The atomic and spin-electronic structure of interfaces and extended structural defects in the Co-based full Heusler alloys

• Main Author: Nedelkoski, Zlatko
• Format: Dissertation
• Language: English
• Published: University of York 2017

The atomic and spin-electronic structure of interfaces and extended structural defects in the Co-based full Heusler alloys is studied. Interfaces between a half-metallic Heusler alloy and metal or semiconductor are fundamental and determine the performance of spintronic devices such as spin valves or devices for spin injection applications. It is shown that for the Co2MnSi/Ag bulk-like terminated interfaces, the interfacial spin-polarisation significantly depends on the atomic plane termination. In addition, on the example of experimentally realised interface, part of a spin valve, it is demonstrated that there is an additional monolayer at the interface, which as shown by the density functional theory calculations can create significantly negative local spin-polarisation, detrimental for the device performance. It is demonstrated that the interfaces between the Heusler alloy and Si, suffer from large interfacial interdiffusion which leads to a gradual decrease of magnetic moment over 2-3 nm region in which the spin-polarisation is also significantly affected. It is shown that even sharp interfaces are not desirable since they lead to reversed spin-polarisation. However, it is demonstrated that the addition of thermodynamically stable Si-Co-Si monolayer provides very high spin-polarisation across all interface layers. An ideal candidate for spin injection applications is found to be the Co2FeAl0.5Si0.5/Ge interface which shows very minor and atomic plane selective interdiffusion that does not affect film’s half-metallic properties, absence of formation of any secondary phases and almost no interfacial strain. Based on models derived from electron microscopy observations, it is demonstrated that this interface retains very high interfacial spin-polarisation. Finally, the atomic structure of an extended structural defect observed in Co2FeAl0.5Si0.5 thin film is revealed by electron microscopy. The performed density functional theory modelling shows that these boundaries reverse the sign of spin-polarisation, hence their presence has to be minimised in order to achieve films with better properties.