Linear Defects Stabilize Magnetic Domain Walls
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leach out of the clay. Very low level radioactive clay could simply be buried.” Komarneni and co-researchers Naofumi Kozai, a visiting scientist from the Japan Atomic Energy Research Institute, and William J. Paulus, master’s degree recipient, now at General Motors Corporation, found that if the mica is only partially filled with radium at the time of disposal, then heating to above 100°C will lock the radium in place. Sodium-4 mica, the researchers found, is easily synthesized by heating kaolinite— a naturally occurring clay with an equal ratio of silicon and aluminum—with magnesium oxide and sodium fluoride to about 815°C. Sodium-4 mica could be used in conventional ion-exchange columns to remove radium from water, but would first need to be pelletized. To immobilize radium from mine or mill tailings, the researchers said that mixing the clay with the tailings is sufficient. They also suggested that the clay could be used to line ponds that receive radium-containing tailing water to prevent migration from the pond, or clay curtains can be placed around tailings to keep the radium inside.
Linear Defects Stabilize Magnetic Domain Walls Ultrathin cobalt films or multilayered structures, magnetized perpendicular to the thin-film planes, have greatly increased storage densities by solving the thermal “flips” (i.e., magnetic spin reversals) problem that leads to the loss of stored information. However, extension of this technique to storage densities larger than 1 Tbit in.-2 suffers from the roughness and mobility of the magnetic domain walls (DWs), which prohibits closer packing of the storage bits. In order to overcome this problem and achieve even higher storage density, researchers from Los Alamos National Laboratory and IBM T.J. Watson and Almaden Research Centers have utilized long-range strain fields caused by introduced linear defects. When ultrathin trilayers (sandwich structures) of Pt(3 nm, top)/Co(0.7 nm)/Pt(2 nm, bottom) are deposited, the domains exist in round shape with rugged walls. The same problem occurs in pattern nucleation sites. The velocity of DWs increases swiftly when the magnetic field is beyond Hc = 750 Oe (the velocity is >180 µm s-1 at H = 854 Oe).
Noting examples that column defects can strongly localize wandering vortex lines in high-temperature superconductors, the researchers installed a line defect in the cobalt layer by clamping the substrate to produce anisotropic tension during the cobalt deposition, and then releasing the sample. As reported in the March 22 issue of Nature, the introduced y-axis-invariant strain field ε(x) influences the behavior of domains through magnetoelastic coupling. First, it accommodates the walls along the linear defect. Second, by increasing the elasticity, it reduces the wall roughness. Finally, it decelerates the motion of DWs to a nearly full stop when the field is high enough. Polar magneto-optic Kerr microscopy images show that the conformance of the DWs to the defect line occurs even at a large distance (~300 µm). The closer the DWs come to the d
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