Gang Xiao - US grants

This Superconducting Quantum Interference Device Magnetometer enables state-of-the-art measurements of the magnetic properties of materials, under study in the Department of Physics, Department of Chemistry, and Division of Engineering at Brown University. The flux detection system and superconducting magnet are capable of measuring magnetization or susceptibility in a wide range of temperatures (1.8-400 Kelvins) and magnetic fields ranging from -5.5 to +5.5 Tesla, with high sensitivity, resolution, accuracy and stability. This instrument is particularly well suited to low magnetic fields and small samples. Its large dynamic range also accommodates strongly ferromagnetic samples in high magnetic fields.

This research deals with the physics of novel superconducting microstructures: uniform submicron particles, mesoscopic interfface structures, and magnetic flux coupled superlattices. The research employs novel vacuum deposition and micro-electronics fabrication methods. The research can lead to improved understanding of certain outstanding problems of high-Tc superconductors and the fabrication of microstructures which exhibit new quantum interference phenomena. Where appropriate, potential applications of the research to superconducting devices will be explored. The submicron particles will be used in experiments to measure the temperature dependence of the penetration depth anisotropy, critical field enhancement and their relation to conduction anisotropy, interlayer coupling, pairing symmetry, and finite size effects. An experiment is proposed to determine the quasiparticle energy spectra, and search for quantum interference effects in mesoscopic structures. The flux coupling structures will potentially provide DC transformers with multiple secondaries and large coupling ratios.

This Engineering Research Equipment Grant will be used by the Division of Engineering and the Center for Advanced Materials Research at Brown University to purchase components, instrumentation, and software in order to convert an existing field- emission scanning electron microscope for use as a direct-write electron-beam lithography tool. The e-beam lithographic capability will support several investigators pursuing individual research projects that represent a broad spectrum of materials, devices, physics, and technology, including fundamental electronic, magnetic, and optoelectronic properties of lower dimensional systems, II-VI and III-V semiconductor heterostructures and devices, ultra-fine magnetic particles, mesoscopic normal-metal superconductor structures, and Moire microscopy of layered microelectronic structures. The conversion project entails installation of beam-blanking hardware in the electron optical column of the microscope, enhancement of the microscope sample stage, construction of a Faraday cup sensor, and configuring the computer and controls for pattern generation and writing. Direct writing of feature sizes at and below 1000 A will be realized. By making use of existing electron optics, this project will provide a significant research capability to a multidisciplinary group in a timely and cost-effective fashion. Specific technological advances that are expected to accrue from the application of this resource include the development of coherent electron transistors, high-density magnetic recording media, optimized blue- green diode lasers, and a versatile materials testing and failure analysis system for microelectronics. These new technologies will emerge from ongoing substantive and comprehensive investigations of the various underlying physical phenomena of quantum confinement and transport, exciton-phonon interaction, and microstructural mechanics.

This is a study of magnetic properties of uniform ultrafine particles and their arrays fabricated by micro-electronics facilities. The objective is to understand outstanding problems such as, magnetic perpendicular anisotropy, magnetic internal structures, finite size effects, and macroscopic magnetic orderings, all are crucial to the potential applications of these novel structures.

9414160 Xiao The program will investigate electronic transport in mesoscopic structures composed of a normal metal (N) and a supercpnductor (S), two states with sharply contrasted electronic states. The objective is to develop scientific understanding of the new phase- coherent electronic transport expected to occur in such N-S structures, and to test, for the first time, recently developed theories which explain quantum transport in such structures. The PI will carry innovative design, fabrication, and transport measurements of three N-S structures: microjunctions, composite wires, and multiply connected structures. Important issues such as interplay between the two types of phase coherence in N and S, and its effects on the quantum transport will be invested. The research will either lead to novel quantum transport mechanism or will modify that in mono-component structures. % % % Mesoscopic structures are obtained when the size of metallic samples is reduced to submicron range. In such systems, the particle wave duality of the electrons plays an important role in the transport of electrical current, and a new less understood electronic mechanism emerges. This research will investigate the electronic transport in mesoscopic structures composed of a normal metal and a superconductor, which are two states with sharply contrasted electronic structures. The PI will investigate the new quantum electronic transport and provide test for new theories. The research includes innovative design and fabrication of microstructures, composites wire, and multiply connected normal- superconductor structures, and transport measurements. Research in this area will benefit the continuing process of device miniaturization and integration. ***

9503701 Xiao A packaged commercial dilution refrigerator will be acquired and its usage will be divided among four research projects requiring experiments at millikelvin temperatures. One project studies quantum phase- coherent transport in normal-superconducting metal mesoscopic structures. The new quantum transport phenomenom reveals itself in the low temperature range. A second project investigates semiconductor devices exhibiting quantum transport effects such as resonant tunneling, ballistic transport over macroscopic distances, and Coulomb blockade. Low temperature transport measurements will be performed on semiconductor nanostructures of adjustable dimensionality and lateral extent. A third project will use the dilution refrigerator to test ultra-sensitive millimeter-wave detectors developed to study the background radiation of the universe. To achieve the required sensitivity, these detectors have to be operated at millikelvin temperatures. The fourth project studies phonon propagation in layered compounds and the effects of disorder on the scattering of phonons in crystalline solids at very low temperatures. %%% The acquisition of a single instrument for experiments impacting a wide range of fields is very cost effective. Quantum transport phenomena play an important role in small structures that are the result of the ever- decreasing dimensions of microelectronic devices. ***

9701579 Xiao In this GOALI supported research program magnetic tunnel junctions (MTJ) will be prepared and studied with ultra-fast laser spectroscopy, near-field optics, and magneto- tunneling. The purpose is to study the fundamental and technological issues essential to the performance and competitiveness of the MTJ. The proposed work is closely linked to activities at IBM Research, Yorktown Heights. IBM will, where appropriate, pay for the support of students for work done at IBM Research. This activity is jointly supported by DMR and the OMA office of the MPS Directorate. %%% Magnetic tunnel junctions are devices which are actively pursued by industry as the new memory and sensor elements such as data storage reading heads and non-volatile magnetic random-access-memory (MRSM) cells. The scientific issues include gaining a more complete understanding of the magnetic behavior of these systems, especially while illuminated by light and the effect of reducing the device size. The GOALI supported work links expertise at the academic institution with activities at IBM Research, Yorktown Heights. IBM will, where appropriate, pay for the support of students for work done at IBM Research. This activity is jointly supported by DMR and the OMA office of the MPS Directorate. ***

9801862 XIAO This award will support five senior and five junior American scientists in condensed matter physics to visit Vietnam for two weeks for the purposes of 1) participating in a joint workshop on high temperature superconductivity, and 2) lecturing and visiting Vietnamese scientific facilities. The American organizer, Professor G. Xiao of Brown University, has selected a team of well-known senior American researchers. The Vietnamese organizer, Dr. Nguyen Van Hieu, is President of the National Center for Science and Technology and Director of the Institute of Materials Science. Through the interactions, which occur during this activity, the American scientists will gain an understanding of condensed matter physics in Vietnam and will form relationships that eventually may lead to collaborative research partnerships.

This individual investigator award is to a professor at Brown University for a project that will explore the physics of magnetoelectronic microstructures. For four decades, the operation of semiconductors has been based on the charge properties of electrons. Now, a new type of electronics- magnetoelectronics - has emerged, which utilizes both the electron spin and charge to enable devices performing novel functions. The objective of this project is to understand and solve some outstanding problems in magnetoelectronic microstructures. The research will focus on systems that are composed of a few magnetic domains. In particular the project will include investigations of magnetization reversal processes, and studies of the origins of magnetically induced electric noise and along with the development of methods to suppress this noise. This research will lead to a more comprehensive understanding of magnetism in microstructures. The results should contribute to the technological development of magnetoelectronics, and to the development of the human resources needed in this critical technology.
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This individual investigator award is to a professor at Brown University for a project that will explore the physics of magnetoelectronic microstructures. This is an important field for the high-tech industries in the new millenium. For four decades, the operation of semiconductors has been based on the charge properties of electrons. Now, a new type of electronics- magnetoelectronics - has emerged, which utilizes both the electron spin and charge to enable devices performing novel functions. Magnetoelectronic devices are increasingly used in areas such as computer information storage, magnetic sensing, communication, and automobiles. The objective of this project is to understand and solve some outstanding problems in magnetoelectronic microstructures. The research will focus on systems that are composed of a few magnetic domains. In particular the project will include investigations of magnetization reversal processes, and studies of the origins of magnetically induced electric noise and along with the development of methods to suppress this noise. This project will lead to a more comprehensive understanding of magnetism in microstructures. The results should contribute to the technological development of magnetoelectronics, which is a critical technology for our Nation's economy. The project will support graduate students, human resources needed for the high-tech industries.
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This Focused Research Group project involves two faculty members and several industrial collaborators who will study ultrafast, spin dependent processes that reflect nonequilibrium magnetization dynamics in ferromagnetic thin films and heterostructures on a picosecond time scale and below. A core question relates to the ultimate "speed limits" of magnetization reversal, which will be approached experimentally by employing all-optical, ultrashort pulse laser techniques. Unlike conventional approaches, which use pulsed magnetic fields to study magnetization switching in storage media, the physics in this research focuses on selective optical excitations of spins within the ordered magnetic medium, so as to modulate the exchange interaction and related electronic correlations by light in an nonthermal manner. In addition to studying optically activated magnetoelectronic processes in laterally uniform magnetic multilayers and exchange biased bi- and multilayers, the project includes the study the dynamics of collective micromagnetic effects in high density planar arrays where the individual submicron magnetic particles are coupled via dipolar (or possibly exchange bias) forces. Thin films of conventional transition metals (Co, NiFe) form the starting materials base for the project work, but a significant component of the research emphasizes selected transition metal oxides, most notably the half metallic ferromagnet CrO2. The research involves students and postdocs in cutting-edge fundamental research that has immediate relevance to current technology. The training prepares student for a variety of careers in academe, industry or government.
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The slowest part of a typical computer is the magnetic hard drive. While there are several steps involved in storing and retrieving data from the thin film disk medium, the process of encoding information into magnetically aligned atoms is reaching its practical limits of speed. In this project work we aim to use ultrashort laser pulses to influence the disk material's magnetic properties and to achieve the reversing the magnetic alignment of groups of atoms in as little as a few trillionth of a second-approximately a hundred times faster than the speed of the process in today's disk drives. The all-optical technique allows the team to investigate the fundamental interactions involved in such fast magnetic switching, and it may lead to extremely fast data storage devices in the future. One specific approach focuses on aiming the laser pulses at a sandwich of two magnetically coupled thin film magnetic films, whose collective interaction determines the overall magnetic properties of the bilayer which is efficient in resisting an externally applied magnetic field. By selectively absorbing the laser radiation at the interface, only a few atomic layers thick, the magnetic coupling between the two materials is abruptly interrupted, freeing one of the layers (the 'free' ferromagnet) to be rapidly reversed by an oppositely-directed static magnetic field, applied from the outside. While the concept could some day be used in fast data storage, the team will be using it mostly to study the basic processes of "flipping ultrasmall compass needles" at unprecedented speeds. Many physicists have studied the reversal of a single atom's magnetic moment, but the collective process of flipping the moments of many thousands of atoms at once is not well understood at a fundamental level. The research involves students and postdocs in cutting-edge fundamental research that has immediate relevance to current technology. The training prepares student for a variety of careers in academe, industry or government

We propose to investigate nanoscale magnetism and spintronics, in particular, systems of controlled self-assembled nanostructures. The objective is to investigate and solve outstanding physics problems that are both basic and essential to applications. We will develop a reliable fabrication process leading to excellent self-assembled nanostructures. We will explore magnetotransport in embedded nanocrystals lattices, in particular, the properties of magnetoresistance (MR) and extraordinary Hall effect (EHE). These systems may offer enhanced magnetotransport properties. The subject of magnetic interactions will be studied with the purposes of engineering magnetic switching fields and achieving large MR and EHE at low magnetic fields. These projects will lead to a more comprehensive understanding of properties of nanoscale magnetic systems. Nanoscale spintronics is important to the future competitiveness of the semiconductor industry in the United States. Our project will support graduate students. We will continue to recruit undergraduates and high school students to experience the wonder of the small world.
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We propose to investigate magnets and quantum spin dependent phenemena in nanometer sized magnets assembled in arrays through self-assembly processes and embedded in metals. In these new systems, many physical properties defy interpretations using traditional theoritical understanding. Our research is designed to foster better understanding of the physics involved and to develop a reliable fabrication process leading to excellent self-assembled nanostructures. The electrical properties of these embedded nanocrystals such as magnetoresistance (MR) and extraordinary Hall effect (EHE) will be measured. These systems are likely to offer enhanced magnetotransport properties. The research will benefit the high-tech industries to overcome roadblocks, which impede the advancement toward smaller, thinner, faster, and cost-effective devices. Our project will support graduate students. We will continue to recruit undergraduates and high school students to experience the wonder of the small world.
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This grant supports a state-of-the-art scanning probe microscope (SPM). Recent advances in scanning probe microscopy technology make possible the direct imaging of a diverse range of nano- and micro-scale materials, including biological macromolecules and micro-organisms in their native solution environment. This appropriately configured AFM will allow imaging of proteins and DNA molecules, the structure of microorganisms such as phages and live bacteria, nanopore single molecule mechanics, the morphology of nano-composites of superconducting and metallic granules, and magnetic domain structures.. The AFM is capable of force measurements as diverse as inter- and intra- molecular interactions and magnetic forces. Exploring such a broad range of applications using a workhorse SPM platform shall enhance the research productivity of all participant investigators, while in the meantime providing essential research training in scanning probe microscopy to a large number of undergraduate and graduate students at Brown. The shared and coordinated use of the multifunctional equipment will yield significant savings for all participants.

The acquisition of a multifunctional SPM facility will greatly enrich the training of a large number of student researchers over the next ten years, forming a cornerstone of their training in cutting edge materials science. Each year, NSF grants at Brown support the research of 10-12 graduate students and a comparable number of undergraduates. Many of these students come to Brown from under-represented groups including women, African Americans and Hispanics. In addition, the PIs have developed new biophysics and biomechanics courses and will incorporate the SPM in their associated laboratory portions. The PIs plan to offer training and research opportunities to local college and high school teachers in an effort to share the excitement of a cutting edge equipment facility with the broader community. The collaborative access to this multifunctional instrument shall likely facilitate new collaborations addressing a rich variety of scientific questions, and thus foster synergistic research activities and new research directions.

Non-technical:
This project will focus on the magnetic properties of the magnetic/electronic nanostructures most relevant to spintronics; an emerging field that employs the electron spin to enable new and unique functions in electronics. Our objective is to understand and solve outstanding physics problems that are both basic and essential to practical applications. We will explore the time dependence of magnetization reversal and the existence of intermediate switching modes, in a variety of nanostructures beneficial to theoretical modeling and application. We will also develop a spintronic metrology technique based on a nanoscale magnetic tunneling junction device. It is anticipated that our research will lead to both scientific understanding and potential applications. The research will benefit the high-tech industry by overcoming roadblocks that impede the evolution towards smaller, faster, and cost-effective devices. Nanoscale spintronics is important for the future competitiveness of the U.S. semiconductor industry. With the investment from NSF, we will train graduate and undergraduate students in the forefront areas of materials science.

Technical:
This project is expected to elucidate the micromagnetic properties of the magnetic/electronic nanostructures most relevant to spintronics, an emerging field that employs the spin degree of freedom to enable new and unique functions in electronics. Our goal is to explore the time dependence of magnetization reversal and the existence of intermediate switching modes, in a variety of nanostructures beneficial to theoretical modeling and application. We will also develop a spintronic metrology technique based on a nanoscale magnetic tunneling junction device that we will design and construct to possess high sensitivity, wide frequency response, and good thermal stability. It is anticipated that our research will lead to both scientific understanding in micromagnetics and potential applications. The research will benefit the high-tech industry by overcoming roadblocks that impede the evolution towards smaller, faster, and cost-effective devices. With the investment from NSF, we will train graduate and undergraduate students in the forefront areas of condensed matter and materials science.

Technical Abstract

This award supports experimental research and education in the field of spintronics. The project entails synthesis of epitaxial magnetic nanostructures for systematic control of the transport and energy spectra of spin-polarized current. The research will be directed toward the understanding of outstanding issues in spintronics, which are both fundamental and essential to applications. In particular, measurements of the single magnetic-domain-wall resistance in single crystal half-metals will be carried out as well as the development of nanostructures that will take advantage of the expected large domain-wall resistance. The project also aims at investigating nanostructures with periodic magnetic domain walls, and using itinerant spin currents to stimulate the modified spin-wave excitations arising from this periodicity. Experimental methods include variable-temperature magnetotransport studies, epitaxial chemical vapor deposition, magnetron sputtering, and submicron lithography. Students will be trained and they will acquire cutting edge skills in advanced experimental techniques and materials processing. There will be efforts to recruit students from underrepresented groups in science, including women and minorities, to participate in this research.

Non-technical Abstract

Semiconductor technology is facing severe challenges in miniaturization. Future electronics will likely rely on the fundamental attribute of electron spin. Spintronics concerns itself with several, equally important issues, related to highly spin-polarized solids and spin transport in microscopic systems. This individual investigator award supports a project to harnesses the electron's spin to create new electronics devices. In particular, measurements of the magnetic-domain-wall resistance in half-metals will be carried out as well as the development of nanostructures that will take advantage of the expected large domain-wall resistance. The project also aims at investigating nanostructures with periodic magnetic domain walls, and using itinerant spin currents to stimulate the modified spin-wave excitations arising from this periodicity. The primary impact of the activity will be the training of human resources. Students will be trained and they will acquire cutting edge skills in advanced experimental techniques and materials processing. There will be efforts to recruit students from underrepresented groups in science, including women and minorities, to participate in this research. This research will generate new knowledge and data on novel spintronic nanostructures. New spintronic devices will be invented and adapted to applications areas where existing solutions are inadequate.

This award supports the acquisition of a High Magnetic Field and Cryogen-Free Physical Property Measurement System (PPMS) at Brown University. It will enable experimental studies of the physical properties of materials including magnetism, electron transport, and thermodynamic properties. The instrument will provide fundamental information about new materials actively being studied by scientists from the disciplines of Physics, Chemistry, and Engineering. Proposed research using the PPMS includes: (1) spintronics based on half-metallic magnetic nanostructures, (2) magnetic nanoparticles for data storage, energy storage and biomedical applications, (3) low dimensional superconducting nanostructures, (4) type-II superconductors, (5) novel superconducting and spin liquid states, and (6) semiconductor hetero-nanowires. By studying physical properties over a wide range of temperatures and fields, scientists at Brown will uncover the quantum mechanical behavior and physical mechanisms underlying many novel and revolutionary materials. With this new instrument, they will have the ability to investigate the ground state properties of their materials directly, rather than rely on mere extrapolations of measurements conducted at room temperature or low magnetic fields. Since the PPMS provides a myriad of physical characterizations, they will be able to develop theoretical models that can then be almost immediately tested by multiple types of measurements and their correlations. The PPMS will open up new research opportunities for the materials research programs, and will help to reveal as yet unexplored physical phenomena while simultaneously producing devices of great practical significance. The proposed research not only will advance the development of material science, but also will lead to prototyping the next generations of spin-based electronics and semiconductor devices, thereby directly influencing the future development of the semiconductor industry.

This instrument will be used as an educational tool for graduate and undergraduate training. With this instrument, they will be able to teach students the fundamental properties of modern materials, develop a deeper understanding of physical mechanisms, and also provide invaluable practical experience in advanced characterization techniques. In particular, undergraduate students will use this sophisticated instrument for their research projects, thereby offering a unique opportunity for high-level investigations. These students will form the next generation of materials scientists in the United States, ensuring the development of the field and helping the US assume and maintain a preeminent role. The proposed research will also generate new basic knowledge and data on novel spintronic, superconducting, and semiconductor materials that can be leveraged to develop potentially revolutionary devices for widespread implementation in multiple arenas including information technology, medicine, and education.

***Technical Abstract***
This award supports experimental research in the field of spintronics. The project will be pursued on two fronts. First, spin-dependent transport in coherent structures possessing tuneable interface discontinuities will be studied using variable-temperature magnetotransport measurements to analyze the electrical and thermal properties of the interface regions and their influence on modified spin current in the material. Second, magnon spin-current coupling between ferromagnetic (FM) and magnetic insulators (FI) will be examined using the same modalities. Since FM/FI interface coupling relies on fundamentally different physics from FM/FM coupling, its study represents a unique experimental opportunity. All samples and devices will be fabricated using epitaxial chemical vapor deposition, magnetron sputtering, and submicron lithography techniques. The proposed research not only will advance the theoretical development of spintronics, but also will serve to prototype the next generations of spin-based electronics. The project will provide students with opportunities to experience advanced spintronics research, and generate new basic knowledge on novel spintronic nanostructures that can be leveraged to develop potentially revolutionary devices.

****Non-Technical Abstract****
This award supports experimental research in the field of spintronics. The project seeks to exploit the unique aspects of the electron's spin to explore novel physical phenomena and to generate potentially revolutionary advancements in nanotechnology and solid-state electronic device design. The proposed research not only will advance the theoretical development of spintronics, but also will serve to prototype the next generations of spin-based electronics. The project will directly affect the local community via educational outreach activities targeted at women and minorities at the high- and middle-school levels, providing individuals from these groups opportunities to experience scientific research first-hand. These individuals and graduate students that will be funded under this proposal will form the next generation of spintronics researchers in the United States, The project will generate new basic knowledge on novel spintronic nanostructures that can be leveraged to develop potentially revolutionary devices for widespread implementation in multiple arenas including information technology, medicine, and education.

Nontechnical Abstract

This award supports experimental research and education to advance basic knowledge in spintronics and magnetism. Spintronics uses electron spin to construct highly performing electronic devices beyond the conventional semiconductor chips. The magnetic entities or particles that the research team investigates are called magnetic skyrmions, tiny magnetic swirls that arise in two-dimensional materials that can be used for computing and information storage. Through the research efforts, the team can effectively control the motion of skyrmions and construct skyrmionic devices, thereby providing a research platform to train the next generation of scientists in spintronics. The principal investigator (PI) plans to develop new fabrication processes and characterization techniques that could lead to revolutionary computing, security, and sensing devices. The PI uses the most sophisticated high resolution magnetic imaging and electronic measurement techniques to uncover new spin-based physical phenomena. The PI plans to train diverse groups of students in materials/devices processing and characterization. The research team strives to make a lasting contribution to the science education of the public and young people. Research on nanoscale physics and devices has positive impacts on our society in areas of computing, information storage and processing, quantum sensing, and medical diagnostics. Skyrmion-enabled devices consume low power or less expensive materials, which mitigates climate change. With great potential in discovery and invention, this research project furthers the United States’ competitive edge in the electronics industry and help maintain leadership in advanced research, manufacturing, and innovation.

Technical Abstract

The objective of this project is to understand the static and dynamic properties of magnetic skyrmions in magnetic multilayers, by exploring the interactions between skyrmions and various excitations including the magnetic field and spin current. The PI aims to achieve an understanding of the static, global and local dynamic behavior of skyrmions, both individually and collectively as clusters. The experimental approach utilizes state-of-the-art sample fabrication, submicron lithography, advanced imaging techniques and highly sensitive electronic measurements. The research team relies on micromagnetic simulations, electron magnetotransport theory, and condensed matter physics on spin-orbit coupling. The team plans to develop new device paradigms with advantages over existing designs in non-volatility of information, probabilistic computing, low power consumption, nanoscale scalability, ultrafast operation, and thermal stability. The project advances basic knowledge in spintronics and physics of topological magnetism. The team plans to develop understanding about single-skyrmion behavior, skyrmion-skyrmion interactions and their interactions with external controls and local variations in the spatial energy landscape. Through the research efforts, the team can effectively control skyrmions and construct skyrmionic devices, providing a research platform to train the next generation of scientists in the critical technological area of spintronics. The PI plans to train and educate diverse groups of students in their acquisition of condensed matter physics knowledge and experimental skills in materials/devices processing and characterization.

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.