Evgeny Y. Tsymbal - US grants

The Materials Research Science and Engineering Center (MRSEC) at the University of Nebraska supports an interdisciplinary research program on Quantum and Spin Phenomena in Nanomagnetic Structures. The MRSEC includes faculty participants representing the departments of physics, mechanical engineering, chemistry, and the school of biological sciences The Center's research is organized into two interdisciplinary research groups (IRGs). IRG1, Nanomagnetism: Fundamental Interactions and Applications, is concerned with the study of exchange and magnetostatic interactions between particles or grains in nanostructures. IRG 2, Spin Polarization and Transmission at Nanocontacts and Interfaces, investigates spin polarization and transport through nanoscale magnetic contacts and at ferromagnetic/ferroelectric structures. The Center's research is aided by extensive collaborations with other universities, government and industrial laboratories that bring in over fifteen additional participants. The Center also maintains shared experimental facilities in support of its research efforts. Education outreach efforts include research experiences for teachers and for faculty-student teams from predominantly undergraduate institutions.

Participants in the Center currently include 16 senior investigators, 2 postdoctoral associates, 14 graduate students, 10 undergraduate students, and 2 support personnel. Professor David J. Sellmyer directs the MRSEC.

This theoretical project explores the fundamental physics of electronic, magnetic and
Transport properties of nanoscale magnetic junctions. The major emphasis is on spin-
Polarized electronic transport in the nanojunctions, in which a small amount of various
Metallic, semiconducting or insulating materials is placed as a contact between two
Ferromagnetic nanowires. First-principles density functional calculations will be
Performed to predict electronic, magnetic and transport chacteristics of these
Nanojunctions. Micromagnetic modeling will be used to understand the magnetic
Structure of nanocontacts and tight-binding models will be developed to tread disordered
Systems and to explore the influence of localized states on magnetotransport in magnetic
Tunnel junctions. These theoretical studies will guide experimental investigations of the
Nanoscale junction fabricated by state-of-the-art nanofabrication techniques and will
Complement the established experimental program at Nebraska. The research will
Facilitate the development of novel magnetoelectronic devices, which merge spin degrees
Of freedom into electronic nanotechnologies.
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This theoretical project explores the fundamental physics of electronic, magnetic and
Transport properties of nanoscale magnetic junctions. The major emphasis is on spin-
Polarized electronic transport in the nanojunctions. These theoretical studies will guide
Experimental investigations of the nanoscale junction fabricated by state-of-the-art
Nanofabrication techniques and will complement the established experimental program at
Nebraska.
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Proposal Number: EPS -1010674
Institution: University of Nebraska
Linked to: EPS-1010094
Institution: University of Puerto Rico
Proposal Title: Collaborative Research: Cyberinfrastructure-enabled
Computational Nanoscience for Energy Technologies

This EPSCoR Research Infrastructure Improvement (RII) Track-2 award builds a consortium of five universities and two computing centers in Nebraska (NE) and Puerto Rico (PR) for collaborative leadership in nanoscience for energy technologies. This consortium brings together the expertise and resources of both the jurisdictions to build a critical mass of computational materials scientists. The project will develop an adaptive cyberinfrastructure that provides access to local and national computing resources and provide research based education for postdoctoral fellows, graduate and undergraduate students in computational nanoscience. The consortium will enable new collaborative cutting-edge research in energy technologies, expand opportunities for research in four-year colleges, and increase the participation of underrepresented groups in STEM fields in both NE and PR.

Intellectual Merit
The research is focused on exploring new functional properties of novel nanomaterials for energy efficient electronics and the development of nanocatalysts for energy applications. Predictive computational modeling efforts will provide routes for testing new ideas and guidance for optimized physical experimentation. The consortium members will work collaboratively in a modern computational environment linked with national resources such as the Open Science Grid and TeraGrid. Computational materials researchers and software developers in NE and PR will collaborate to create an Open Source Code Library (OSCL) of electronic structure and quantum chemistry codes and facilitate the design of new materials and nanostructures for efficient energy applications.

Broader Impacts
The NE-PR consortium will expand and enhance the computational capabilities, networking possibilities for research and educational activities at the universities of NE and PR. The OSCL will serve as a repository of nanoscience codes which will benefit the computational nanoscience community. Overall, the consortium will bring together human resources and facilities of NE and PR across institutional, geographical and cultural boundaries and enable new collaborative cutting-edge research in energy technologies. This project will train a diverse, well-prepared, and internationally competent STEM workforce necessary to sustain the nation's competitive edge. The participation of Hispanic students in the cyber-workforce will be increased. The project includes a strong dissemination component aimed at engaging low-income, first generation pre-college students and teachers at underserved school districts, as well as raising the awareness of the general public about nanoscience.

****Nontechnical abstract****

Ferroic materials are characterized by switchable magnetic or electric polarizations which make them interesting for advanced technological applications. The Materials Research Science and Engineering Center (MRSEC) at the University of Nebraska-Lincoln supports an interdisciplinary research project named Polarization and Spin Phenomena in Nanoferroic Structures (P-SPINS). This project is centered on studies of new ferroic materials and structures at the nanoscale aimed at developing the fundamental understanding of their properties and related phenomena important for information processing and storage, energy harvesting, and advanced electronics. P-SPINS's education and outreach programs encourage gifted young people to pursue scientific careers, broaden the participation of underrepresented groups in science, and improve materials literacy among the general public.

****Technical abstract****
P-SPINS is organized into two interdisciplinary research groups (IRGs). IRG1 "Magnetoelectric Materials and Functional Interfaces" is focused on magnetoelectricity in complex functional heterostructures and its unconventional use beyond the realm of static equilibrium and linear response. This IRG synergistically explores dynamic strain-driven phase transitions in magnetoelectric bulk materials and thin films, voltage-controlled entropy changes, magnetoelectric heterostructures for ultra-low power devices with memory and logic functions, and electrical tuning of interface magnetic anisotropy and exchange bias. IRG2 "Polarization-Enabled Electronic Phenomena" exploits ferroelectric polarization as a state variable to realize new polarization-enabled electronic and transport properties of novel oxide, organic, and hybrid heterostructures. This IRG investigates ferroelectrically induced resistive switching effects, modulation of electronic confinement at the hybrid ferroelectric/semiconductor and organic interfaces, dipole ordering in molecular ferroelectric structures, and manipulation of polarization-enabled electronic properties. To address these challenges in materials research P-SPINS relies on interdisciplinary collaborations, extensive use of shared facilities, partnerships with national laboratories and international institutions and interactions with industrial companies to leverage the expected scientific innovations for potential technological advances. As an integral part of the center, P-SPINS maintains a portfolio of signature education and outreach activities designed to increase the number, quality, and diversity of individuals pursuing and succeeding at careers in materials science.

There is a critical need for new technologies as the semiconductor industry reaches the limits of how small a transistor can be made and how much power can be used in an increasingly small space. This project will meet this need through the development of novel memory and logic devices. Continual interaction between academia and the semiconductor industry will ensure in new semiconductor device concepts that lead to faster and better electronics that use significantly less energy than current approaches. These advances will exploit the unusual magnetic properties of magnetoelectrics, a special class of materials that tie together magnetism and voltage. An important aspect of the devices will be their nonvolatility, a feature that makes them prime candidates for use in the emerging Internet of Things. Nonvolatility refers to the property that once written, information can be recovered, even if electrical power has been absent for an extended period. An example of such a situation is the shutdown of a computer. A computer equipped with this type of "instant on" circuitry will restart to the exact state when power failed. Nonvolatility will also lead to energy savings by enabling electronics to operate longer on smaller batteries with less need for recharge. Reducing the energy cost of consumer electronics could also lead to some world-wide energy savings, as new less energy expensive electronics become available.

This project develops novel device concepts to greatly extend the practical limits of energy-efficient computation, focusing primarily on magnetoelectric materials, enabling interfacial magnetism to be reversibly switched by voltage. This approach to the writing of magnetic information via voltage will result in a significant reduction in energy consumption, while improving the computing speed of integrated circuit technologies. To enable electronic applications based on these devices to come to fruition, the new concepts must allow for miniaturization, inexpensive fabrication on a huge scale, and long working lifetimes. Just as for conventional electronic circuits, to ensure reliable operations, the new devices will be capable of operating repeatedly at well above room temperature. By exploiting more than just electrical charge in each device, these new devices will have more function than a simple transistor, which in turn, will present new opportunities for the development of circuit ideas that go beyond existing technologies ? ideas that will also be explored as this research program develops.

Nontechnical:
Ferroelectrics such as hafnium oxide have promising applications in data storage and memory processing. These materials have a spontaneous electrical polarization that can be switched by applying an electric field, akin to ferromagnetism. The PIs will study the fundamental properties of hafnium oxide and the performance of hafnium oxide-based devices. The ultimate aim is to develop ferroelectric memory devices with superior performance. Graduate and undergraduate students will benefit from education and training in the science and technology of ferroelectric materials and devices. Students from underrepresented groups will be provided valuable educational experiences that enrich their professional development. Outreach programs aim at K-12 students and Nebraska residents through open public events, such as the State Science Olympiad and Nanocamp. These activities will integrate research with community engagement.

Technical:
The discovery of ferroelectricity in HfO2 films has recently attracted enormous interest due to its best compatibility with CMOS technology among known ferroelectrics. Thin films of HfO2-based ferroelectric materials exhibit robust switchable polarization and low leakage, and thus have a huge potential for being used in nonvolatile memories and ferroelectric field-effect transistors with enhanced performance. To realize this potential in practice, systematic studies of the structure-property-device performance relationship in these materials are required. In contrast to conventional perovskite ferroelectrics, the mechanism of ferroelectricity in HfO2-based films is far from being understood. The microstructure and strain conditions likely control the formation of the ferroelectric phase in polycrystalline HfO2 films, but there is no clear method for stabilizing it in a monocrystalline film. This proposal aims at achieving a comprehensive understanding of how the interplay between the film microstructure and interfacial stress affects the stability of the ferroelectric phase, polarization reversal dynamics, electronic transport behavior, and the related device performance. This project studies (1) theory-driven fabrication of the HfO2-capacitor structures with controlled microstructure and electromechanical boundary conditions stabilizing the ferroelectric state; (2) the effect of film microstructure on the polarization reversal mechanism in the epitaxial, textured, and polycrystalline HfO2-based films; and (3) the polarization-controlled resistive switching in the HfO2-based ferroelectric tunnel junctions as a function of the ferroelectric barrier microstructure.

The effect of epitaxial strain on stabilizing the ferroelectric phase in single-crystalline orthorhombic and rhombohedral HfO2 films will be explored using theoretical modeling based on density functional theory and verified by electrical and structural characterization of the HfO2 films grown using pulsed laser deposition on the substrates and buffer layers. The time-bias-dependent switching behavior will be investigated as a function of the HfO2 films microstructure via direct observation of the domain structure evolution by piezoresponse force microscopy (PFM) and measurements of the device-level integrated transient currents by pulse testing methods. Local probe microscopy in conjunction with structural characterization methods will be used to relate the tunneling electroresistance effect to the polarization state stability and microstructure of the HfO2-based tunnel junctions. Thus, the intellectual merit of this project will be elucidation of the relationship between the polar phase stability, film microstructure and interface strain, clarification of the mechanism of polarization reversal, and demonstration and quantification of the resistive switching in the HfO2-based ferroelectric films and device structures.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.