Research - Daniel H. Reich
Some of my current research interests include:
Biological Applications of Magnetic Nanoparticles
Magnetic Nanoparticle Probes of Complex Fluids
Quantum Magnetism in Low-Dimensional Systems
In addition to the information on this page, further details can be found at the website of the JHU Materials Research Science and Engineering Center (MRSEC).
Biological Applications of Magnetic Nanoparticles
The integration of biology and the physical sciences at the nanoscale has the potential to revolutionize many areas of science and technology. The nanometer size scale is a crucial one in biology, as the dimensions of large biomolecules such as proteins and DNA, as well as those of many important sub-cellular structures, fall in this range. Recent advances in materials research have made it possible to engineer materials on these same nanometer lengthscales, and thus it is now possible to begin to design devices and artificial structures that can interact with cells and biomolecules in fundamentally new ways.
Magnetic microparticles and nanoparticles are playing an ever-increasingly important role in the study of soft matter, whether as probes of complex fluids, or for biophysical studies of living cells' responses to applied forces and stress. Recent advances are now enabling the design and fabrication of magnetic nanoparticles that can be tailored for a variety of specific applications. For example, we are developing new biological and soft matter probes using asymmetric, multisegment magnetic nanowires. These nanoparticles' multisegment architecture, high aspect ratio, and the ability to vary the aspect ratio and juxtaposition of dissimilar segments allows them to be given a wide range of magnetic, optical, and other physical properties. In addition, differences in the surface chemistry between segments can be exploited to selectively bind different ligands to those segments, giving the particles spatially resolved surface functionality. Our recent work includes: (i) magnetic carrier applications that exploit the nanowires to achieve large scale trapping of heterotypic cell pairs for cell-cell interaction studies, and (ii) experiments that probe the non-local contractile response of cells to local forces, using magnetic nanowire-based soft actuator microarrays.
Recent publications:
N. J. Sniadecki, C. M. Lamb, Y. Liu, C. S. Chen, and D. H. Reich, "Magnetic microposts for mechanical stimulation of biological cells: Fabrication, characterization, and analysis," Rev. Sci. Instrum. 79, 044302 (2008).
N. J. Sniadecki, A. Anguelouch, M T. Yang, C. M. Lamb, Z. Liu, S. B. Kirschner, Y. Liu, D. H. Reich, and C. S. Chen, "Maagnetic microposts as an approach to apply forces to living cells," PNAS 104 14553-14558 (2007).
A. Hultgren, M. Tanase, E. J. Felton, K. Bhadriraju, A. K. Salem, C. S. Chen, and D. H. Reich, "Optimization of Yield in Magnetic Cell Separations Using Magnetic Nanowires of Different Lengths," Biotech. Prog. 21, 509-515 (2005)
M. Tanase, E. J. Felton, D. S. Gray, A. Hultgren, C. S. Chen, and D. H. Reich, "Assembly of multicellular constructs and microarrays of cells using magnetic nanowires," Lab on a Chip 5, 569-696 (2005)
Magnetic Nanoparticle Probes of Complex Fluids
Fluids composed of complex molecules can display a variety of surprising and unusual properties that are never seen in ordinary fluids like water. We are using novel magnetic nanoparticles to probe the local dynamics and microrheology of complex fluids. Examples include wormlike micelles, where magnetic nanoparticle microrheology yields new insight into the non-linear response of these systems, studies of nanometer thin films and adsorbed protein monolayers at air-water interfaces, where the magnetic nanoparticle approach enables us to access features of these systems that are not readily probed by bulk methods, and liquid crystals where interactions of anisotropic particles with the nematic order parameter allow novel approaches to nanoparticle assembly.
More information on:
Magnetic nanoparticles in liquid crystals
Nonlinear microrheology of wormlike micelles.
Recent Publications
M.H. Lee, C. P. Lapointe, D. H. Reich, K. J. Stebe, and R. L. Leheny, "Interfacial Hydrodynamic Drag on Nanowires Embedded in Thin Oil Films and Protein Layers," Langmuir 25, 7976-7982 (2009).
C. Lapointe, D. H. Reich, and R. L. Leheny, "Manipulation and organization of ferromagnetic nanowires by patterned nematic liquid crystals," Langmuir 24, 11175-11181 (2008).
N. Cappallo, C. LaPointe, D. H. Reich, and R. L. Leheny, "Nonlinear microrheology of wormlike micelle solutions using ferromagnetic nanowire probes," Phys. Rev. E 76, 031505 (2007).
C. Lapointe, A. Hultgren, D. M. Silevitch, E. J. Felton, D. H. Reich, and R. L. Leheny, "Elastic Torque and the Levitation of Metal Wires by a Nematic Liquid Crystal," Science 303, 652 (2004).
Levitation of a nickel wire within a twisted nematic liquid crystal (LC). A: The LC orientation (green), twists uniformly as a function of height to match the directions of planar anchoring at the upper and lower surfaces. When a magnetic field aligns a nanowire parallel to LC at some height in the cell (e.g., at the center of the cell in the figure), the elastic energy becomes a function of the wire's height, leading to a levitating force on a wire resting on the bottom substrate. B: Measurements demonstrate the height of a wire initially resting at the bottom rises as a function of time in response to this force.
here is currently a strong interest in spin dependent transport in organic semiconductors (OSCs) for magnetoelectronic devices. There are also conflicting results and proposed mechanisms in such systems. To explore these issues, we are carrying out studies of organic spin valves.
First, we studied spin transport in four OSCs, including electron-carrying Alq3 and hole-carrying CuPc, that show substantial magnetoresistance (MR) effects up to room temperature, and electron-carrying PTCDA and CF3-NTCDI that show much weaker spin transport effects due to perhaps rougher interfaces. DSEcond, mcrostructural studies of Co/Alq3/Fe spin-valves using Auger electron spectroscopy, x-ray reflectometry and polarized neutron reflectometry (PNR) results show that larger MR effects are associated with smaller FM/Alq3 interface width (both chemical and magnetic) and a magnetically dead layer at the Alq3 /Fe interface. The PNR data also show that the Co layer on top of the Alq3, adopts a multidomain magnetic structure.
Our work demonstrates that it is possible to use transition metal FMs to inject spin-polarized current into OSCs at room temperature, if we can control the subtle differences in the microstructure of the samples, especially at the FM/Alq3 interfaces.
Recent Publications
Y. Liu, T. Lee, H. E. Katz, and D. H. Reich, "Effects of carrier mobility and morphology in organic semiconductor spin valves," J. Appl. Phys. 105, 07C708 (2009).
Y. Liu, S. M. Watson, T. Lee, J. M. Gorham, H. E. Katz, J. A. Borchers, H. D. Fairbrother, and D. H. Reich, "Correlation between microstructure and magnetotransport in organic semiconductor spin-valve structures," Phys. Rev. B 79, 075312 (2009).
Quantum Magnetism in Low-Dimensional Systems
Insulating magnetic systems are very clean systems in which to study fundamental questions in condensed matter physics. They generally have well-characterized microscopic interactions described by the Heisenberg model of localized, interacting spins on a lattice, and yet display a full panoply of complex many-body phenomena. In antiferromagnets with small spin (generally S=1/2 or S=1), anisotropies in the spin-spin coupling that render the system effectively one- or two-dimensional by decoupling it into an array of chains or layers, can enhance the destabilization of long-range order by quantum fluctuations, and lead to a variety of novel and unusual behaviors.
My research focuses on experimental studies of low-dimensional metallo-organic materials. These generally have spin-spin interaction energies of approximately 1 meV (J/K_B ~10 K), and so by applying magnetic fields H ~10 T, it is possible to introduce Zeeman energies into the system that are large compared to J. In many cases this enables one to radically change the nature of the ground state of the system, and for example, to probe a variety of quantum critical phenomena.
In one recent example, we have studied the magnetic field dependence of spin excitations in the S=1/2 linear chain antiferromagnet copper pyrazine nitrate (CuPzN). The S=1/2 antiferromagnetic spin chain is one of the most important model systems in many-body physics, displaying Luttinger liquid and quantum critical behavior. In a magnetic field it remains critical along a line in parameter space, a highly unusual situation for quantum critical systems. We have used inelastic neutron scattering to map out the full spectrum in the partially magnetized quantum critical state of CuPzN, and show for the first time several long-sought features of this state. [Phys. Rev. Lett. 91, 037205 (2003)]
Through measurements on other spin-1/2 chain systems, we have shown that in the presence of a staggered magnetic field, the excitations of the spin-1/2 chain coalesce into sharply defined bound states, and their spectrum acquires a gap, in quantitative agreement with predictions for novel excitations known as solitons and breathers, that are solutions of the quantum sine-Gordon field theory [ Phys. Rev. Lett. 93, 017204 (2004); Phys. Rev. B 71, 094411 (2005)].