Successful PhD defense of Jiho Yoon

The current induced manipulation of chiral spin textures is of great interest for fundamental research and technological applications. Of particular interest are magnetic non- volatile memories formed from synthetic antiferromagnetic racetrack nanowires in which chiral composite domain walls (DWs), that act as data bits, can be efficiently moved by current pulses. However, overcoming the trade-off between energy efficiency, namely a low threshold current density and high thermal stability, remains a major challenge for developing integrated chips with high reliability and low power consumption. In this thesis, we experimentally show that chiral domain walls in a synthetic antiferromagnet (SAF) -ferromagnet (FM) lateral junction, formed by local plasma oxidation, are highly stable against large magnetic fields, whilst the domain walls can be efficiently moved across the junction by current. Our new approach takes advantage of field-induced global energy barriers in the engineered energy landscape of the junction that are added to the local energy barrier. One of our most important results is that we demonstrate that thermal fluctuations are energetically equivalent to a magnetic field effect, thereby, increasing the energy barrier, and further stabilizing the DW in the junction at higher temperatures, which is in sharp contrast with conventional FMs or SAFs. We find that the threshold current density can be effectively decreased by tilting the junction’s angle across the racetrack while keeping a high domain wall stability. Furthermore, we demonstrate that our approach can effectively create nanoscopic domain bits which is the key element for the racetrack memory technology. To achieve this, we created a novel structure wherein a FM region is spatially confined between adjacent SAF regions and confirmed that a magnetic bit can efficiently formed and shifted.
This thesis clearly shows how the aforementioned trade-off between efficiency and stability can be broken which will guide the future development of reliable DW-based memory, logic, and beyond.