Disorder Engineering of Ferroic Properties

Abstract

Worldwide energy consumption is expected to increase 50% by the year 2050, with as much as 25% of that being lost to waste heat from electronic devices. Multiferroic materials have the potential to mitigate this heating and volatility in computational devices by allowing voltage control of a magnetic state, virtually eliminating waste heat from “always-on” Si-based technologies. This places multiferroic devices among the most competetive post-silicon technologies considering energy and delay. Multiferroic systems, however, are extremely rare, hindering the realization of new technologies based on these materials. This dearth of materials can be mitigated through use of multiferroic composite systems, where, for instance, a piezoelectric layer is coupled to a magnet through strain, but further challenges exist in maximizing the coupling between layers in the composite, an effort that has seen relatively little work. Additionally, existing engineering techniques utilize atomic-scale or crystal-scale ordering to access magnetic coupling in materials, but chemical and structural disorder is an oft explored technique that has been shown to lead to novel and colossal functional properties. This thesis explores the use of disorder as primary phenomenon to both synthesize new ferroics and enhance material properties for superior functionalities, an orthogonal vector to addressing the scarcity of state-of-the-art materials. By using low temperature epitaxial growth to stabilize the disordered, α-Fe-like, phase of Fe1-xGax out to high, metastable concentrations of Ga, both the increased spin-lattice coupling of Fe and Ga and the lattice softening associated with the phase transition can be leveraged without the formation of intermetallic phases that detract from functionality. With this technique, epitaxial kinetic freezing of disorder, I have demonstrated a means to boost magnetostrictive coupling by as much as 10x relative to bulk, allowing us to show record magnetoelectric performance in a device based on the material. Additionally, I have shown that the phase space of ferroic materials can be extended using entropy as a driving force to stabilize materials with novel chemistries. By leveraging the large configurational entropy from the inclusion of many atomic species, the formation of a random, solid solution crystal can be achieved, potentially overriding other thermodynamic considerations. At elevated temperatures, a large entropic contribution to the Gibbs’ free energy will stabilize the formation of a single phase, even in excess of an unfavorable heat of mixing. This metastability can be further controlled with modern thin film techniques, allowing further access to a large class of materials that have been shown to possess unusual and colossal functional properties. For the first time, I have shown strong magnetism in these new systems, as well as shown that it is strongly correlated to structure and chemistry. These new magnetic oxides provide a platform to investigate and tailor interplay between charge, lattice and spin via disorder for functional properties by the design of disorder. The goal of the work presented here is to understand how engineered disorder plays a role in the tunability of functional properties. I show that low-temperature epitaxy can be used as a tool to access new, disordered, regions of the phase diagram, significantly enhancing the functional properties when compared to the thermodynamic phase. Additionally, the engineering of disorder through chemistry and processing conditions can be used to further tune magnetic phenomena, introducing new order parameters to optimize the system.