Emergent interfacial ferromagnetism in perovskite oxide heterostructures [electronic resource]
Keywords: Materials Science and Engineering.

The rapid technological advancement of electronics in my lifetime has embedded digital technologies in nearly every facet of commercial and social life. To pursue the continued development of electronics, novel device architectures and correspondingly novel materials will be required. This dissertation focuses on emergent interfacial ferromagnetism in perovskite oxides. In particular, the model system of LaNiO3/CaMnO3 (LNO/CMO) is used to comprehensively explore the factors involved in generating and manipulating emergent ferromagnetism at the interface between these materials. First, CMO thin films are investigated for strain-induced ferromagnetism. While I find that the antiferromagnetic transition temperature is tunable via strain, ferromagnetism is not stabilized. The possibility of generating strain-induced ferromagnetic CMO is still open, but these results indicate that in the range of strains studied, CMO is an excellent candidate for emergent interfacial ferromagnetism as it remains antiferromagnetic. Next, I investigate the mechanisms responsible for generating interfacial ferromagnetism in LNO/CMO. I find that the ferromagnetism can be divided into two regimes. In metallic superlattices, Mn--Mn double exchange is the dominant ferro- magnetic mechanism. However, using x-ray magnetic circular dichroism I determine that the ferromagnetism in insulating superlattices is associated with Ni--Mn superexchange. It is shown that this interfacial ferromagnetism is induced by charge transfer to interfacial Ni as a result of polar compensation. Resonant soft x-ray scattering is used to show that the excess charge is shown to be confined to Ni at the interface. A model is proposed for the generation of excess charge from oxygen vacancies in the CMO layer. This is the first conclusive demonstration of emergent interfacial ferromagnetism arising from polar compensation. Third, interfacial oxygen octahedral coupling is investigated as a potential mechanism for tuning interfacial ferromagnetism. Synchrotron x-ray diffraction is used to examine the oxygen sub-structure. It is shown that interfacial ferromagnetism in LNO/CMO can be tuned by controlling the CMO crystal symmetry and orientation by varying the LNO and CMO layer thicknesses. These effects can be used to selectively tune double-exchange ferromagnetism in metallic superlattices. It is also shown that interfacial symmetry mismatch can inhibit this tuning mechanism. These results demonstrate a new framework for manipulating interfacial ferromagnetism. Finally, I study (111)-oriented LNO/CMO superlattices. To date, there have been no reports of (111)-oriented CMO-based superlattices, yet it is expected that (111)-oriented superlattices may exhibit enhanced interfacial coupling. It is demonstrated that well-ordered, high quality (111)-oriented superlattices can be fabricated for LNO/CMO. The magnetometry and x-ray spectroscopy results for (111)-oriented superlattices are consistent with enhanced superexchange. In thin superlattices, the magnetic moment per interfacial unit is more than doubled. The transition temperature is increased by at least 50 K compared to (001)-oriented superlattices. (111)-oriented superlattices are found to exhibit exchange bias. The exchange bias results are discussed in the context of related exchange-biased systems, LaNiO3/LaMnO3 and CaRuO3/CaMnO3. These results suggest that both LNO and CMO may play a role in biasing the interfacial ferromagnetic layer.