Investigation of the Dzyaloshinskii-Moriya interaction and perpendicular magnetic anisotropy in magnetic thin films and nanowires

Material development has led to the advancement of the three different categories of device applications, namely logic, sensors and memory. In recent years, there has been a vast increase in research and development towards ultra-high density and highly energy efficient magnetic memory devices. One of the driving factors of this development has been the uncovering of a multitude of physical phenomena observed at the interface between a Heavy Metal (HM) layer and a Ferromagnetic (FM) material. Here two such interfacial phenomena were probed, namely perpendicular magnetic anisotropy and the interfacial Dzyaloshinskii-Moriya interaction. Both these phenomena have been shown to enhance the storage density and the power efficiency in novel magnetic memory devices. Moreover, in such HM/FM thin films, chiral magnetic spin textures have been shown to be stabilised which are interesting from both a fundamental and a technological perspective for use as a platform for future devices. In this work material thin films of the form HM/FM/MgO were investigated in which the HMs studied were Ta, W, Pt, Pd and WxTa1-x and the FM materials were different alloys of CoFeB namely, Co20Fe60B20, Co60Fe20B20, Co40Fe40B20 and Co. The structural symmetry in these heterostructures was broken at the interface between the HM and the FM in order to facilitate the interfacial Dzyaloshinskii-Moriya interaction. Typical thickness of the FM ranged from 0.6 nm – 1 nm. Growth processes using both D.C. and R.F. magnetron sputtering were used to deposit thin films primarily on Si/SiO2 substrates. It is shown here that the deposition conditions as well as post deposition processes used to deposit the material stack have a dramatic effect on magnetic properties such as domain wall pinning. A higher deposition power of 1500 W was shown to result in smoother domain structures and reduced pinning. The variation of the coercive fields was investigated for different symmetric and non-symmetric structures and found to be much smaller in FM alloys as compared to pure Co-based structures. Post deposition annealing was shown to lead to at least double the coercivity for a range of different heterostructures. Several different means of tuning the magnetic anisotropy were studied. A decrease in the magnetic anisotropy was observed for increasing FM thicknesses. The influence of a seed layer has been shown to affect the underlying domain structure as well as the anisotropy, with Ta and W exhibiting a larger anisotropy than Pd for a given FM. The more conventional out-of-plane magnetised bubble domains are replaced with worm like domains for a Pd seeded multilayer. Also, perpendicular magnetic anisotropy has been shown to be dependent on the composition of the ferromagnetic alloy under consideration. The Dzyaloshinskii-Moriya interaction was studied in optimised thin films of W/Co20Fe60B20/MgO using two different magnetic field based methods namely the field driven asymmetric magnetic domain expansion and the magnetic domain strip annihilation. The values for the interfacial DMI were found to be (0.63 ± 0.05) mJm-2 and (0.73 ± 0.5) mJm-2 respectively. Chiral magnetic spin textures called magnetic skyrmions were studied in thin films of W/Co20Fe60B20/MgO and Ta/Co20Fe60B20/Ta/MgO structures using synchrotron based Scanning Transmission X-ray Microscopy and the Magneto-optical Kerr microscopy techniques. Charge current induced motion for both the material stacks were demonstrated using current pulses along a nanowire. A very low pinning of the magnetic domains is observed with the insertion of a Ta ultra-thin dusting layer. This thereby, highlights the subtle interfacial effects that govern the magnetic properties in such multi-layered material stacks