October 11, 2006 Wednesday 4:00 PM
Meyer 6th Floor Conference Room
Bill Bailey
Columbia University
Control of Spin Relaxation in Ferromagnetic Ultrathin Films
Ferromagnetic moments M precess around applied fields H until they are aligned by relaxation. The spin relaxation process, occurring at intrinsic rate G (damping constant alpha), is formally similar to momentum relaxation in the electrical resistivity in metals, but is far more poorly understood. I will describe how G can be both raised and lowered in thin-film ferromagnetic materials, both epitaxial and polycrystalline, through impurity content. I will emphasize our recent discovery of a ferromagnetic alloy, Fe1-xVx, with lower relaxation rate (35 Mhz) than pure iron (57 Mhz), the elemental solid analogous in relaxation to copper in conduction. Finally, I will describe our development of ultrafast x-ray techniques to investigate phase lags between elemental moments in precession, proposed as the microscopic origin of relaxation.
October 18, 2006 Wednesday 4:00 PM
Meyer 6th Floor Conference Room
Dmitry Garanin
CUNY Lehman College
The Phonon Bottleneck Revisited
The problem of the phonon bottleneck in the relaxation of two-level systems (spins) to a narrow group of resonant phonons via emission-absorption processes is investigated from the first principles. It is shown that the kinetic approach based on the Pauli master equation is invalid because of the narrow distribution of the phonons exchanging their energy with the spins. This results in a long-memory effect that can be best taken into account by introducing an additional dynamical variable corresponding to the nondiagonal matrix elements responsible for spin-phonon correlation. The resulting system of dynamical equations describes the phonon-bottleneck plateau in the spin excitation, as well as a gap in the spin-phonon spectrum for any finite concentration of spins. On the other hand, it does not accurately render the line shape of emitted phonons and still needs improving.
October 25, 2006 Wednesday 4:00 PM
Meyer 6th Floor Conference Room
George Fytas
Max Planck Institut
Elastic Excitations and Phononic Band Gaps in Colloidal Crystals at Hypersonic Frequencies
Diffraction of photons by periodic structures can display frequency band gaps around Bragg resonances associated to the lattice constant where the propagation of light is forbidden. Soon after the birth of these photonic crystals with periodic variations of dielectric constant, theoretical and experimental work embarked on the propagation of acoustic waves in structures with periodic variations of density and/or sound velocities. The experimental realization of phononic crystals was up to now restricted to manually fabricated structures with macroscopic spacing and hence band gaps in the sub MHz frequency range. In contrast to the sonic and ultrasonic crystals, the study of hypersonic crystals at the submicron scale imposes substantial demand on fabrication and characterization techniques, which are currently being developed. The phononic properties of fabricated fcc colloidal crystals formed by self-assembly during vertical lifting deposition of spherical submicrometer particles e.g. polystyrene (diameter d=256nm and 307nm, polymethylmethacrylate (d=330nm)) with subsequent fluid infiltration (e.g. silicon oil and low molar mass polydimethylsiloxane) were investigated by high resolution Brillouin light scattering spectroscopy in the GHz frequency range. The dispersion relation between the frequency and the wave vector (k|| parallel to the fcc (111) plane) of the thermally excited acoustic waves has revealed two phononic band gaps: (i) a Bragg -gap occurring at the boundary of the first Brillouin zone and (ii) a hybridization-gap resulting from the interaction of particle eigenmodes with the acoustic mode of the effective medium. Crystallinity is a prerequisite for the appearance only of the Bragg-gap. Depending on the particle size and the speed of sound in the infiltrated fluid, the frequency and the width of the Bragg-gap can be tuned. The elastic parameters of the particles can be extracted from their vibration eigenfrequencies in the multipole resonance spectrum of the corresponding dry opal, whereas the particle size polydispersity is reflected in the line shape of the lowest-frequency mode. Since hypersonic crystals can simultaneously exhibit phononic and photonic band gaps in the visible spectral region, the technological applications could range from tunable filters and heat management to acoustic-optical devices.
November 1, 2006 Wednesday 4:00 PM
Meyer 6th Floor Conference Room
Mark Goulian
University of Pennsylvania
TBA
November 7, 2006 Tuesday 4:00 PM
Meyer 6th Floor Conference Room
Emil Yuzbashyan
Rutgers University
TBA
November 15, 2006 Thursday 4:00 PM
Meyer 6th Floor Conference Room
Hanna Salman
Rockefeller University
Response of Bacterial Culture to a Temperature Gradient
We study the response of bacterial culture in a one-dimensional temperature gradient. The bacteria accumulate near their natural temperature due to thermotaxis. The maximum of the bacterial density profile drifts to lower temperature with a velocity proportional to the initial concentration of bacteria (typical velocity 0.5 m/sec). At a high concentration of bacteria, above 108cells/cm3, a new mode develops from the initial density increase in the form of a sharp pulse moving at a faster velocity (~3.5 m/sec). This mode is a result of a positive feedback mechanism provided by inter-bacterial communication. The theoretical model shows good agreement with the experimental results.
November 29, 2006 Wednesday 4:00 PM
Meyer 6th Floor Conference Room
Richard Kiehl
University of Minnesota
TBA
December 13, 2006 Wednesday 4:00 PM
Meyer 6th Floor Conference Room
Bill Gelbart
University of California, Los Angeles
TBA
PAST EVENTS
October 5, 2006 Thursday 2:00 PM
Meyer 6th Floor Conference Room
Alex Lobkovsky
Massachusetts Institute of Technology
Catastrophe and Rescue: Stochastic Dynamics of Cracks in a Microtubule
October 4, 2006 Wednesday 4:00 PM
Meyer 6th Floor Conference Room
Aditi Mitra
New York University
Quantum Systems in a Nonequilibrium Steady State
September 27, 2006 Wednesday 4:00 PM
Meyer 6th Floor Conference Room
Eric Furst
University of Delaware
Lazer Tweezer Microrheology and Micromechanics of Colloidal Gels
At the heart of the current push in nanotechnology lies the promise that manipulating matter at the nanoscale will deliver unique functionality and utility for applications ranging from advanced photonic materials to new medical therapies. Our research focuses on the roles nanoscale structures and interactions play in determining the properties of soft materials and complex fluids, such as polymers, biopolymers and colloidal suspensions. These constitute a range of economically important materials, from consumer care products to specialty performance coatings, ceramics and biomimetic polymer scaffolds; therefore, understanding their underlying physical characteristics enables molecular and nanometer scale manipulation with the aim of engineering useful and novel properties. We study these materials using emerging experimental methods, including laser tweezer manipulation, microrheology and confocal microscopy.
To illustrate our approach, I will present unique experiments that provide a critical missing link between the macroscopic mechanical properties of colloidal gels and the particle nanoscale interfacial phenomena from which these properties ultimately originate. Gels form when colloidal particles aggregate to form an open, space-filling network. The elasticity, yield behavior and stability of materials made from colloidal gels depend on the mechanical strength of the gel microstructure; that is, the tendency of the network to deform and rupture under applied stresses. Using the micromanipulation and piconewton force sensing capabilities of laser tweezers, we have led the investigation of the properties of colloidal gels by developing methods to directly assemble model aggregates and measure their bending rigidity and failure mechanics. From this knowledge, we develop models that accurately describe the dependence of the gel elastic properties on solution conditions, and even find regimes where the boundary friction between particles determines the yield stress. These studies provide the insight necessary to control gel rheology by tailoring the chemical and nanometer scale properties of the colloidal particle surfaces.
August 30, 2006 Wednesday 2:00 PM
Meyer 6th Floor Seminar Room
Pedro Miguel Reis
City College of New York
The Venation Network in Leaves as Anticracks?
Thus far, existing models of venation in leaves are entirely biochemical, involving hormonal diffusive processes. These are, however, unable to capture some crucial structural features of vein networks such as the existence of reconnection loops. We propose a novel scenario for venation patterning which involves an interplay between stress induced mechanical instabilities in the plant tissue and its genetic properties. Our hypothesis is that ?anti-cracks? (regions of stress focusing under compression - a novel concept we are developing, guided by both experiments and theory) may be important in the vein patterning process. Inspired by physical experiments of fracture in drying gels and anticracks in compressed solid foams, we speculate that during the growth process the plant tissue may sensor such regions of localized stress, thereby leading to mechanical induced gene expression. At the interface of mechanics and genetics, this is an new and exciting conceptual framework for the study of plant tissue, which we are exploring in close collaboration with plant development biologists.
This work is funded by the European Union's NEST-ADVANTURE project MechPlant.
July 20, 2006 Thursday 2:30 PM
8th Floor SCM Conference Room
Eva Schotz
Princeton University
Germ Layer Formation in Vertebrate Embryos: A Viscoelastic Description in Terms of the Differential Adhesion Hypothesis
Gastrulation is the central process in embryonic development in which a previously unstructured cell conglomerate rearranges to form an embryo with a distinct head-to-tail, left-to-right and bottom-up morphology. During this process, large populations of cells have to rearrange to form the three germ layers ectoderm, mesoderm and endoderm.
Zebrafish is the model organism for studying vertrebrate gastrulation because embryos are transparent. In zebrafish, mesendodermal progenitor cells internalize at the onset of gastrulation and, once internalized, migrate as a sheet along the overlying ectodermal progenitor cells, but in the opposite direction. This implies that the adhesive contact between the two cell layers must be regulated in a highly dynamic manner so that the mesendodermal progenitor cells can not only compensate for the oppositely directed movement of the adjacent tissue, but create some net movement toward the animal pole of the gastrula.
A possible mechanism for processes like this has been formulated in the 'Differential Adhesion Hypothesis" (DAH), which proposes that cell motility behaviors are a consequence of cell sorting and that this cell sorting results from different adhesive properties of different cell types. The cell-cell cohesiveness as well as ?adhesiveness between different cell types is directly correlated to tissue surface and interfacial tensions.
I will present results from in vitro experiments, testing the validity of the DAH on Zebrafish embryonic tissues and extract relevant physical quantities. Furthermore, I will address the question of the relevance of these in vitro experiments regarding the developmental processes in vivo.
June 20, 2006 Tuesday 11:00 AM
Meyer 424B
Oliver Klein
CEA Orme des Merisiers
This presentation will review the advantage and disadvantage of using the Magnetic Resonance Force Microscopy technique towards the study of nanoelements. The main difficulty concerns the influence of the tip on the measurement [1]. For weak coupling, one can have a quantitative understanding [2] of the spectral distorsion induced by the stray field of the tip on the confinement of the spin-waves. Then MRFM offers the possibility to perform spectroscopy on individual nano-element. This will be illlustrated by a detailed characterization of the normal modes in submicron non-ellipsoidal element. The lowest energy mode is found to be a localized mode and not the uniform precession. Another advantage is that MRFM detects the dynamics of the longitudinal component of the magnetization [3]. This is important because it represents the energy strored in the spin system, which is proportional to the intrinsic relaxation rate. We will illustrate this with a recent study of the saturation regime in ferrite sample where surprisingly an increased linewidth coincides with a diminution of the relaxation rate [4].
[1] V. Charbois, V. V. Naletov, J. Ben Youssef, and O. Klein, Appl. Phys. Lett. 80, 4795 (2002).
[2] V. V. Naletov, V. Charbois, O. Klein, and C. Fermon, Appl. Phys. Lett. 83, 3132 (2003).
[3] O. Klein, V. Charbois, V. V. Naletov, and C. Fermon, Phys. Rev. B (Rapid Comm.) 67, 220407 (2003).
[4] G. de Loubens, V. V. Naletov, and O. Klein, Phys. Rev. B (Rapid Comm.) 71, 180411 (2005).
April 27, 2006 Thursday 2:00 PM
Meyer 8th Floor Conference Room
Mirjam Leunissen
University of Utrecht
TBA
April 13, 2006 Thursday 2:00 PM
Meyer 8th Floor Conference Room
Armand Ajdari
ESPCI
TBA
March 29, 2006 Wednesday 2:00 PM
Meyer 424B
Roberto Piazza
Politecnico di Milano
Particle Thermophoresis in Liquids
In the presence of a thermal gradient, macromolecular solutes or dispersed particles drift to the cold or to the hot side: this effect is known as thermophoresis, and is the counterpart in colloidal suspensions (?nanofluids?) of the Soret effect (or thermal diffusion) in simple fluid mixtures. Thermal diffusion plays a crucial role in many naturally occurring convective processes, from thermohaline convection in oceans to component segregation in volcanic lava and crystal growth. Thermophoresis of colloids or macromolecules is an even stronger effect: For instance, recent experiments have shown that biopolymer thermophoresis leads to convective patterns where the local macromolecular concentration may be strongly amplified, suggesting to exploit this effect as a new separation method. Yet, the microscopic mechanism underlying particle thermophoresis is still poorly understood;
Here I shall review recent experimental results on colloid thermophoresis obtained by our group on a large class of colloid and macromolecular systems, ranging from micellar solutions to polymers, proteins, latex particles, DNA. These results allow extracting some basic ideas about the microscopic nature of thermophoresis, and evaluating the influence of collective effects due to interparticle interactions. In addition. they suggest a universal nature for the temperature dependence of thermal diffusion in aqueous dispersions.
I shall also present a tentative general hydrodynamic model of particle thermophoresis in liquids, recently developed in collaboration with Alberto Parola.
Finally, I shall speculate about possible practical applications of thermophoresis as a separation method, in particular in relation to microfluidics.
REFERENCES
[1] R. Piazza and A. Guarino, /Soret effect in interacting micellar solutions/, Phys. Rev. Lett. *88*, 208302 (2002)
[2] S. Iacopini and R. Piazza, /Thermophoresis in protein solutions/, Europhys. Lett., 63, 247 (2003)
[3] R. Rusconi, L. Isa, and R. Piazza, /Thermal lensing measurement of particle thermophoresis in aqueous dispersions/, Journal of the Optical Society of America B, *21*, 605 (2004)
[4] R. Piazza, /Thermal forces: colloids in temperature gradients/, J. Phys: Cond.Matter *16*, S4195 (2004)
[5] A. Parola and R. Piazza, /Particle thermophoresis in liquids/, Eur. Phys. J. E 15, 255 (2004)
[6] S. Iacopini, R. Rusconi, and R. Piazza. /The ?Macromolecular Tourist?: Universal Temperature Dependence Of Thermal Diffusion In Aqueous Colloidal Suspensions/. Eur. Phys. J. E. *19*, 59 (2006)
March 24, 2006 Friday 11:00 AM
Meyer 424B
Oliver Waldman
University of Bern
Quantum Tunneling of the Neel Vector Antiferromagnetic Rings
March 20, 2006 Monday 4:00 PM
Meyer 433
Jerome Buerki
University of Arizona
Stability and Structural Dynamics of Metal Nanowires
I will present the nanoscale free-electron model, which provides a continuum description of metal nanostructures. I will argue that surface and quantum-size effects are the two dominant factors in the energetics of metal nanowires, and that much of the phenomenology of nanowire stability and structural dynamics can be understood based on the interplay of these two competing factors. In particular, I will discuss the exceptional linear stability of wires with certain magic conductance values, and the universal equilibrium shapes of metal nanowires, as revealed by non-linear dynamical simulations. I will conclude the talk with a brief discussion of the lifetimes of these metastable structures, including some experimental consequences.
March 3, 2006 Friday 11:00 AM
Meyer 424B
Corneliu Nistor
University of Texas
Magneto-optic Studies of Field-driven Spin Dynamics in Permalloy Thin Films and Nanostructures
February 28, 2006 Tuesday 2:30 PM
Meyer 424B
Luca Cipelletti
Universit?ontpellier de France
TBA
February 3, 2006 Friday 11:00 AM
Meyer 424B
Ari Mizel
Penn State University
Invasion of the Quantum Probe: Identifying Quantumness in a Nanostructure
With the recent surge of interest in quantum computation, it has become very important to develop clear experimental tests for ``quantum behavior'' in a system. This issue has been addressed in the past in the form of the inequalities due to Bell and those due to Leggett and Garg. These inequalities concern the results of ideal projective measurements, however, which are experimentally difficult to perform in many proposed qubit designs, especially in many solid state qubit systems. Here, we show that weak continuous measurements, which are often practical to implement experimentally, can yield particularly clear signatures of quantum coherence.
January 17, 2006 Tuesday 2:00 PM
Meyer 639
Remy Dreyfus
ESPCI
Magnetic Filaments and Low-Reynolds Swimming
The talk is devoted to chains of magnetic particles. First, I show how chains of magnetic particles allow us to measure forces between colloids. Then I describe the design and the fabrication of the first microscopic artificial swimmers. Flexible magnetic filaments are created by aligning magnetic particles under a magnetic field and gluing them together by means of PAA or DNA strands. Their behaviour under a magnetic field is investigated. Subjected to an oscillating magnetic field, the filaments experience deformations so that a wave propagates along them, thereby generating a propulsive force. Theoretical and numerical predictions are validated experimentally.
December 9, 2005 Friday 11:00 AM
Meyer 424B
Daniel Worledge
IBM Research Division
Spin Flop Switching for Magnetic RAM
Magnetic random access memory (MRAM) has the potential to be as fast as SRAM and as dense as DRAM, in addition to being non-volatile, making it a contender to be the future universal memory. However, research on MRAM has revealed two fundamental challenges: half select errors (how to write one bit without writing neighboring bits), and activation energy errors (how to prevent thermal energy from accidentally writing bits). Recently a new device, with two antiferromagnetically coupled magnetic free layers which are written by spin flop switching, has been proposed that has the potential to eliminate both of these problems. Using a single domain model, this talk will elucidate the magnetic behavior of this new device, including a discussion of the various types of easy axis hysteresis loops, the critical switching curve as a function of word and bit line fields, the role of thickness asymmetry, and the dependence of activation energy on applied field. In particular, it is shown that the field required to switch the bit under half select can be many times larger than the field required for full select, and furthermore that the activation energy initially increases under application of a half select field. Experimental data will be used to illustrate the operation of the device, with a special emphasis on optimizing the free layer material, spacer thickness, device size, and device shape, in order to reduce the operating point to below 50 Oe. Furthermore, high-speed pulsed measurements used to estimate the activation energy will be presented. Finally, a new idea for scaling involving the use of parallel coupling to reduce the switching field will be theoretically and experimentally discussed.
November 17, 2005 Thursday 11:00 AM
Meyer 424B
Luis Hueso
University of Cambridge
Half-Metallic Manganites, Magnetic Domain Walls and Carbon Nanotubes
I will discuss electrical transport between half-metallic manganite electrodes connected by native domain walls and, more exotically, carbon nanotubes. Manganites are mixed-valent oxides of manganese that display interesting physical properties such as ?ectronic?hase separation, and metallic phases with almost fully spin polarised conduction electrons. Native domain walls. In a phase separated thin film manganite device, magnetic domain walls can be created in ferromagnetic percolating pathways. The magnetoresistance of these devices was tested in different geometries, yielding qualitatively different results. Moreover, the changes in resistance-area product are large enough to suggest that the domain walls display mesoscopic phase separation at the wall centres. Manganite-carbon nanotube-manganite devices. Ferromagnetic metallic manganite electrodes connected by a carbon nanotube also display magnetoresistance. This demonstrates micron-scale spin coherence in the nanotube, and spin injection between an inorganic half-metallic crystal and the organic molecule.
June 23, 2005 Thursday 11:00 AM
Meyer 424B
Carl Patton
Colorado State University
Envelope Solitons in Magnetic Films
Solitons are ubiquitous to many physical systems. These include shallow and deep water waves, nonlinear electrical circuits, fiber optics, and Bose Einstein condensates. The nonlinear and dispersive spin wave properties in magnetic thin films and the ready availability of extremely low loss single crystal films of yttrium iron garnet (YIG) make this an ideal model system for the study of envelope solitons. This talk will present a comprehensive yet tutorial overview of work at Colorado State University on microwave magnetic envelope (MME) solitons in YIG film waveguides. Be prepared to think about wave packets, linear and nonlinear, in ways you probably have never thought about them before.
The talk will be organized (time permitting) as follows: I. Solitons and envelope solitons - what is a soliton; some history; some pictures. II. Soliton basics - nonlinearity; chirp and dispersion compensation; the nonlinear Schr?ger equation. III. Spin waves in magnetic thin films - the spin wave soliton experiment; YIG films; spin waves; mode flavors in thin films; the dispersion D and nonlinearity N in thin films. IV. Microwave magnetic envelope solitons - pulse driven bright solitons; higher order solitons, phase; thresholds; decay; dark solitons. V. YIG film feedback rings and other special effects - soliton trains; solitons as natural eigenmodes; cloning and trapping; modulational instability and fractals.
June 10, 2005 Friday 2:00 PM
Meyer 424B
Gregoire de Loubens
Saclay, CEA
FMR Spectrum of an Individual Spin-valve Device Obtained by FerroMagnetic Resonance Force Microscopy
The study of the dynamics of spintronics devices and in particular the influence of a spin polarized current on the emission spectrum of magnetic multilayers is of considerable interest [1-4]. I have used FerroMagnetic Resonance Force Microscopy to measure the coherent excitation spectrum of individual GMR devices. This technique [5-7] uses a magnetic probe fixed at the free end of a cantilever to measure the change in the static component of the dipolar field of the sample induced by the magnetization precession at resonance.
FMRFM is the only technique that has the sensitivity to detect the FMR signal of an individual nanopillar. I will present results obtained on Permalloy samples in presence of a spin polarized current. I will also show the dynamics modifications due to nonlinear effects that become dominant at large excitation power [8].
References :
[1] J.C. Slonczewski, J. Magn. Magn. Mater. 159, 1 (1996)
[2] L. Berger, Phys. Rev. B 54, 9353 (1996)
[3] X. Waintal, E. B. Myers, P.W. Brouwer, and D.C. Ralph, Phys. Rev. B 62, 12317 (2002)
[4] S.I. Kiselev, J.C. Sankey, I.N. Krikorotov, N.C. Emley, R.J.Schoelkopf, R.A. Buhrman, and D.C. Ralph, Nature 425, 380 (2003)
[5] Z. Zhang, P.C. Hammel, and P.E. Wigen, Appl. Phys. Lett. 68, 2005 (1996)
[6] M.M. Midzor, P.E. Wigen, D. Pelekhov, W. Chen, P.C. Hammel, and M. Roukes, J. Appl. Phys. 87, 6493 (2000)
[7] V. Charbois, V. V. Naletov, J. Ben Youssef, O. Klein, Appl. Phys. Lett. 80, 4795 (2002)
[8] G. de Loubens, V.V. Naletov et O. Klein, cond-mat/0405301, to be published in Phys. Rev. B (Rapid Comm.)
May 25, 2005 Wednesday 11:00 AM
Meyer 424B
Sara Majetich
Carnegie Mellon University
Magnetic Properties of Self-Assembled Nanoparticle Assemblies
We describe the preparation and properties of highly monodisperse iron, cobalt, and iron platinum nanoparticles, and nanostructures made from them. All of the surfactant-coated nanoparticles are prepared by high temperature solution chemistry methods. We examine two case studies that illustrate the potential of these particles for improving our understanding of nanomagnetism.
In the first, the magnetostatic interactions among monodisperse Fe nanoparticles are varied by changing the particle size, spacing, and degree of structural ordering. The length scale structural order is quantified by small angle x-ray scattering (SAXS), and the length scale of magnetic order is determined from small angle neutron scattering (SANS). These results are correlated with the macroscopic magnetic properties of the assemblies. We find that spin glass-like or mictomagnet behavior dominates when the length scale for structural ordering is less than 300 nm, or about 30 particle diameters.
In the second example we describe FePt nanoparticles that have been suggested for magnetic data storage media. A key advantage of using self-assembled nanoparticle arrays in data storage media would be the high degree of uniformity in the magnetic response. This depends on uniformity in the grain size and interparticle separation and the degree of crystallographic orientation. To be useful for storage media, the arrays must also have long-range structural order. It remains a challenge to prepare nanoparticles with all of these features simultaneously, but significant progress has been made in addressing these problems individually. We report our findings regarding the mechanism and size-dependence of the fcc to L10 phase transformation in FePt nanoparticles, and compare the results with the behavior in thin films.
