Research

Magnetics and Spintronics

"Spintronics" is an emerging set of technology utilizing the electron spin degree of freedom. Proposed devices and technologies include, but are not limited to, spin torque magnetic random access memory (ST-MRAM), domain wall storage and logic, next generation hard disk drives, radio frequency sensors, and magnetic nanoparticles for drug delivery and biomedical imaging. New devices are continually being proposed as new physics emerges from experimental measurements, especially as device sizes are reduced and we enter new physical regimes where confinement and interfacial effects become important.

Spin is an intrinsic property (technically speaking, an intrinsic form of angular momentum) that all elementary particles possess. Electrons are spin-1/2, which means, in practical terms, that each electron can exist in one of two possible states. When many electrons are together, such as in a solid, the electrons interact via electromagnetic fields. These interaction strengths determine the properties of a given material, for example how the electrons move when subjected to an external force and how the spins align. The electron spin is directly related to the electron magnetic moment, so spintronic technology generally refers to devices utilizing magnetism - usually ferromagnets are a major component of spintronic applications.

A ferromagnet (for example, iron) is a material in which the electrons have a net spin alignment - i.e. more electron spins have the same value than have a different value. The total magnetic moment is simply the spin imbalance times a factor that relates spin to magnetic moment. Some newly discovered physics describing the interaction between spin-polarized conduction electrons in a metal and the magnetic moment in a ferromagnet have brought exciting new possibilities for devices. How this interaction works - the governing parameters, the basic phenomenology - is what I study. I hope the new information gained from my research leads to improvements in data storage, logic, and other applicable arenas. Aside from applications, the experimental data generated from my research helps us understand some fundamental many-body properties of metals, electron transport, and the like.

My research will involve new, high-precision measurements on systems of interest to spintronics. Topic include:

1. Spin transport in heavy metals: Does spin current in strongly spin-orbit coupled metals like platinum obey the standard diffusion equations? If not, why? What are the dominant electron scattering mechanisms and sites? How are momentum scattering and spin scattering related in ultrathin films?
2. Origins of magnetic damping: Is magnetic damping fundamentally of the Gilbert or Landau-Lifshitz form? Are there microscopic mechanisms that dominate damping in a large ferromagnet? How does the form of damping change as you approach the Curie temperature? Is damping a purely local phenomenon? By studying confined spin excitations in microstructures, we can answer these basic questions.

1. Magnetodynamics of multiferroics: Can we control magnetic switching with electric voltages in multiferroic heterostructures, and over what timescales? Does ferroelectricity affect the ferromagnetic damping? What do the dynamic ferromagnetic modes look like?
2. Spin Hall Effect: Is the SHE any different at high frequency? Can we use the SHE to excite coherent magnetic dynamics in large structures, including phase-locking of nonlinear local oscillators? Phase locking and synching of nonlinear oscillators can be the basis for neural networks and pattern recognition systems.

1. Conversion of spin transfer to mechanical torque: Spin can be absorbed by the crystal lattice, and the change in angular momentum results in a mechanical torque. Can we activate MEMS amd NEMS devices with spin transfer? Can we use MEMS and NEMS devices to measure spin transport properties of materials?

1. Dilute magnetic semiconductors: Integration of spintronic logic and storage devices with conventional semiconductor technology may require magnetic semiconductors. Magnetic semiconductors are generally created by heavily doping a conventional semiconductor (e.g. GaAs) with a traditionally magnetic transition metal (e.g. Mn). How does growth and concentration affect the magnetic and transport properties? Can we optimize the properties? How do the electron bands transform when heavily doped with transition metals?
2. Spin Caloritronics: How do degrees of freedom associated with spin conduct heat? Thermal conduction can also be used to study interaction strengths between subsystems in a solid - including spin, electron momentum, and lattice momentum (phonons).
3. Emergent phenomena: Disorder introduced into a system is random, yet large-scale patterns often emerge from disorder - think about how conductivity emerges as a single parameter that describes the effect of disordered scattering sites in a solid. Does any such order, or otherwise simple physics, emerge from magnetic disorder, introduced defects, or controlled defects in artificial systems (including artificial spin ices, magnonic and photonic crystals)?

My research involves precision measurements over a range of materials parameters. This allows us to correlated microscopic structure with macroscopic properties - the main goal of condensed matter physics. The precision needed for next-generation experiments requires development of the proper instrumentation to study these systems, including fast-time MOKE systems, frequency-domain MOKE, and temperature-variable FMR. I am also interested in developing the proper theoretical and numerical modeling for data interpretation. Adding additional interaction terms based on newly-discovered effects to the equations of motion must be done properly, and results must be interpreted properly within that theoretical framework. Adding terms to large-scale numerical code can be tricky, but needs to be done so we can make good predictions.

Biologic/Molecular Sensors and Devices

My work at BU and with a biomedical device start-up has opened me to the possibility of applied research. Semiconductor nanowires, functionalized nanomagnets, and magnetic nanowires can be used to detect biomolecules for disease, toxin, and health markers. Such research involves device optimization and improvements based on complicated microelectrofluidic physics such as electrophoretic particle focusing, modifying Debye screening, and other effects. In many of these situations the physics is described for spherical, uniform, single-species particles, in a uniform environment, but needs to be extended for the disorder in particle size, shape, and concentration that appear in biological samples. In this sense, the research is towards a fundamental understanding of disorder, and how to model it (a general goal of condensed matter physics).

The Si-based biochemical sensors are I currently design are, effectively, field effect transistors controlled by binding of biological nanoparticles (“analytes,” e.g. proteins, virus fragments, disease markers) to sensing molecules (e.g. antibodies, DNA fragments) specific to a given analyte. The sensors consist of a thin semiconductor layer, coated in thin oxide, patterned into nanowires or other shapes upon which the sensing molecules are bound. Metal electrodes are deposited on the semiconductor to access conductance measurements. “Functionalizing,” or attaching the sensing molecules to the semiconductor, follows a well-established chemical procedure. Individual sensors can be refunctionalized with different sensing molecules if necessary. When the analyte binds to the sensor, it can change the configuration of the sensing molecule or simply can be brought close enough (within the Debye length) to the semiconductor to influence the semiconductor electrically. Binding then is measured as a change in conductance of the semiconductor nanowire. (The exact mechanism of conductance change, whether field-effect due to analytes, field effect due to sensor conformation change, strain-effect, or something else, is not well understood at present. My research will be able to address some of these more basic questions.)

My research will involve improvement of sensor performance by utilizing microfluidic and electrophoretic physics. Methods of increasing the Debye length, particle focusing, and generating electroosmotic flow through applications of external electric fields are well-known for spherical, uniform dielectric particles in standard ionic fluids. I will extend this research to complicated nanoparticle analytes (proteins, etc.) in biological fluids. This will involve correlating known analyte properties (e.g. size, charge) of the analytes with sensitivity, binding frequency, and external field profiles.

I anticipate additional biochemical metrology to be available through the same samples and measurements. In principle, configurational and charge-state changes in a protein, DNA strand, or other biomolecule due to environmental changes (pH, temperature, etc.) can be measured using these sensors. Because the semiconductor conductance is extremely sensitive to local charge distributions, any configurational changes in a bound protein will affect the measured conductance. Energy barriers between configurational states and average state lifetimes will then also be directly accessible through time and frequency domain measurements.

I also expect a large numerical component to this work, simulating entire device structures including the microfluidics with multiphysics software such as ComSol. Test sensors can be produced by collaborators, such as my colleagues at Boston University, or can be made using advanced fabrication facilities at nearby institutions. Because the measurements are all electrical, at low to moderate (DC to high kHz) frequency, these projects are straight-forward and suitable for undergraduates with limited graduate student and postdoc supervision. The students will learn about basic transport measurements, fluid dynamics, biological chemistry. Motivated students may also become involved in device fabrication, including design, cleanroom techniques, and thin film deposition. This broad array of practical education opportunities will serve students well for future science, research, and development careers in industry, as well as preparation for PhD studies. Further information in this research statement includes specifics on a subset of experiments that I plan on performing.

Some experiments and design considerations:

1. Dipole separation: can we enhance sensitivity by applying ac fields, thereby inducing electric dipole moments within a charged or dielectric biomolecule?
2. Focusing: We can focus uniform, spherical dielectric particles in ionic fluids with spatially-varying, ac electric fields. Can we do the same for complicated molecules? How does the optimal frequency depend on the molecule and its internal structure?
3. Thermal and other effects: Can we determine how biomolecular properties change as a function of temperature? As a function of pH or salt concentration?
4. Integrated simulations: These devices are complex, multiphysics structures that need to account for ionic microfluidics, molecular bonding, semiconductor transport, temperature. Can we develop multiphysics modules in ComSol, MatLab, Mathematica, or other languages to predict device performance?
5. New sensors based on different physics, including magnetism. I will leave this vague until patents have been filed or funding has been secured.

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