[Show abstract][Hide abstract] ABSTRACT: Cells respond to mechanical forces whether applied externally or generated internally via the cytoskeleton. To study the cellular response to forces separately, we applied external forces to cells via microfabricated magnetic posts containing cobalt nanowires interspersed among an array of elastomeric posts, which acted as independent sensors to cellular traction forces. A magnetic field induced torque in the nanowires, which deflected the magnetic posts and imparted force to individual adhesions of cells attached to the array. Using this system, we examined the cellular reaction to applied forces and found that applying a step force led to an increase in local focal adhesion size at the site of application but not at nearby nonmagnetic posts. Focal adhesion recruitment was enhanced further when cells were subjected to multiple force actuations within the same time interval. Recording the traction forces in response to such force stimulation revealed two responses: a sudden loss in contractility that occurred within the first minute of stimulation or a gradual decay in contractility over several minutes. For both types of responses, the subcellular distribution of loss in traction forces was not confined to locations near the actuated micropost, nor uniformly across the whole cell, but instead occurred at discrete locations along the cell periphery. Together, these data reveal an important dynamic biological relationship between external and internal forces and demonstrate the utility of this microfabricated system to explore this interaction.
Proceedings of the National Academy of Sciences 10/2007; 104(37):14553-8. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Experiments are reported to characterize the viscous drag on Ni wires of diameter of 350 nm and lengths of 5 μm<L<30 μm confined to the air interface of glycerol/water mixtures upon which very thin (30–150 nm thick) silicone oil films are deposited. The sensitivity of the observed drag to the film viscosity demonstrates the utility of the wires as highly sensitive probes of interfacial shear rheology. The dependence of the drag on wire length is analyzed in terms of recent theoretical predictions for the hydrodynamic drag on an anisotropic particle confined to an interfacial film.
[Show abstract][Hide abstract] ABSTRACT: Microfabricated poly(dimethylsulfoxide) PDMS post arrays provide a method for mapping the distribution of contractile forces produced by adherent cells grown on the tips of the posts through optical tracking of the posts' deflections (1). We describe a new technique whereby large local mechanical stimuli can be applied to such cells via magnetic torques applied to anisotropic magnetic nanowires embedded in selected posts. These magnetic post arrays thus serve both as actuators and detectors of cellular contractile response. Experiments characterizing the performance of these post arrays will be presented along with results on global changes in cell contractility induced by magnetic forces applied to a single post under a cell. (1) J. Tan et al. PNAS 100, 1484 (2003).
[Show abstract][Hide abstract] ABSTRACT: Transport across a CrC2/Ag/YBCO interface was studied using a flip-chip configuration. The results were interpreted in the Andreev reflection scenario.
It is shown that the surface spinpolarization of CrO2 film, even after exposing to air, remained close to 100% to theT
c of YBa2Cu3O7−x
, a temperature limited by this method.
Journal of Superconductivity 08/2005; 18(4):499-502.
[Show abstract][Hide abstract] ABSTRACT: Epitaxial ruthenium oxide (RuO2) thin films have been grown on (100) TiO2 substrates by chemical vapor deposition at temperatures as low as 300 °C using tris(2,2,6,6-tetramethyl-3,5-heptanedionato)ruthenium [Ru(TMHD)3] as a precursor with oxygen carrier gas. These films exhibit low resistivity, with room-temperature values as low as ∼40 μΩ cm. The surface morphology, epitaxial strain and resistivity as a function of film thickness have been compared with those of similarly deposited epitaxial CrO2 films on TiO2. The temperature dependence of the resistivity for both set of films can be fit well using a combination of the Bloch-Gruneisen formula for electron–phonon scattering and additional scattering terms, including magnon scattering in the case of CrO2.
[Show abstract][Hide abstract] ABSTRACT: The detection of ferromagnetic Ni nanowires with integrated giant magnetoresistance sensors is investigated for possible use in magnetic biosensing. Single nanowires with magnetic moments at least as small as 1.5×10<sup>-10</sup> emu are readily detectable in a zero-applied field. The signal is a linear function of the number of nanowires, making the sensor suitable for use as a particle counter. With increasing wire length, the magnitude of the sensor signal increases at first and then appears to saturate. The signal shows significant dependence on the angle of the wire with respect to the sensing axis and changes sign when the wire's moment is antiparallel to the sensing direction.
IEEE Transactions on Magnetics 08/2004; · 1.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Epitaxial (1 0 0) thin films of CrO2 were fabricated by chemical vapour deposition technique onto TiO2 (1 0 0) single-crystal substrates. X-band (9.5 GHz) ferromagnetic resonance studies reveal that magnetic anisotropy parameters in the series prepared using CrO2Cl2 liquid precursor show only slight variations with thickness, contrary to their strong dependence observed in the CrO3-based films. The different behaviour was attributed to different morphologies of the films prepared using solid and liquid precursors.
Journal of Magnetism and Magnetic Materials 05/2004; · 2.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have applied photoexcitation by ultrashort laser pulses to single crystal thin CrO2 films to trigger coherent transient magnetization rotation on a subnanosecond time scale, in macroscale single domains. Moreover, by applying the photoexcitation by pairs of temporally separated pump pulses, the transient precession of the magnetization can be phase controlled, depending on the time separation between the pulses. The mechanism behind the photoexcitation originates from the modulation of the magnetocrystalline anisotropy by nonthermal hot electron spins.
[Show abstract][Hide abstract] ABSTRACT: We have fabricated single crystal chromium dioxide (CrO2) films using chemical vapor deposition with chromyl chloride (CrO2Cl2) as a liquid precursor. Their electrical and magnetic properties have been investigated. These films are atomically smooth with a rms roughness of less than 5 Å for 1000 Å thick films. We have obtained a spin polarization of P=98.4%, as determined by the technique of point-contact Andreev reflection. Magnetization and resistivity measurements on these films are in good agreement with those measured on films made with the CrO3 solid precursor. The process using the liquid precursor is superior to other existing techniques for the preparation of single crystal single- and multilayers containing CrO2, especially the structure of magnetic tunnel junctions.
Physical Review B 10/2001; 64(18). · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Chromium dioxide (CrO<sub>2</sub>) thin films of different
thicknesses (110-2600 Å) have been grown epitaxially on
(100)-oriented TiO<sub>2</sub> substrates by the chemical vapor
deposition technique. The thicker films, with a Curie temperature of 395
K, exhibit in plane magnetocrystalline anisotropy with the magnetic easy
axis along the c-axis direction. At low temperatures, the easy axis is
deflected from the c- toward the b-axis in the thickness range 1500-600
Å. For even smaller thicknesses, the c-axis and the magnetic easy
axis coincide again. However, at 300 K, the c-axis is always the
magnetic easy axis in every film studied. Resistivity measurement in the
temperature range of 4.2 to 420 K has also been performed. Temperature
dependence of the resistivity is highly anisotropic between the b- and
the c-axis, due to thickness induced anisotropic strain in the CrO<sub>2
IEEE Transactions on Magnetics 08/2001; · 1.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Andreev reflection at a Pb/CrO(2) point contact has been used to determine the spin polarization of single-crystal CrO(2) films made by chemical vapor deposition. The spin polarization is found to be 0.96 +/- 0.01, which confirms that CrO(2) is a half-metallic ferromagnet, as theoretically predicted.