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.focal adhesions ͉ magnetic nanowires ͉ mechanotransduction ͉ microfabrication ͉ traction forces

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.

We present measurements of the magnitude of Néel “orange-peel” coupling due to interface roughness in a series of magnetic tunneling junction devices. Results from magnetometry and transport measurements are shown to be in good agreement with the theoretical model of Néel. In addition, we have used transmission electron microscopy to directly probe the sample interface roughness and obtain results consistent with the values obtained by magnetometry and transport methods.

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Experiments are reported to characterize the viscous drag on Ni wires of diameter of 350nm and lengths of 5μm<L<30μm confined to the air interface of glycerol/water mixtures upon which very thin (30–150nm 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.

We have applied photoexcitation by ultrashort laser pulses to single crystal thin CrO(2) 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.

The magnetic switching behavior of micron-size magnetic tunnel junctions has been studied in two-dimensional magnetic fields. By measuring junction resistance, we obtain information about the magnetization state of the free ferromagnetic layer. Magnetic properties of this layer are explored using the Stoner–Wohlfarth rotational model as a starting point. We use geometric parameters of the critical curves to obtain information about interlayer coupling and domain structure effects in the free layer.

Epitaxial chromium dioxide (CrO2) films have been grown using chemical vapor deposition on (100) TiO2 substrate with chromyl chloride (CrO2Cl2) as a liquid precursor. The films are extremely smooth (rms roughness less than 4.6 Å for a 1000-Å-thick film) and have the largest spin polarization (P=98.4%) yet observed, as determined by point contact Andreev reflection. Magnetization switching properties of the films are close to those of a single-domain particle. Preliminary results on the in situ growth of exchange-biased CrO2/Cr2O3 multilayers are also reported. Although a bias field is observed, it is much smaller in comparison with the coercivity of the CrO2 film.