UCLA

This work focuses on the engineering and tailoring the interfaces of both ferromagnetic multilayer films based on (Iron Gallium) FeGa as well as the surfaces of ferroelectric (Hafnium Oxide) HfO2 for applications toward magnetoelectric applications, which offer the promise of efficient control of magnetism at the nanoscale. In this work, two key materials challenges of the respective ferromagnetic and ferroelectric materials toward integration in composite magnetoelectric devices are discussed: development of ferromagnetic materials with strong magnetomechanical coupling and ferroelectric materials with robust ferroelectric properties at the nanoscale. First, while FeGa is a well-known magnetostrictive material that could be a candidate for integration for strain-mediated magnetoelectric devices, the challenge remains that it is lossy at high frequencies. On the other hand, while ferroelectric HfO2 has gained interest due to its emergent and ferroelectricity at the nanoscale that circumvents traditional limitations of ferroelectric materials and is CMOS compatible, it remains a challenge to fully understand how to stabilize the ferroelectric phase.

To address the former, this work investigated how the influence of an underlayer and a multilayering structure can be used to enhance the soft magnetic properties of FeGa films. It was found that a NiFe underlayer serves to influence the microstructure of the FeGa films, resulting in a smaller grain size and enhanced texture, which yielded a smaller coercivity while retaining a strong magnetostriction. It was also observed that the saturation magnetostriction is maintained for the FeGa films. Furthermore, a multilayering strategy that uses NiFe as an interlayer to form FeGa/NiFe bilayers was investigated to achieve a composite with a further decrease in coercivity and lower high frequency losses – specifically for a multilayer consisting of ten bilayers of FeGa (10 nm) / NiFe (2.5 nm). Additionally, the multilayering strategy combined with an insulating interlayer was shown to be a useful strategy to achieve a composite with an even lower coercivity meets the necessary criteria of magnetic softness and low loss necessary for integration in magnetoelastic and high frequency antenna devices.To address the latter, density functional theory was used to understand the relationship between the ferroelectric polarization and the surface composition to stabilize the orthorhombic ferroelectric phase of HfO2. It was found that the surface composition plays a critical role in the ferroelectric stability of orthorhombic HfO2 thin films, which can enable stable polarization without a critical thickness limit under an open-circuit boundary condition. It was found that a relatively oxygen-rich positively polarized surface can effectively screen the polarization to stabilize the orthorhombic phase. In contrast, stoichiometric HfO2 surfaces that cannot screen the polarization lead to an ionic depolarization towards a nonpolar monoclinic phase. This highlights the importance of controlling the surface composition for the stability of ferroelectricity in HfO2 and points towards control of the surface composition as a mechanism for optimizing the ferroelectric performance of HfO2-based thin films.

This work provided two routes for the development and engineering of ferromagnetic and ferroelectric materials that can overcome key material challenges for the integration toward magnetoelectric devices with robust and efficient performance.