Criteria for ferromagnetic–insulator–ferromagnetic tunneling

The Rowell criteria, commonly used to identify tunneling in magnetic tunnel junctions (MTJ), are scrutinized. While neither the exponential-thickness dependence of the conductivity nor fits of non-linear transport data are found to be reliable tunneling criteria, the temperature-dependent conductivity does remain a solid criterion. Based on experimental studies of the bias and temperature-dependent resistance and magnetoresistance of MTJs, with and without shorted barriers, a new set of criteria is formulated.     

Reversal modes in asymmetric Ni nanowires

We have investigated the evolution of the magnetization reversal mechanism in asymmetric Ni nanowires as a function of their geometry. Circular nanowires are found to reverse their magnetization by the propagation of a vortex domain wall, while in very asymmetric nanowires the reversal is driven by the propagation of a transverse domain wall. The effect of shape asymmetry of the wire on coercivity and remanence is also studied. Angular dependence of the remanence and coercivity is also addressed. Tailoring the magnetization reversal mechanism in asymmetric nanowires can be useful for magnetic logic and race-track memory, both of which are based on the displacement of magnetic domain walls. Finally, an alternative method to detect the presence of magnetic drops is proposed.     

How can humans, in air, hear sound generated underwater (and can goldfish hear their owners talking)?

The air/water interface at the top of a body of water is often treated from below as a pressure release boundary, which it closely matches. The small discrepancy in that match, however, is enough to enable humans in air to hear sounds generated underwater, which would not be possible across a pressure release boundary. A discussion of this phenomenon, designed for teaching purposes and using no more acoustics than would be contained in a first-year undergraduate syllabus in acoustics, leads to a discussion of whether goldfish can hear their owners speaking. The analysis is then used to illustrate the care needed when comparing sound levels in air and water, a process which continues to lead to erroneous statements in the media and some academic articles.     

Self focusing of acoustically excited Faraday ripples on a bubble wall

A theoretical explanation is presented to explain pattern formation during the generation of Faraday waves on a bubble wall. The theory derives the Hamiltonian formulation of the nonlinear bubble dynamics. The nonlinear Schrödinger equation for the envelope of surface modes on the bubble wall has been obtained. The solitary wave solution predicts that the shape distortions should be localized near the equator of the bubble.     

Packet routing and job-shop scheduling inO(congestion+dilation) steps

In this paper, we prove that there exists a schedule for routing any set of packets with edge-simple paths, on any network, inO(c+d) steps, wherec is the congestion of the paths in the network, andd is the length of the longest path. The result has applications to packet routing in parallel machines, network emulations, and job-shop scheduling.This research was conducted while the authors were at MIT. Support was provided by the Defense Advanced Research Projects Agency under Contract N00014-87-K-825, the Office of Naval Research under Contract N00014-86-K-0593, the Air Force under Contract OSR-86-0076, and the Army under Contract DAAL-03-86-K-0171. Tom Leighton is supported by an NSF Presidential Young Investigator Award with matching funds provided by IBM.     

Sign patterns of inverse-positive matrices

We identify the sign patterns which occur among the real, nonsingular, entrywise nonzero matrices whose inverses are entrywise positive.