Enhancement of Functionality in Complex Oxides Through Growth in the [111] Direction PDF Download

Author: Margaret Marie Kane
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Languages : en
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Book Description
Transition metal oxides are a particularly interesting class of materials because they exhibit tunable properties due to the coupling of the lattice, spin, orbital, and charge degrees of freedom. In this thesis, I explore the potential to tune their properties through growth in the [111] direction. The enhancement of functionality of materials grown in the (111)-orientation could expand the current toolbox used to design novel computing devices. The growth of LaNiO3 thin films on (111)-oriented LaAlO3 results in a unique strain state in the first few unit cells at the substrate/film interface. This strain, consisting of an expansion of the out-of-plane lattice parameter in the first few interfacial unit cells, results in insulating behavior at the substrate/film interface, the emergence of the anomalous Hall effect, and changes in the chemical valence. Notably, these changes are distinct from those observed in ultrathin (001)-oriented LaNiO3 films. This makes it clear that the strain introduced by the substrate in (111)-oriented films has the potential to greatly alter the properties of the first few unit cells of film grown on it. These thin films are also indirectly observed to be antiferromagnetically ordered. While antiferromagnetism can be difficult to directly measure, I provide evidence that these films are entirely magnetically ordered and that there is no net ferromagnetic moment. These results suggest growth in the [111] direction alters the magnetic structure of LaNiO3. Next, I explore the enhanced interfacial ferromagnetism enabled in (111)-oriented CaMnO3/CaRuO3 superlattices. These superlattices have a larger total moment and a thicker interfacial ferromagnetic phase present in the CaMnO3 as compared to (001) superlattices as well as the emergence of ferromagnetism in CaRuO3 layers. Further, an enhancement of the Curie temperature shows that growth direction can change this important engineering specification: the temperature at which magnetic ordering occurs. These changes are likely due to the differences in the orbital overlap across the (111) interface and suggest that magnetic structure can be tuned by growth direction--particularly for G-type antiferromagnets. In sum, the superlattices support the conclusion that growth of complex oxides in the (111) orientation results in an enhancement of the functionalities observed in (001)-oriented films.