Investigation of Chiral Spin Textures in Pt/Co/Ni Based Magnetic Superlattices

The work presented in this thesis investigates material systems and conditions to stabilize chiral spin textures such as domain walls and Skyrmions in Pt/Co/Ni based asymmetric superlattices. We show that various spin textures can be selectively controlled by tuning magnetic properties such as Dzyaloshinskii-Moriya Interaction (DMI), Perpendicular Magnetic Anisotropy (PMA), Saturation Magnetization (M_s) etc. This is done using Lorentz Transmission Electron Microscopy (LTEM), a technique that relies on deflection of an electron beam through a magnetic thin film to image magnetic features at a high resolution. This thesis provides pathways for further exploration of Pt/Co/Ni based magnetic thin films in optimizing domain wall and Skyrmion based computing and memory devices. 

First, we compare Pt/Co/Ni the system supports stabilization of N´eel Skyrmions and efficient movement of Dzyaloshinskii domain walls (DW) against a less established system Pt/Ni/Co system. We leverage the multi-layer system to compare the role of the Pt/Co interface when on the bottom vs top of the asymmetric repeat unit. In [Pt/Co/Ni]_M LTEM confirmed the existence of chiral N´eel domain walls and Skyrmions, and hence a large interfacial DMI. However, [Pt/Ni/Co]_M showed a large drop in PMA and DMI as LTEM showed only Bloch walls. It is possible that higher adatom energy of Pt results in a higher degree of intermixing at the interface when deposited onto Co leading to these observations. When perpendicular fields were applied, this system stabilized a large number of Bloch Skyrmions along with topologically trivial type II bubbles. The high bubble density was beneficial to examine the role of topological protection in stability of spin textures. By simultaneously observing trivial and non-trivial bubbles, we report lower perpendicular field for annihilation of the former. 

Second, we show that Bloch chirality preference in magnetic domain walls and Skyrmions can originate from the interplay between an interfacial Dzyaloshinskii Moriya interaction (DMI) and a perpendicular magnetic field. This is starkly evident in Pt/Ni/Co thin films with DMI lower than the critical strength required to form purely N´eel walls and Skyrmions. While conventional DMI does not break energetic symmetry of the two Bloch chiralities, the energy barrier to switch between two Bloch chiralties becomes asymmetric. We also demonstrate that the growth of favorable ’worm’ domains is mediated by vertical Bloch line (VBL) movements pursuant to DMI vector sign and applied field direction. The preference for any chiral Bloch Skyrmions due to interfacial DMI has not previously been seen or explained in the magnetics community and offers newer pathways to explore memory devices based on Bloch chirality switching. 

Third, we present Ir-based Synthetic Antiferromagnets (SAFs) composed of asymmetric building blocks of Pt/[CoNi]_M/Ir, that allows for simultaneous control of interfacial Dzyaloshinskii-Moriya interaction (iDMI) and interlayer exchange coupling (IEC). Using Lorentz TEM, we explore configurations of DWs in absence and in presence of different magnitudes and sign of iDMI. We demonstrate conditions to directly observe these DWs with LTEM and study their “untangling” under in-situ perpendicular magnetic fields. These thin film SAFs could prove to be viable for spintronics based sensing and memory devices, where chiral DW configurations can be stabilized and manipulated by strong DMI coupled with strong antiferromagnetic exchange coupling.