A number of CMU faculty members explore the physics of nanostructured electronic and magnetic materials, both for application in electronic and/or magnetic devices, and to understand the properties of such structures, experimentally and theoretically. Fabrication methods include self-assembly, as occurs for quantum dots or epitaxial thin films, as well as photolithography and electron-beam lithography. The nanostructures are studied on the nm-scale using transmission electron microscopy, low-energy electron microscopy, atomic force microscopy and scanning tunneling microscopy. Additionally, measurements of electrical, magnetic, and optical properties elucidate the electronic states in the nanostructures and provide a basis for understanding their potential application in devices. Theoretical approaches include electronic band structure and total energy calculation as well as geometrical and topological analysis.
Several aspects of the physics of nanostructures differ from larger, bulk materials. Their electronic states are determined by quantum mechanics, and confinement in the nanostructures produces an increased energy separation between states. Transport through states will in general occur either in a diffusive or ballistic regime, and the small length scale in nanostructures tends to produce more ballistic transport. Surfaces and interfaces contribute strongly to nanostructure properties, leading to novel states such as “topological insulators”. In magnetic materials, large magnetocrystalline anisotropy is needed in order to reduce the domain size below that available in bulk materials. Thin magnetic multilayers are often used for this purpose, although again the role of interfaces (and surfaces) is not well understood. Another crucial issue is the means by which spins are flipped in these magnetic nanostructures using spin-polarized electrical currents and the spin torque effect.
In addition to the interesting fundamental properties, nanoelectronic and nanomagnetic materials hold considerable promise for applications. The increase in computing power made possible through shrinking device dimensions over the past decades is now reaching physical limitations, including lithographic precision, tunneling (leakage) through thin gate oxide layers, size of magnetic bits, etc. Nano-scale devices utilizing novel quantum materials may provide new means of overcoming such limitations. Extensive collaboration in these types of studies exists with faculty in the Engineering departments of Carnegie Mellon University. Also, facilities in the Carnegie Mellon Nanofabrication Facility and Data Storage Systems Center as well as in the Department of Materials Science & Engineering provide an excellent complement to the equipment within the Physics department used to pursue this work.
Jim Bain's interested in storage systems architectures and performance specifications, as they set the requirements for storage devices. Specifically, he is interested in how systems specifications set the performance requirements for recording heads - the transducers that must deliver energy to the recording medium and thus change its state in a reproducible way. Within information storage systems like hard disk drives, tape drives, etc., there are core technologies in thin film materials and devices that are used to fabricate heads and media. Prof Bain studies the fabrication of and characterization of these materials and devices. Examples of recent areas of activity include thin film FeCo alloys for high magnetization write poles, field emission assisted magnetic probe recording with a scanning tunneling microscopy, and nanostructured magnetic disk media using self-organized nanomasks.
Randall Feenstra's research deals with the nanoscale aspects of semiconductor surfaces and interfaces. Techniques such as scanning tunneling microscopy and low-energy electron microscopy are used for imaging surfaces and determining spectroscopic information about the electronic energy levels. Semiconductor surfaces for study are prepared in ultra-high vacuum using methods of cleaving or film deposition (molecular beam epitaxy). Heterostructures consisting of multiple layers of different types of material are also investigated, with the goal of understanding how the structure of the device (including imperfections and defects) determines its electronic properties. Recent studies have been performed on gallium nitride and epitaxial graphene.
Sara Majetich is interested in the fundamental physics of magnetic nanoparticles that have very uniform sizes, and in their possible applications in data storage media, high speed electronics, and biomedicine. Monodisperse nanoparticles coated with organic surfactants can self-assemble into ordered arrays. The collective magnetic behavior of the arrays has been studied using electron holography and Lorentz microscopy to image domains, and using polarized small angle neutron scattering to investigate the magnetic shell structure due to symmetry breaking at the particle surface. Monolayer arrays are also used as nanoscale templates for pattern transfer into thin films, which could be used to prepare arrays of magnetic devices for data storage media or microwave generation. Her group has recently demonstrated the ability of conductive atomic force microscopy to detect the state of a magnetic tunnel junction nanopillar, and switch it using a spin polarized current. They are also developing a method to control the motion of nanoparticles with magnetic tweezers that could later be used to probe within living cells.
Elias Towe pursues research in basic optical and quantum phenomena in materials for applications in novel photonic devices that enable a new generation of information processing systems for communication, computation, and sensing. His group is also interested in understanding new pathways and fundamental mechanisms for solar energy conversion devices. Current focus is on the use of phenomena (such as three-dimensional quantum-confinement effects in nanometer-scale structures) in the study of novel devices. Examples include: quantum-dot infrared detectors and imaging sensors, electrically-pumped photonic crystal micro-cavity lasers with quantum-dot active regions, multi-spectral solar energy conversion devices, plasmonic bio-sensors, and fluorescence bio-sensing devices.
Michael Widom's research utilizes first-principles electronic density functional theory calculations to model the structure and stability of materials. By combining quantum-based total energy calculations with classical statistical mechanics one can model the free energy of bulk phases and nanostructures in order to predict structures and properties of compounds as functions of chemical composition and temperature. Recent studies considered the structures of thin films at surfaces and grain boundary complexions, as well as nanocrystalline models of bulk metallic glass.
Di Xiao's research interests lie in quantum condensed matter theory. One major direction of his research is to understand and predict material properties (transport, magnetic, and optical) from the viewpoint of Berry phase and topology. In particular, he is interested in topological phenomena arising from spin-orbit coupling and many-body interactions. These phenomena are often characterized by novel electromagnetic responses, which may be useful for applications in quantum electronics and quantum computing.
Jimmy Zhu's research in the area of magnetic recording technology deals with (i) novel recording mechanisms that enables area storage density exceeding 1 Tbits/in2 for hard disk drive applications, and (ii) novel perpendicular thin film media microstructures. He is also interested in innovative designs of magnetic random access memory (MRAM). The research focuses on novel MRAM designs that offer robust and repeatable magnetic switching characteristic, low operation power capability, and sufficient thermal-magnetic stability. Micromagnetic modeling on computers is utilized to aid the design process and the devices are fabricated using the state-of-the-art e-beam and optical lithographic fabrication technology. His group is also working to understand noise in nano-magnetic systems, arising from thermally excited magnetization precession or spin current induced chaotic spin waves. Theoretical analysis and experimental measurements are performed in order to obtain a good understanding of the noise and the corresponding underlying physics.