319 research outputs found
On the representation of the search region in multi-objective optimization
Given a finite set of feasible points of a multi-objective optimization
(MOO) problem, the search region corresponds to the part of the objective space
containing all the points that are not dominated by any point of , i.e. the
part of the objective space which may contain further nondominated points. In
this paper, we consider a representation of the search region by a set of tight
local upper bounds (in the minimization case) that can be derived from the
points of . Local upper bounds play an important role in methods for
generating or approximating the nondominated set of an MOO problem, yet few
works in the field of MOO address their efficient incremental determination. We
relate this issue to the state of the art in computational geometry and provide
several equivalent definitions of local upper bounds that are meaningful in
MOO. We discuss the complexity of this representation in arbitrary dimension,
which yields an improved upper bound on the number of solver calls in
epsilon-constraint-like methods to generate the nondominated set of a discrete
MOO problem. We analyze and enhance a first incremental approach which operates
by eliminating redundancies among local upper bounds. We also study some
properties of local upper bounds, especially concerning the issue of redundant
local upper bounds, that give rise to a new incremental approach which avoids
such redundancies. Finally, the complexities of the incremental approaches are
compared from the theoretical and empirical points of view.Comment: 27 pages, to appear in European Journal of Operational Researc
Magnetic domain wall propagation in a submicron spin-valve stripe: influence of the pinned layer
The propagation of a domain wall in a submicron ferromagnetic spin-valve
stripe is investigated using giant magnetoresistance. A notch in the stripe
efficiently traps an injected wall stopping the domain propagation. The authors
show that the magnetic field at which the wall is depinned displays a
stochastic nature. Moreover, the depinning statistics are significantly
different for head to head and tail-to-tail domain walls. This is attributed to
the dipolar field generated in the vicinity of the notch by the pinned layer of
the spin-valve
Light-induced magnetization reversal of high-anisotropy TbCo alloy films
Magnetization reversal using circularly polarized light provides a new way to
control magnetization without any external magnetic field and has the potential
to revolutionize magnetic data storage. However, in order to reach ultra-high
density data storage, high anisotropy media providing thermal stability are
needed. Here, we evidence all-optical magnetization switching for different
TbxCo1-x ferrimagnetic alloy composition and demonstrate all-optical switching
for films with anisotropy fields reaching 6 T corresponding to anisotropy
constants of 3x106 ergs/cm3. Optical magnetization switching is observed only
for alloys which compensation temperature can be reached through sample
heating
Wide range and tunable linear TMR sensor using two exchange pinned electrodes
A magnetic tunnel junction sensor is proposed, with both the detection and
the reference layers pinned by IrMn. Using the differences in the blocking
temperatures of the IrMn films with different thicknesses, crossed anisotropies
can be induced between the detection and the reference electrodes. The pinning
of the sensing electrode ensures a linear and reversible output. It also allows
tuning both the sensitivity and the linear range of the sensor. The authors
show that the sensitivity varies linearly with the ferromagnetic thickness of
the detection electrode. It is demonstrated that an increased thickness leads
to a rise of sensitivity and a reduction of the operating range
Fragmentation of magnetism in artificial kagome dipolar spin ice
Geometrical frustration in magnetic materials often gives rise to exotic,
low-temperature states of matter, like the ones observed in spin ices. Here we
report the imaging of the magnetic states of a thermally-active artificial
magnetic ice that reveal the fingerprints of a spin fragmentation process. This
fragmentation corresponds to a splitting of the magnetic degree of freedom into
two channels and is evidenced in both real and reciprocal space. Furthermore,
the internal organization of both channels is interpreted within the framework
of a hybrid spin-charge model that directly emerges from the parent spin model
of the kagome dipolar spin ice. Our experimental and theoretical results
provide insights into the physics of frustrated magnets and deepen our
understanding of emergent fields through the use of tailor-made magnetism.Comment: 9 pages, 5 figures. Published version available on the Nat. Comm. web
site:
http://www.nature.com/ncomms/2016/160513/ncomms11446/full/ncomms11446.htm
Magnetic SAW RFID Sensor Based on Love Wave for Detection of Magnetic Field and Temperature
Magnetic field measurement including a temperature compensation is essential
for a magnetic field sensor. This study investigates a magnetic surface
acoustic wave (MSAW) sensor in a reflective delay line configuration with two
acoustic propagation paths with and without magnetic field sensitive layer. The
delay in path with sensitive layer leads to magnetic field detection and the
one without enable temperature measurement and thus compensation for the first
path. The developed sensor is based on a ZnO/LiNbO Ycut (X-direction)
layered structure as Love wave platform. Love wave as a shear wave being more
favorable for magnetic detection. Co-Fe-B is considered as sensitive layer to
detect magnetic field changes and is deposited on the top of ZnO, but only on
one of the two paths. We combined an original configuration of connected IDTs
with a high electromechanical coupling coefficient (K) mode to improve the
signal amplitude. The achieved sensor exhibits a high temperature and magnetic
field sensitivity of -63 ppm/C and -781 ppm/mT, respectively. The
temperature compensation method for magnetic field measurement is demonstrated
using a differential measurement by subtracting the delay times obtained for
the two paths with and without the sensitive layer. Finally, The sensor
exhibited good repeatability at various temperatures. Moreover, the device
developed allows in addition to the multisensor functionality, the radio
frequency identification (RFID) which is necessary for the deployment of sensor
networks
Truncus arteriosus repair: Influence of techniques of right ventricular outflow tract reconstruction
AbstractFifty-six consecutive patients underwent total correction of truncus arteriosus. Median age at repair was 41 days, with a range of 2 days to 8 months. In 71% the operation was done in the first 2 months of life. Nine patients had complex forms of truncus and 11 patients had aortic insufficiency. The truncal aortic root was transected, which provides a clear exposure of the coronary ostia. The aorta was reconstructed by direct end-to-end anastomosis, and the truncal valve was preserved in every case. Several different techniques were used for pulmonary reconstruction, including three types of anatomic reconstruction of the pulmonary valve with a trisigmoid leaflet system and two types of nonanatomic reconstruction. The anatomic techniques included use of 33 Dacron valved conduits, eight homograft valved conduits, and one porcine aortic root bioprosthesis. The nonanatomic reconstructions included direct anastomosis to the right ventricle in nine patients and insertion of autologous pericardial valved conduits in five. The hospital mortality was 16% (9/56; 95% confidence limits, 2% to 30%). Multivariate analysis outlines two independent incremental risk factors for hospital death: nonanatomic pulmonary valve reconstruction techniques and age younger than 1 month. The hospital mortality was 7.1% in the group with anatomic pulmonary valve reconstruction versus 43% in the group with nonanatomic pulmonary valve reconstruction (p = 0.015). The hospital mortality was 5.7% in those older than 1 month versus 33% in those younger than 1 month of age (p = 0.04). There were two late deaths. The actuarial freedom from reoperation and angioplasty at 7 years was 100% for patients receiving pericardial conduits, 80% for those undergoing direct anastomosis, 77% for those receiving Dacron conduits, and only 43% for those receiving homografts (p = 0.02). In conclusion, anatomic reconstruction of the pulmonary valve seems important at the time of the operation, age younger than 1 month remains an incremental risk factor, and the truncal valve can be preserved. (J THORAC CARDIOVASC SURG 1996;111:849-56
- …