provides compelling evidence that orbital currents can serve as an efficient and robust readout mechanism, marking a paradigm shift from spin-based approaches and opening a new frontier in antiferromagnetic orbitronics.
Third, comparative studies on CoO/Cr and CoO/Cu/Pt trilayers show that the effect is robust and not limited to the oxidation of the Cu. Because the surface oxidation of Cu is difficult to control, it is essential to identify mechanisms and materials that are suitable for real-world applications. In CoO/Cr, we find that the readout signal is enhanced by a factor of five compared with measurements on CoO/Pt. Moreover, the enhanced readout signal in CoO/Cu/Pt bilayers indicates that the effect can as well originate from interface-driven orbital-current generation, rather than solely from oxidation of the Cu layer or other extrinsic effects. This result demonstrates the feasibility of manufacturing stable, oxidation-free devices and lays the groundwork for further investigations into interface effects and the precise readout mechanism.
By demonstrating spin-orbit-torque-based switching and introducing a novel readout mechanism based on orbital currents, this work constitutes a paradigm shift in our understanding of spin- and orbital-driven phenomena in antiferromagnets. These results open new avenues for the field of orbitronics and establish
orbital angular momentum as a critical tool in developing faster, more energyefficient, and scalable magnetic technologies.