Alexandria Will-Cole’s PhD Proposal Review

“Morphology, Magnetism, and Transport in Nanomaterials and Nanocomposites”

Abstract:

Magnetic thin film materials and bilayer composites enable unprecedented new applications, ranging from magnetic-based microelectromechanical systems (magnetoelectric sensors, ultracompact magnetoelectric antennas, etc.), terahertz emitters, to spin-orbit-torque driven magnetic memories. Here we focus on two subdisciplines within magnetics – magnetoelectrics and spintronics heterostructures.

The first aspect of the talk is focused on magnetoelectrics. Strain-mediated magnetoelectric coupling (i.e., voltage/electric field control of magnetism, or magnetic field control of electrical polarization) in bilayer composites has received heightened attention in the research community for applications in memory, motors, sensors, communication etc. The composite ME effect is dependent on the magnetostrictive effect (magnetic-mechanical coupling) and the piezoelectric effect (electrical-mechanical coupling), and therefore to improve the composites each constituent phase needs to be optimal. Here we demonstrate the feasibility of machine learning, specifically Bayesian Optimization methods, to optimize ferromagnetic materials, specifically (Fe100−y Gay)1−xBx (x=0–21 & y=9–17) and (Fe100−y Gay)1−xCx (x=1–26 and y=2–18) to demonstrate optimization of structure-property relationships, specifically the compositional effect on magnetostriction and ferromagnetic resonance linewidth. Following the materials optimization study, we present voltage control of ultrafast demagnetization in ME heterostructure of (Fe81Ga19)88B12/ Pb(Mg1/3Nb2/3)O3–PbTiO3. Previous studies implement multiple strategies to tune ultrafast demagnetization namely via the laser pump wavelength, fluence, polarization, and pulse duration as these control the total absorbed energy into the film. Here we present an alternate strategy to tune ultrafast demagnetization with application of an electric field in the ME heterostructure to induce magnetic axis rotation. Additionally, we studied magnetic anisotropy changes and E-field tuning behavior following ultrafast demagnetization.

The second aspect of this talk is focused on spintronics heterostructures, namely ferromagnetic (FM)/topological insulator (TI) or ferrimagnetic insulator (FI)/topological insulator (TI) bilayer composites, and TI sputter growth and characterization. Bilayer FM/TI and FI/TI heterostructures are promising for spintronic memory applications due to their low switching energy and therefore power efficiency. TIs have been grown with molecular beam epitaxy (oriented, epitaxial films) and RF magnetron sputtering (amorphous to crystalline oriented films) and have demonstrated large spin-to-charge conversion efficiencies. However, the reactivity of TIs with FM films is often overlooked in the spin-orbit-torque literature, even though there are reports that it is energetically favorable for topological insulators to react with metals and form interfacial layers. Here we present the interfacial reaction and antiferromagnetic phase formation between MBE-grown Sb2Te3 and sputtered Ni80Fe20 films. Since FM/TI interfaces are highly reactive and form novel interfacial phases, which can encourage spin memory loss, it is critical to explore heterostructures with cleaner interfaces. Recently, we synthesized chemically stable Y3Fe5O12/Bi2Te3 films, which should have a chemically sharp interface. We present preliminary structural and magnetic characterization, followed by proposed experiments to study proximity induced magnetization in these bilayer composites. Concurrent to our investigation spintronic heterostructures, we seek to optimize sputter deposition of TIs. However, sputtering TIs requires enhanced control over defects/stoichiometry as these influence bulk transport. We present preliminary results and propose experiments to elucidate structure-transport relationships, such that we can provide strategies to controllably suppress bulk conduction to access topologically protected surface states.

Committee:
Prof. Nian X. Sun (advisor)
Prof. Don Heiman (co-advisor)
Prof. Yongmin Liu
Dr. A. Gilad Kusne
Dr. Todd Monson