36,908 research outputs found
Domain wall propagation due to the synchronization with circularly polarized microwaves
Finding a new control parameter for magnetic domain wall (DW) motion in
magnetic nanostructures is important in general and in particular for the
spintronics applications. Here, we show that a circularly polarized magnetic
field (CPMF) at GHz frequency (microwave) can efficiently drive a DW to
propagate along a magnetic nanowire. Two motion modes are identified: rigid-DW
propagation at low frequency and oscillatory propagation at high frequency.
Moreover, DW motion under a CPMF is equivalent to the DW motion under a uniform
spin current in the current perpendicular to the plane magnetic configuration
proposed recently by Khvalkovskiy et al. [Phys. Rev. Lett. 102, 067206 (2009)],
and the CPMF frequency plays the role of the current
Effects of turbulent dust grain motion to interstellar chemistry
Theoretical studies have revealed that dust grains are usually moving fast
through the turbulent interstellar gas, which could have significant effects
upon interstellar chemistry by modifying grain accretion. This effect is
investigated in this work on the basis of numerical gas-grain chemical
modeling. Major features of the grain motion effect in the typical environment
of dark clouds (DC) can be summarised as follows: 1) decrease of gas-phase
(both neutral and ionic) abundances and increase of surface abundances by up to
2-3 orders of magnitude; 2) shifts of the existing chemical jumps to earlier
evolution ages for gas-phase species and to later ages for surface species by
factors of about ten; 3) a few exceptional cases in which some species turn out
to be insensitive to this effect and some other species can show opposite
behaviors too. These effects usually begin to emerge from a typical DC model
age of about 10^5 yr. The grain motion in a typical cold neutral medium (CNM)
can help overcome the Coulomb repulsive barrier to enable effective accretion
of cations onto positively charged grains. As a result, the grain motion
greatly enhances the abundances of some gas-phase and surface species by
factors up to 2-6 or more orders of magnitude in the CNM model. The grain
motion effect in a typical molecular cloud (MC) is intermediate between that of
the DC and CNM models, but with weaker strength. The grain motion is found to
be important to consider in chemical simulations of typical interstellar
medium.Comment: 20 pages, 10 figures and 2 table
Spin transfer torque enhancement in dual spin valve in the ballistic regime
The spin transfer torque in all-metal dual spin valve, in which two
antiparallelly aligned pinned ferromagnetic layers are on the two sides of a
free ferromagnetic layer with two thin nonmagnetic spacers in between, is
studied in the ballistic regime. It is argued that, similar to the results in
the diffusion regime, the spin transfer torque is dramatically enhanced in
comparison to that in a conventional spin valve although no spin accumulation
exists at the magnetic-nonmagnetic interfaces. Within the Slonczewski's
approach, an analytical expression of the torque on the free magnetic layer is
obtained, which may serve as a theoretical model for the micromagnetic
simulation of the spin dynamics in dual spin valve. Depending on the
orientation of free layer and the degree of electron polarization, the spin
transfer torque enhancement could be tens times. The general cases when
transmission and reflection probabilities of free layer are different from zero
or one are also numerically calculated.Comment: 8 pages, 5 figure
Euler equation of the optimal trajectory for the fastest magnetization reversal of nano-magnetic structures
Based on the modified Landau-Lifshitz-Gilbert equation for an arbitrary
Stoner particle under an external magnetic field and a spin-polarized electric
current, differential equations for the optimal reversal trajectory, along
which the magnetization reversal is the fastest one among all possible reversal
routes, are obtained. We show that this is a Euler-Lagrange problem with
constrains. The Euler equation of the optimal trajectory is useful in designing
a magnetic field pulse and/or a polarized electric current pulse in
magnetization reversal for two reasons. 1) It is straightforward to obtain the
solution of the Euler equation, at least numerically, for a given magnetic
nano-structure characterized by its magnetic anisotropy energy. 2) After
obtaining the optimal reversal trajectory for a given magnetic nano-structure,
finding a proper field/current pulse is an algebraic problem instead of the
original nonlinear differential equation
Domain wall propagation through spin wave emission
We theoretically study field-induced domain wall (DW) motion in an
electrically insulating ferromagnet with hard- and easy-axis anisotropies. DWs
can propagate along a dissipationless wire through spin wave emission locked
into the known soliton velocity at low fields. In the presence of damping, the
mode appears before the Walker breakdown field for strong out-of-plane magnetic
anisotropy, and the usual Walker rigid-body propagation mode becomes unstable
when the field is between the maximal-DW-speed field and Walker breakdown
field.Comment: 4 pages, 4 figure
Optimal time-dependent polarized current pattern for fast domain wall propagation in nanowires: Exact solutions for biaxial and uniaxial anisotropies
One of the important issues in nanomagnetism is to lower the current needed
for a technologically useful domain wall (DW) propagation speed. Based on the
modified Landau-Lifshitz-Gilbert (LLG) equation with both Slonczewski
spin-transfer torque and the field-like torque, we derive the optimal spin
current pattern for fast DW propagation along nanowires. Under such conditions,
the DW velocity in biaxial wires can be enhanced as much as ten times compared
to the velocities achieved in experiments so far. Moreover, the fast variation
of spin polarization can help DW depinning. Possible experimental realizations
are discussed.Comment: 4 pages, 1 figur
Mining Frequent Graph Patterns with Differential Privacy
Discovering frequent graph patterns in a graph database offers valuable
information in a variety of applications. However, if the graph dataset
contains sensitive data of individuals such as mobile phone-call graphs and
web-click graphs, releasing discovered frequent patterns may present a threat
to the privacy of individuals. {\em Differential privacy} has recently emerged
as the {\em de facto} standard for private data analysis due to its provable
privacy guarantee. In this paper we propose the first differentially private
algorithm for mining frequent graph patterns.
We first show that previous techniques on differentially private discovery of
frequent {\em itemsets} cannot apply in mining frequent graph patterns due to
the inherent complexity of handling structural information in graphs. We then
address this challenge by proposing a Markov Chain Monte Carlo (MCMC) sampling
based algorithm. Unlike previous work on frequent itemset mining, our
techniques do not rely on the output of a non-private mining algorithm.
Instead, we observe that both frequent graph pattern mining and the guarantee
of differential privacy can be unified into an MCMC sampling framework. In
addition, we establish the privacy and utility guarantee of our algorithm and
propose an efficient neighboring pattern counting technique as well.
Experimental results show that the proposed algorithm is able to output
frequent patterns with good precision
Microflow valve control system design
A design synthesis for a microflow control system is presented based on the interrogation of an analytical model, testing, and observation. The key issues relating to controlling a microflow using a variable geometry flow channel are explored through the implementation and testing of open and closed-loop control systems. The reliance of closed-loop systems on accurate flow measurement and the need for an open-loop strategy are covered. A valve and control system capable of accurately controlling flowrates between 0.09 and 400 ml/h and with a range of 900:1 is demonstrated
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