Interfacial Tuning of the Magnetoresistance and Magnetic Coupling in Metal/Oxide Thin Films

Abstract

We live in a society defined by constant growth. This is exemplified by information technologies which, over a time period of only a few decades, have transformed our way of life. This rapid expansion has relied heavily on there being ”plenty of room at the bottom”. However, as we are faced with the physical limitations of continuous miniaturization, there is a need for a new paradigm to support the growing computing power requirements of our society. The field of spintronics provides us with several opportunities to increase the functionality of electronic devices in a sustainable way. After the discovery of the giant magnetoresistance and the tunneling magnetoresistance, which have revolutionized data storage technologies, scientists have continued to unravel novel effects and propose new applications that exploit the interplay of charge and spin degrees of freedom in electronic devices. In the last decade, a growing number of ways to measure and manipulate the magnetic state, which is used to store data, have arisen. One common thread in modern spintronics is the importance of interfaces. In this thesis, we deal with a specific type of interface, that is both well-known, and often overlooked: ferromagnet/oxide interfaces. Specifically, we explore two main areas: firstly, magnetotransport and current-induced SOTs, and secondly, the tuning of magnetic anisotropies to induce chiral effects. To investigate the resulting physical phenomena, both parts of this work have involved the development of new fabrication techniques as well as data acquisition and analysis methodologies. To investigate the magnetotransport of ferromagnet/oxide interfaces, we study model systems consisting of Fe/oxide heterostructures epitaxially grown on MgO(001). Our investigation reveals that current-induced effects that are typical of heavy metal/ ferromagnet bilayers, such as spin-orbit torques and unidirectional magnetoresistance, also exist in Fe/oxide systems. In addition, the crystallinity of the Fe is imprinted onto these effects, resulting in a dependence on the orientation of the current relative to the crystal lattice. Our results show that charge-spin conversion effects do not require the presence of heavy metals as the source of spin currents. Moreover, they show that subtle differences in the morphology and chemical composition of the interfaces can produce sizable changes of the spin-orbit torques and unidirectional magnetoresistance. Besides playing a role in the magnetotransport properties, oxide interfaces can also influence the magnetic anisotropy of ferromagnets. In this thesis, we exploit the interface-mediated anisotropy in Pt/Co/AlOx to induce what we coined ”lateral chiral coupling”. This coupling requires nanoscale ontrol over the magnetic anisotropy as it makes use of the Dzyaloshinskii–Moriya interaction, which has a very short range. For this purpose, we combined state-of-the-art fabrication techniques to develop a new process, which opened the door to the fabrication of laterally coupled in-plane and out-of-plane magnetized regions of the same layer. The effect of the lateral chiral coupling is manifold, leading to the exchange bias of adjacent in-plane and out-of-plane magnetized regions and the antiferromagnetic coupling of out-of-plane nanomagnets mediated by an in-plane spacer region. Moreover, the lateral coupling enables field-free magnetization switching using spin-orbit torques, as well as domain nucleation sites with reconfigurable energy barriers. The results of this thesis highlight the potential of ferromagnet/oxide interfaces to tune the magnetic ground state of singlelayer ferromagnets as well as their magnetotransport properties and current-induced spin-orbit torques.