Recently in Classical mechanics and electromagnetism Category

Magnetic moments don’t necessarily point in the same direction everywhere in a ferromagnet. More often, domains of different orientations coexist, separated by thin domain walls. Moving those walls with spin-polarized current is potentially a convenient way to write bits to magnetic random-access memory or to shuttle sequences of bits to and fro in three-dimensional memory devices. But such applications require that domain walls be moved quickly and with minimal current. Unfortunately, the materials best suited to yield such highly mobile domain walls are also the most susceptible to Walker breakdown, a turbulence-triggering instability that slows domain-wall speeds to a crawl. Now, researchers led by Gilles Gaudin and Ioan Mihai Miron of Spintec laboratory in Grenoble, France, have figured out a way around that problem. They crafted 500-nm-wide nanowires consisting of cobalt, the active ferromagnetic layer, sandwiched between platinum and aluminum oxide, as shown here. The resulting inversion asymmetry produces an out-of-plane electric field that gives rise to a fortuitous spin–orbit coupling: As electrons pass along a nanowire, their spins tend to tilt to one side, producing a magnetic torque that stabilizes the domain wall even at large current densities. Unconstrained by Walker breakdown, the domain walls reached speeds of up to 400 m/s, more than fast enough for memory applications. The researchers say they’ll now work toward achieving comparable speeds with less current. (I. M. Miron et al., Nat. Mater. 10, 419, 2011.)—Ashley G. Smart

Like their optical-tweezer cousins, magnetic tweezers have become a standard tool for stretching individual biological molecules to gain insight into their physical properties and behavior. Magnetic tweezers (and optical ones, too) can also apply torques to rotate or twist specimens. Last year, Sean Sun of the Johns Hopkins University and colleagues developed a technique to control and measure the applied torque with magnetic tweezers. In their approach a biomolecule, such as DNA, was connected to the middle of a 2-µm-long nanorod that was magnetically latched at one end to a superparamagnetic bead. In a dipole magnetic field, the bead held the nanorod and the top of the molecule in place while the experimenters introduced a controlled amount of twist by rotating the substrate bound to the molecule's other end. Nynke Dekker and her coworkers at the Delft University of Technology have now presented a new take on magnetic torque tweezers, this time using standard spherical beads. A nonmagnetic bead 1 µm in diameter served as a landmark on the 2.8-µm-diameter superparamagnetic bead to which it was attached (a third, nonmagnetic reference bead corrected for mechanical drift). The larger bead was tethered to a substrate by a single DNA molecule, and the beads were placed in a slightly asymmetric dipole magnetic field. Rotating the magnet twisted the DNA, and by monitoring the beads' orientation the team could extract the DNA's torsional stiffness and the torque. With their tweezer setup, Dekker and colleagues could document torque-induced twisting, buckling, and denaturing of DNA under a wide range of torques and stretching forces and could study protein–DNA interactions. (J. Lipfert et al., Nat. Meth., in press, doi:10.1038/nmeth.1520.)—Richard J. Fitzgerald

For thousands of years, children have delighted in hoop rolling. Certainly, most of them have not considered that the rings are subject to gravitational and inertial forces; in any case, the hoops are stiff enough that they maintain their circular form despite those forces. But what happens to a rolling hoop that’s not so stiff? John Bush of the MIT mathematics department, along with visiting student Pascal Raux and colleagues, has answered that question in a recent study of more general systems—rolling bands that may be wider than they are high. Bush and company’s work was both experimental and theoretical. In their experimental investigations they took pictures of a vinyl polysiloxane loop placed on the inner surface of a rotating drum. The figure shows how the form of a representative loop changes as the drum speed is increased; blue corresponds to low speeds; red, high. In their theoretical work, the investigators confirmed the intuitive idea that the rolling band deforms as the inertial or gravitational force overwhelms the internal stiffness force. Indeed, if either gravity or inertial effects are strong enough, the top of the band can make contact with the bottom; new forces then come into play and the team’s analysis is no longer valid. Rolling droplets, tumbling blood cells, and carbon nanotubes deformed by van der Waals forces, the authors note, all display similar shapes to the rolling ribbons; the dynamics of those varied systems may be elucidated by the relatively simple ribbon study. (P. S. Raux et al., Phys. Rev. Lett., in press.)—Steven K. Blau

In the late 1940s, Hendrik Casimir proposed that two perfectly conducting parallel plates should feel a feeble attractive force between them, due to the zero-point energy of the surrounding electromagnetic field and its dependence on the plates' positions. (See the article by Steve Lamoreaux, Physics Today, February 2007, page 40.) About a decade later, Evgeny Lifshitz and colleagues generalized Casimir's work to real conductors and dielectrics and found that the force persisted. In most cases the proposed force was still attractive, but for some configurations—a high-permittivity material and a low-permittivity material separated by a medium of intermediate permittivity—it could be repulsive. In fact, the repulsive Casimir-Lifshitz force is responsible for liquid helium's tendency to climb the walls of its container: The container repels the ambient vapor, and the liquid rises to fill the gap. Now, a group of researchers led by Harvard University's Federico Capasso have observed a repulsive Casimir-Lifshitz interaction between two solid objects, a silica surface and a 40-μm-diameter gold-coated sphere, immersed in bromobenzene. To monitor the force, they attached the sphere to an atomic force microscope cantilever and measured the cantilever's deflection using a light beam and a split-quadrant photodetector, as shown in the figure. A repulsive force of a few tens of piconewtons was measurable when the objects were brought within 40 nm of each other, and it increased as their separation decreased. The researchers suggest that the force they observed could levitate a solid within a liquid, which may lead to very low-friction sensors of force and torque. (J. N. Munday, F. Capasso, V. A. Parsegian, Nature 457, 170, 2009.) —Johanna L. Miller

The loops and folds that result when a sheet, tape, or wire crumples are of practical and theoretical interest. Engineers want to predict how structures deform under stress; physicists want to reduce diverse crumpling behavior to a few simple principles. Toward that second aim, Norbert Stoop, Falk Wittel, and Hans Herrmann of ETH Zürich have conducted an experimental study of one elementary system: a length of metal wire stuffed from two opposing directions into a cylindrical container so shallow that the crumpling is two-dimensional. At the start of each run, the wire spanned the container in a straight line. Two counterrotating drums then pushed more and more of the wire into the container until, having bent to form a loop, the wire touched the side. What happened next, the researchers found, depended on the wire's elasticity and on the friction between the wire and the container. When friction is high, the wire adopted near-symmetric looping patterns, which the researchers termed classical. When friction is low and the wires are stiff and springy (the researchers used steel), the wire adopted spiral patterns. Floppy, soft wires (solder) adopted messy, asymmetric patterns, which the researchers termed plastic. By adjusting the elasticity and friction in their experiment, the researchers could delineate the three regimes in a morphological phase diagram. And, as the figure shows, they could reproduce the three phases with a simple continuum model. The ETH team anticipates their phase diagram could prove useful in characterizing the packing of DNA inside viral capsids and other crumpling systems. (N. Stoop, F. K. Wittel, H. J. Herrmann, Phys. Rev. Lett., in press.) ― Charles Day