We have fabricated several 50 omega characteristic impedance low-pass metal powder filters. The filters are made with bronze or copper metal powder with varying amounts of metal powder in a metal powder/epoxy mixture. Our goal is to make a filter with a characteristic impedance Z = 50 omega at frequencies up to 10 GHz. Using a 78% bronze powder/epoxy mixture in a suitable geometry, we achieved an impedance Z = 54 omega at 4.2 K, with a cutoff frequency fc approximately/= 0.3 GHz and an attenuation A = Vout/Vin=0.0001 (-80 dB) at 10 GHz. We also made several non-50 omega low-pass bronze powder filters with fc = 1 MHz and A = 0.0001 at 10 MHz. Fabrication details and performance data will be presented for both types of filter.

We experimentally demonstrate the use of a superconducting transmission line, shorted at both ends, to stabilize the operation of a tunable flux qubit. Using harmonic-oscillator stabilization and pulsed dc operation, we have observed Larmor oscillations with a single shot visibility of 90%. In another qubit, the visibility was 60% and there was no measurable visibility reduction after 35 ns.

Solving the stochastic Landau-Lifshitz equation numerically, we compute as a function of time $t$ the probability per unit time, ${P}_{s}(t)$, that a classical, single-domain magnetic particle with an easy uniaxial anisotropy and a collinear applied magnetic field will reverse its magnetization (``switch'') via thermal activation over the energy barrier. The ${P}_{s}(t)$ curves increase with $t$ for small $t$, achieving a maximum at some time ${\ensuremath{\tau}}_{P}$ before decaying exponentially with time constant ${\ensuremath{\tau}}_{D}$ at long time, as per the standard Neel-Brown picture. Both ${\ensuremath{\tau}}_{P}$ and ${\ensuremath{\tau}}_{D}$ increase (the latter exponentially) with increasing barrier height; ${\ensuremath{\tau}}_{P}$ grows logarithmically with ${\ensuremath{\tau}}_{D}$, consistent with a recent phenomenological ``energy-ladder'' model, and experiments on submicron-sized magnetic thin films.

We introduce a new Josephson junction circuit for which quantum operations are realized by low-bandwidth, nearly adiabatic magnetic-flux pulses. Coupling to the fundamental mode of a superconducting transmission line permits a stabilization of the rotation angle of the quantum operation against flux noise. A complete scheme for one-qubit rotations, and high-visibility Ramsey-fringe oscillations, is given. We show that high visibility depends on passing through a portal in the space of applied fluxes, where the width of the portal is proportional to the ramp-up rate of the flux pulse.

Time-resolved measurements of spin-transfer-induced (STI) magnetization reversal were made in current-perpendicular spin-valve nanomagnetic junctions subject to a pulsed current bias. These results can be understood within the framework of a Landau-Lifshitz-Gilbert equation that includes STI corrections and a Langevin random field for finite temperature. Comparison of these measurements with model calculations demonstrates that spin-transfer induced excitation is responsible for the observed magnetic reversal in these samples.

We report direct experimental evidence of an energy threshold for spin-transfer-induced magnetic excitations in Co/Cu/Co nanopillars from temperature dependent measurements of current-induced excitations. The current threshold of excitations decreases when the temperature is increased from 4.2 to 300 K. However, the product of the current threshold and the pillar resistance stays almost temperature independent up to a temperature of similar to150 K. This behavior supports the energy-threshold mechanism for spin-transfer-induced magnetic excitations.

The use of accelerator hardware for micromagnetics simulations is described, along with some initial results. The accelerator calculates the dipole interactions at 400 gigaflops, allowing large simulations to be performed with arbitrary geometries. Two research programs are highlighted, the simulation of a curved MRAM cell and the simulation of the write head in a computer disk drive.

We have obtained CCD photometric observations of the near‐contact binary AX Dra during two observing seasons from 2001 March through May and from 2003 April and May. From these measurements, the seasonal light curves of AX Dra for 2001 and 2003 were compiled, and a total of 13 new timings of minimum light were calculated. An analysis of the resulting O - C diagram reveals evidence for a periodic change with a semiamplitude of 0.0061 days and a period of 56.1 yr, but this conclusion depends entirely on the photographic plate estimates around 1960. Our light curves display the conventional O’Connell effect and small‐scale year‐to‐year light variability. If these light variations are produced by a stellar spot, a cool‐model spot on the secondary star satisfies both seasonal light curves quite well. The same representation applies to the only other published photoelectric light curves.

We present a method to measure the effective field contribution to spin-transfer-induced interactions between the magnetic layers in a bilayer nanostructure, which enables spin current effects to be distinguished from the usual charge-current-induced magnetic fields. This technique is demonstrated on submicron $\mathrm{Co}∕\mathrm{Cu}∕\mathrm{Co}$ nanopillars. The hysteresis loop of one of the magnetic layers in the bilayer is measured as a function of current while the direction of magnetization of the other layer is kept fixed, first in one direction and then in the opposite direction. These measurements show a current-dependent shift of the hysteresis loop which, based on the symmetry of the magnetic response, we associate with spin transfer. The observed loop shift with applied current at room temperature is reduced in measurements at $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We interpret these results both in terms of a spin-current-dependent effective activation barrier for magnetization reversal and a spin-current-dependent effective magnetic field. From data at $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, we estimate the magnitude of the spin-transfer-induced effective field to be $\ensuremath{\sim}1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{2}∕\mathrm{A}$, about a factor of 5 less than the spin torque.