264 research outputs found
Magnetic anisotropies and magnetization reversal of the CoCrFeAl Heusler compound
Magnetic anisotropies and magnetization reversal properties of the epitaxial
Heusler compound CoCrFeAl (CCFA) deposited on Fe and Cr
buffer layers are studied. Both samples exhibit a growth-induced fourfold
anisotropy, and magnetization reversal occurs through the formation of stripy
domains or 90 degree domains. During rotational magnetometric scans the sample
deposited on Cr exhibits about 2 degree sharp peaks in the angular dependence
of the coercive field, which are oriented along the hard axis directions. These
peaks are a consequence of the specific domain structure appearing in this
particular measurement geometry. A corresponding feature in the sample
deposited on Fe is not observed.Comment: 11 pages, 7 figure
Huge quadratic magneto-optical Kerr effect and magnetization reversal in the CoFeSi Heusler compound
CoFeSi(100) films with L2 structure deposited onto MgO(100) were
studied exploiting both longitudinal (LMOKE) and quadratic (QMOKE)
magneto-optical Kerr effect. The films exhibit a huge QMOKE signal with a
maximum contribution of up to 30 mdeg, which is the largest QMOKE signal in
reflection that has been measured thus far. This large value is a fingerprint
of an exceptionally large spin-orbit coupling of second or higher order. The
CoFeSi(100) films exhibit a rather large coercivity of 350 and 70 Oe for
film thicknesses of 22 and 98 nm, respectively. Despite the fact that the films
are epitaxial, they do not provide an angular dependence of the anisotropy and
the remanence in excess of 1% and 2%, respectively
Brillouin light scattering study of CoCrFeAl and CoFeAl Heusler compounds
The thermal magnonic spectra of CoCrFeAl (CCFA) and
CoFeAl were investigated using Brillouin light scattering spectroscopy
(BLS). For CCFA, the exchange constant A (exchange stiffness D) is found to be
0.48 erg/cm (203 meV A), while for CoFeAl the corresponding values
of 1.55 erg/cm (370 meV A) were found. The observed asymmetry in the
BLS spectra between the Stokes and anti-Stokes frequencies was assigned to an
interplay between the asymmetrical profiles of hybridized Damon-Esbach and
perpendicular standing spin-wave modes, combined with the optical sensitivity
of the BLS signal to the upper side of the CCFA or CoFeAl film
Ion beam induced modification of exchange interaction and spin-orbit coupling in the CoFeSi Heusler compound
A CoFeSi (CFS) film with L2 structure was irradiated with different
fluences of 30 keV Ga ions. Structural modifications were subsequently
studied using the longitudinal (LMOKE) and quadratic (QMOKE) magneto-optical
Kerr effect. Both the coercivity and the LMOKE amplitude were found to show a
similar behavior upon irradiation: they are nearly constant up to ion fluences
of ion/cm, while they decrease with further
increasing fluences and finally vanish at a fluence of
ion/cm, when the sample becomes paramagnetic. However, contrary to this
behavior, the QMOKE signal nearly vanishes even for the smallest applied
fluence of ion/cm. We attribute this reduction of the
QMOKE signal to an irradiation-induced degeneration of second or higher order
spin-orbit coupling, which already happens at small fluences of 30 keV Ga
ions. On the other hand, the reduction of coercivity and LMOKE signal with high
ion fluences is probably caused by a reduction of the exchange interaction
within the film material
New Measurements of Fine-Scale CMB Polarization Power Spectra from CAPMAP at Both 40 and 90 GHz
We present new measurements of the cosmic microwave background (CMB)
polarization from the final season of the Cosmic Anisotropy Polarization MAPper
(CAPMAP). The data set was obtained in winter 2004-2005 with the 7 m antenna in
Crawford Hill, New Jersey, from 12 W-band (84-100 GHz) and 4 Q-band (36-45 GHz)
correlation polarimeters with 3.3' and 6.5' beamsizes, respectively. After
selection criteria were applied, 956 (939) hours of data survived for analysis
of W-band (Q-band) data. Two independent and complementary pipelines produced
results in excellent agreement with each other. A broad suite of null tests as
well as extensive simulations showed that systematic errors were minimal, and a
comparison of the W-band and Q-band sky maps revealed no contamination from
galactic foregrounds. We report the E-mode and B-mode power spectra in 7 bands
in the range 200 < l < 3000, extending the range of previous measurements to
higher l. The E-mode spectrum, which is detected at 11 sigma significance, is
in agreement with cosmological predictions and with previous work at other
frequencies and angular resolutions. The BB power spectrum provides one of the
best limits to date on B-mode power at 4.8 uK^2 (95% confidence).Comment: 19 pages, 17 figures, 2 tables, submitted to Ap
Stability of bulk Ba2YCu3O(7-x) in a variety of environments
Small bars of ceramic Ba2YCu3O(7-x) were fabricated and subjected to environments similar to those that might be encountered during some NASA missions. These conditions include ambient conditions, high humidity, vacuum, and high fluences of electrons and protrons. The normal state resistivity or critical current density (J sub c) were monitored during these tests to assess the stability of the material. When normal state resistivity is used as a criterion, the ambient stability of these samples was relatively good, exhibiting only a 2 percent degradation over a 3 month period. The humidity stability was shown to be very poor, and to be a steep function of temperature. Samples stored at 50 C for 40 min increased in normal state resistivity by four orders of magnitude. Kinetic analysis indicates that the degradation reaction is second order with water vapor concentration. It is suspected that humidity degradation also accounts for the ambient instability. The samples were stable to vacuum over a period of at least 3 months. Degradation of J sub c in a 1 MeV electron fluence of 9.7 x 10 to the 14th e(-)/sq cm was determined to be no more than about 2 percent. Degradation of J sub c in a 8.7 x 10 to the 14th p(+)/sq cm of 42 MeV protons was found to be grain size dependent. Samples with smaller grain size and initial J sub c of about 240 A/sq cm showed no degradation. while that with larger grain size and an initial J sub c of about 30 A/sq cm degraded to 37 percent of its original value
Instrumental and Analytic Methods for Bolometric Polarimetry
We discuss instrumental and analytic methods that have been developed for the
first generation of bolometric cosmic microwave background (CMB) polarimeters.
The design, characterization, and analysis of data obtained using Polarization
Sensitive Bolometers (PSBs) are described in detail. This is followed by a
brief study of the effect of various polarization modulation techniques on the
recovery of sky polarization from scanning polarimeter data. Having been
successfully implemented on the sub-orbital Boomerang experiment, PSBs are
currently operational in two terrestrial CMB polarization experiments (QUaD and
the Robinson Telescope). We investigate two approaches to the analysis of data
from these experiments, using realistic simulations of time ordered data to
illustrate the impact of instrumental effects on the fidelity of the recovered
polarization signal. We find that the analysis of difference time streams takes
full advantage of the high degree of common mode rejection afforded by the PSB
design. In addition to the observational efforts currently underway, this
discussion is directly applicable to the PSBs that constitute the polarized
capability of the Planck HFI instrument.Comment: 23 pages, 11 figures. for submission to A&
Are Devaney hairs fast escaping?
Beginning with Devaney, several authors have studied transcendental entire
functions for which every point in the escaping set can be connected to
infinity by a curve in the escaping set. Such curves are often called Devaney
hairs. We show that, in many cases, every point in such a curve, apart from
possibly a finite endpoint of the curve, belongs to the fast escaping set. We
also give an example of a Devaney hair which lies in a logarithmic tract of a
transcendental entire function and contains no fast escaping points.Comment: 22 pages, 1 figur
Miniature Low-Noise G-Band I-Q Receiver
Weather forecasting, hurricane tracking, and atmospheric science applications depend on humidity sounding of atmosphere. Current instruments provide these measurements from groundbased, airborne, and low Earth orbit (LEO) satellites by measuring radiometric temperature on the flanks of the 183-GHz water vapor line. Miniature, low-noise receivers have been designed that will enable these measurements from a geostationary, thinned array sounder, which is based on hundreds of low-noise receivers that convert the 180-GHz signal directly to baseband in-phase and in-quadrature signals for digitization and correlation. The developed receivers provide a noise temperature of 450 K from 165 to 183 GHz (NF = 4.1 dB), and have a mass of 3 g while consuming 24 mW of power. These are the most sensitive broadband I-Q receivers at this frequency range that operate at room temperature, and are significantly lower in mass and power consumption than previously reported receivers
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