The following list the seed research projects from 2017 to 2019.
THIN FILM CHROMIUM OXIDE PEROVSKITES
Riccardo Comin, Assistant Professor, Department of Physics
Transition metal oxides (TMOs) have found use in various technologies. There has been mounting interest to harness the spin-charge interplay of TMOs and engineer new data storage devices relying on all-electrical (current or field) writing operations.
The relationship between magnetic order and electronic transport in TM-based compounds suggest that ferromagnets (FM) are typically metals, while antiferromagnets (AFM) are insulators. Compounds as such these are on the verge between different magnetic and electronic ground states, an ideal platform to design materials that are highly tunable by external parameters such as doping or magnetic field. Further, in these materials, the realization of room-temperature metallic conduction with robust AFM order could pave the way to new oxide-based magnetic access memories for fast magnetization switching using spin-transfer torque.
We propose to explore focuses on a family of transition metal oxides with a rich phenomenology: chromium-based oxide perovskites (chromites). We propose to study the combined charge and spin response in Cr-based perovskite thin films in the multidimensional domain spanned by strain, dimensionality (thickness), and doping (chemical substitution or oxygen removal). The novelty of our work lies in the exploration of a relatively new class of materials, and the impact of the proposed work is in the synergistic feedback between synthesis and characterization of the charge and spin response to reveal the driving forces behind the complex phenomenology of chromites.
Zhu, Z.H., Strempfer, J., Rao, R.R., Occhialini, C.A., Pelliciari, J., Choi, Y., Kawaguchi, T., You, H., Mitchell, J.F., Shao-Horn, Y., and Comin, R. "Anomalous Antiferromagnetism in Metallic RuO2 Determined by Resonant X-ray Scattering." Physical Review Letters, 122(1): Article 017202, January 2019. <DOI: 10.1103/PhysRevLett.122.017202>
SYNTHESIS AND STUDY ON THE SPIN-CHARGE INTERACTION IN TOPOLOGICAL SEMIMETAL/FERROMAGNET HETEROSTRUCTURES
Luqiao Liu, Assistant Professor, Department of Electrical Engineering and Computer Science
The main focus of the proposed work will be to (1) develop the synthesis process which can seamlessly integrate topological semimetal thin film with ferromagnet electrode, and (2) study the mutual interaction between charge and spin at the topological semimetal/ferromagnet interface. For the first part of the proposed efforts, various growth techniques such as sputtering and molecular beam epitaxy will be employed and the obtained film stacks will be characterized. For the second part, nanoscale devices will be fabricated for the magneto-electrical transport measurement. It is expected that the successful implementation of the proposed topological semimetal/ferromagnet heterostructure could be used to reduce the energy consumption (by more than a factor of 100x) of magnetic random access memories (MRAM), which has been extensively studied as a promising beyond CMOS technology for replacing existing electronic memory and logic devices. In the meantime, through the proposed study, deeper understanding will be gained on the spin and charge transport properties at the topological material/ferromagnet interface, which can lay a solid physical ground for the future development of electronic systems such as topological quantum computer, where the mutual interaction between topological ordering and magnetic ordering plays important roles.
(2019) Spin-Pumping and Spin-to-Charge Conversion in Topological Dirac Semimetal/Ferromagnet Bilayer Structure
(2018) Room-temperature spin-orbit torque switching induced by a topological insulator
Finley, J., Lee, C.H., Huang, P.Y., and Liu, L.Q. "Spin-Orbit Torque Switching in a Nearly Compensated Heusler Ferrimagnet." Advanced Materials, 31(2): Article 1805361, January 2019. <DOI: 10.1002/adma.201805361>
BOTTLEBRUSH HYDROGELS AS TUNABLE TISSUE ENGINEERING SCAFFOLDS
Robert Macfarlane, Assistant Professor, Department of Materials Science and Engineering
Tissue engineering (TE) is a promising method to grow artificial tissues for biological and biomedical applications, typically implemented using a porous, flexible, and biocompatible scaffold for cells so that, upon growth and proliferation, they ultimately form a continuous three-dimensional biomaterial1,2. However, living cells and tissues are complex constructs, and synthesizing scaffolds that properly interact with them remains a challenge; scaffolds need to simultaneously be (1) biocompatible, (2) mechanically matched to the native tissue, (3) porous enough to allow for nutrient flow and tissue development, and (4) capable of presenting molecular signals that promote cell growth and viability. Therefore, while hydrogels are a promising tool for medicine and biology, several key limitations in these biomedical technologies can only be addressed via advances in the field of materials science.
Here, we will develop methods to synthesize new BBP architectures, crosslink them into gels, and characterize how different design variables affect the resulting gel physical, chemical, and mechanical properties. Our lab is uniquely suited to study these materials, as we possess the requisite polymer synthesis and characterization capabilities necessary, and have proven expertise in manipulating soft material structure at the nanoscale via controlled polymer synthetic strategies10. Additional support from other member of CMSE IRG II will aid in our characterization capabilities.
A LITHIUM SOLID-STATE MEMRISTOR - MODULATING INTERFACES AND DEFECTS FOR NOVEL LI-IONIC OPERATED MEMORY AND COMPUTING ARCHITECTURES
Jennifer L.M. Rupp, Assistant Professor, Department of Materials Science and Engineering
This project focus is on the research of lithium ionic carrier and defect kinetics in oxides to design material architectures and interfaces for novel "Li-operated memristors as alternative memory and non-binary computing architectures". The digital revolution relies on fast and efficient data collection, storage, and information transfer. Since the early days of computers, information is processed in logic elements that are built up from electronically controlled transistors based on binary states. However, further down-scaling of transistors will soon be prohibited by physical limits as well as their increasing power demand. Here, the use of ionically-controlled memristors – nanometersized and analog – could allow for the realization of highly functional, low-energy circuit elements operating on multiple resistance states and to encode information beyond binary.
Memristors are resistive elements whose structure is typically composed of a transition metal oxide thin film sandwiched between two metallic electrodes. The application of a sufficiently high electric field induces a non-volatile resistance change linked to locally induced redox processes in the oxide. Through varying the voltage amplitude and duration, several distinct resistance levels can be achieved in the memristor by formation of either conductive filaments with ionic carriers (mostly O2−, Ag+ or Cu2+) or their charge/defect accumulation at the oxide/electrode interfaces in the device. The fingerprint of a memristor is an hysteretic current-voltage profile, which depends on the magnitude, polarity and time range of the applied voltage to the metal/oxide/metal structure, defining the redistribution of ionic carriers and defects in the device. Key challenges to replace today`s electronic transistors by ionic memristors are the low retention of addressable resistance states (caused by e.g. unstable charge potentials at the interfaces) and lack in understanding of charge/mass transfer kinetics at high electric fields driving future switching times and energy consumption. Only limited defect chemistry and ion migration kinetics studies exist for oxides and interfaces under high electric fields; also the nature of switching ions (e.g. from host lattice vs. other mobile ions require attention).
The idea of this research is to design, fabricate and investigate Li-based memristors based on monolayers and heterostructures of Li-oxides with controllable space charge potentials at their interfaces. For this, as the first part of this research, we make and study Li-oxide films with variable capacitance, operation voltage range and Li-ionic transfer kinetics under high electric field strengths and probe their memristive function. We will then assess their Li space-charge potentials at the interfaces, structural stability, retention and defect formation as they form the backbone to the Li-heterostructure memristor concept in the second project part. Methods for thin film fabrication and for probing structure-defect-carrier properties of Li-oxides and their interfaces by ex-situ techniques and also novel in-operando electrochemistry/wavelength-dependent Raman spectroscopy are discussed. Innovative and interdisciplinary opportunities for collaboration with CMSE/MIT faculty are highlighted.
The innovation in this research will be in making the first Li-memristors/resistive switches based on heterostructures with systematically altered space charge potentials by alternations of width and extension of high Li-capacitive monolayers and low Li capacitive/fast conductive monolayers. The outcome of this research will produce systematic model experiments to understand role of Li/defect space charges for memristors and a new strategy to increase retention by number of Li heterostructure interfaces on resistive switching and potentially number of addressable states.