Understanding and utilizing non-volatile properties of materials
The Rackham Predoctoral Fellowship supports outstanding doctoral students who are actively working on dissertation research and writing. It seeks to support students working on dissertations that are unusually creative, ambitious and impactful.
Congratulations Sieun!
Abstract: Rashba spin current generation emerges in heterostructures of ferromagnets and transition metal dichalcogenides (TMDs) due to an interface polarization and associated inversion symmetry breaking. Recent work exploring the synthesis and transfer of epitaxial films on the top of low layer count 2D materials reveals that atomic potentials from the underlying substrate interface are not completely screened. The extension of this transparency effect to other interfacial phenomena, such as the Rashba effect and associated spin torques, has not yet been demonstrated. Here, we report enhanced spin transfer torques from the Rashba spin current in heterostructures of permalloy (Py) and WSe2. We show that insertion of up to two monolayers of WSe2 enhances the spin transfer torques in a Rashba system by up to 3×, without changing the fieldlike Rashba spin−orbit torque (SOT), a measure of interface polarization. Our results indicate that low layer count TMD films can be used as an interfacial “scattering promoter” in heterostructure interfaces without quenching the original polarization.
Full text available from ACS Publications
Sieun and Nguyen both gave virtual talks at ACerS Electronic Materials and Applications (EMA) last week. Congratulations! Their abstracts are included below.
Sieun’s Abstract: Rutile GeO2: an ultra-wide-band-gap semiconductor for power electronics. Ultra-wide-band-gap (UWBG) semiconductors have tantalizing advantages for power electronics as their wider band gaps enable higher breakdown voltages. A handful of materials such as AlN/AlGaN, -Ga2O3, and diamond have been developed for UWBG semiconducting devices, however, they are still facing numerous challenges, such as doping asymmetry and/or inefficient thermal conduction. In our work, we have identified rutile GeO2 (r-GeO2) to be a promising, yet unexplored UWBG (4.68 eV) semiconductor. Our first-principles calculations predict shallow ionization energies for donors such as Sb, As, and F, a phonon-limited electron mobility of 289 cm2 V-1s-1, and a breakdown electric field of 7.0 MV cm-1, which lead to a higher Baliga figure of merit than
-Ga2O3. We also predicted that p-type doping is promising in r-GeO2: the calculated ionization energy for Al acceptors is 0.45 eV and the calculated phonon-limited hole mobility is 27 cm2 V-1s-1. r-GeO2 also has superior thermal conductivity (45 W m–1 K–1 (calculated) and 51 W m–1 K–1 (experiment)) relative to
-Ga2O3. Though the thin-film synthesis of r-GeO2 has remained challenging due to its highly metastable amorphous phase, we demonstrate the first synthesis of single crystalline epitaxial thin films of r-GeO2 on a sapphire substrate using ozone-assisted molecular beam epitaxy. Our work motivates further exploration of r-GeO2 as an alternative UWBG semiconductor that can overcome the limitations of the current state-of-the-art UWBG materials.
Nguyen’s abstract: Magnetic properties of thin single crystal Cr2O3 films. Magnetoelectric materials show potential for low-power spintronics via the electric field control of magnetization. Antiferromagnet Cr2O3 is a room temperature magnetoelectric yet the existence of twin domains in thin films grown on metallic electrodes leads to high leakage current and low dielectric breakdown fields. By using an isostructural epitaxial oxide electrode, V2O3, recent studies have shown the possible elimination of these twin domains. Dielectric properties of 200 nm thick films show improved performance, however, for next generation logic and memory the films must be scaled down. Here we present the electrical endurance and magnetic properties of very thin (30-60 nm) single crystal Cr2O3 films grown by pulsed laser deposition onto V2O3 buffered (0001) oriented Al2O3 substrates. A 60 nm single crystal thin film has bulk-like resistivity (1012 Ωcm) and significantly improved breakdown voltage (150-300 MV/m). From magnetometry of a Cr2O3/ferromagnet heterostructure, the blocking temperature is found to be at 285 K, higher than twinned films with similar or greater thickness in literature. Further, Second Harmonic Generation confirms bulk magnetoelectric order of our single crystal thin film at room temperature. These results indicate the importance of crystallinity to realize bulk like properties in very thin films at room temperature.
Abstract: For next-generation technology, magnetic systems are of interest due to the natural ability to store information and, through spin transport, propagate this information for logic functions. Controlling the magnetization state through currents has proven energy inefficient. Multiferroic thin-film heterostructures, combining ferroelectric and ferromagnetic orders, hold promise for energy efficient electronics. The electric field control of magnetic order is expected to reduce energy dissipation by 2–3 orders of magnitude relative to the current state-of-the-art. The coupling between electrical and magnetic orders in multiferroic and magnetoelectric thin-film heterostructures relies on interfacial coupling though magnetic exchange or mechanical strain and the correlation between domains in adjacent functional ferroic layers. We review the recent developments in electrical control of magnetism through artificial magnetoelectric heterostructures, domain imprint, emergent physics and device paradigms for magnetoelectric logic, neuromorphic devices, and hybrid magnetoelectric/spin-current-based applications. Finally, we conclude with a discussion of experiments that probe the crucial dynamics of the magnetoelectric switching and optical tuning of ferroelectric states towards all-optical control of magnetoelectric switching events.
Full Text available from Physical Sciences Reviews
Abstract: We present data for epitaxial thin films of the prototypical entropy-stabilized oxide (ESO), Mg0.2Ni0.2Co0.2Cu0.2Zn0.2O, that reveals a systematic trend in lattice parameter and properties as a function of substrate temperature during film growth with negligible changes in microstructure. A larger net Co valence in films grown at substrate temperatures below 350 °C results in a smaller lattice parameter, a smaller optical band gap, and stronger magnetic exchange bias. Observation of this phenomena suggests a complex interplay between thermodynamics and kinetics during ESO synthesis; specifically thermal history, oxygen chemical potential, and entropy. In addition to the compositional degrees of freedom available to ESO systems, subtle nuances in atomic structure at constant metallic element proportions can strongly influence properties, simultaneously complicating physical characterization and providing opportunities for property tuning and development.
Full text available from Physical Review Materials