Resonant switching using spin valves

Using micromagnetics we demonstrate that the r.f. field produced by a spin valve can be used to reverse the magnetization in a magnetic nanoparticle. The r.f. field is generated using a current that specifically excites a uniform spin wave in the spin valve. This current is swept such that the chirped-frequency generated by the valve matches the angular dependent resonant frequency of the anisotropy-dominated magnetic nanoparticle, as a result of which the magnetization reversal occurs. The switching is fast, requires currents similar to those used in recent experiments with spin valves, and is stable with respect to small perturbations. This phenomenon can potentially be employed in magnetic information storage devices or recently discussed magnetic computing schemes

Dynamic magnetic response of infinite arrays of ferromagnetic particles

Recently developed techniques to find the eigenmodes of a ferromagnetic particle of arbitrary shape, as well as the absorption in the presence of an inhomogeneous radio-frequency field, are extended to treat infinite lattices of such particles. The method is applied to analyze the results of recent FMR experiments, and yields substantially good agreement between theory and experiment

Ultraviolet and visible photometry of asteroid (21) Lutetia using the Hubble Space Telescope

The asteroid (21) Lutetia is the target of a planned close encounter by the Rosetta spacecraft in July 2010. To prepare for that flyby, Lutetia has been extensively observed by a variety of astronomical facilities. We used the Hubble Space Telescope (HST) to determine the albedo of Lutetia over a wide wavelength range, extending from ~150 nm to ~700 nm. Using data from a variety of HST filters and a ground-based visible light spectrum, we employed synthetic photometry techniques to derive absolute fluxes for Lutetia. New results from ground-based measurements of Lutetia's size and shape were used to convert the absolute fluxes into albedos. We present our best model for the spectral energy distribution of Lutetia over the wavelength range 120-800 nm. There appears to be a steep drop in the albedo (by a factor of ~2) for wavelengths shorter than ~300 nm. Nevertheless, the far ultraviolet albedo of Lutetia (~10%) is considerably larger than that of typical C-chondrite material (~4%). The geometric albedo at 550 nm is 16.5 +/- 1%. Lutetia's reflectivity is not consistent with a metal-dominated surface at infrared or radar wavelengths, and its albedo at all wavelengths (UV-visibile-IR-radar) is larger than observed for typical primitive, chondritic material. We derive a relatively high FUV albedo of ~10%, a result that will be tested by observations with the Alice spectrograph during the Rosetta flyby of Lutetia in July 2010.Comment: 14 pages, 2 tables, 8 figure

The triaxial ellipsoid dimensions, rotational pole, and bulk density of ESA Rosetta target asteroid (21) Lutetia

We seek the best size estimates of the asteroid (21) Lutetia, the direction of its spin axis, and its bulk density, assuming its shape is well described by a smooth featureless triaxial ellipsoid, and to evaluate the deviations from this assumption. Methods. We derive these quantities from the outlines of the asteroid in 307 images of its resolved apparent disk obtained with adaptive optics (AO) at Keck II and VLT, and combine these with recent mass determinations to estimate a bulk density. Our best triaxial ellipsoid diameters for Lutetia, based on our AO images alone, are a x b x c = 132 x 101 x 93 km, with uncertainties of 4 x 3 x 13 km including estimated systematics, with a rotational pole within 5 deg. of ECJ2000 [long,lat] = [45, -7], or EQJ2000 [RA, DEC] = [44, +9]. The AO model fit itself has internal precisions of 1 x 1 x 8 km, but it is evident, both from this model derived from limited viewing aspects and the radius vector model given in a companion paper, that Lutetia has significant departures from an idealized ellipsoid. In particular, the long axis may be overestimated from the AO images alone by about 10 km. Therefore, we combine the best aspects of the radius vector and ellipsoid model into a hybrid ellipsoid model, as our final result, of 124 +/- 5 x 101 +/- 4 x 93 +/- 13 km that can be used to estimate volumes, sizes, and projected areas. The adopted pole position is within 5 deg. of [long, lat] = [52, -6] or[RA DEC] = [52, +12]. Using two separately determined masses and the volume of our hybrid model, we estimate a density of 3.5 +/- 1.1 or 4.3 +/- 0.8 g cm-3 . From the density evidence alone, we argue that this favors an enstatite-chondrite composition, although other compositions are formally allowed at the extremes (low-porosity CV/CO carbonaceous chondrite or high-porosity metallic). We discuss this in the context of other evidence.Comment: 9 pages, 8 figures, 5 tables, submitted to Astronomy and Astrophysic

Near-Infrared Mapping and Physical Properties of the Dwarf-Planet Ceres

We study the physical characteristics (shape, dimensions, spin axis direction, albedo maps, mineralogy) of the dwarf-planet Ceres based on high-angular resolution near-infrared observations. We analyze adaptive optics J/H/K imaging observations of Ceres performed at Keck II Observatory in September 2002 with an equivalent spatial resolution of ~50 km. The spectral behavior of the main geological features present on Ceres is compared with laboratory samples. Ceres' shape can be described by an oblate spheroid (a = b = 479.7 +/- 2.3 km, c = 444.4 +/- 2.1 km) with EQJ2000.0 spin vector coordinates RA = 288 +/- 5 deg. and DEC = +66 +/- 5 deg. Ceres sidereal period is measured to be 9.0741 +/- 0.0001 h. We image surface features with diameters in the 50-180 km range and an albedo contrast of ~6% with respect to the average Ceres albedo. The spectral behavior of the brightest regions on Ceres is consistent with phyllosilicates and carbonate compounds. Darker isolated regions could be related to the presence of frost.Comment: 11 pages, 8 Postscript figures, Accepted for publication in A&

Physical properties of ESA Rosetta target asteroid (21) Lutetia: Shape and flyby geometry

Aims. We determine the physical properties (spin state and shape) of asteroid (21) Lutetia, target of the ESA Rosetta mission, to help in preparing for observations during the flyby on 2010 July 10 by predicting the orientation of Lutetia as seen from Rosetta. Methods. We use our novel KOALA inversion algorithm to determine the physical properties of asteroids from a combination of optical lightcurves, disk-resolved images, and stellar occultations, although the latter are not available for (21) Lutetia. Results. We find the spin axis of (21) Lutetia to lie within 5 degrees of ({\lambda} = 52 deg., {\beta} = -6 deg.) in Ecliptic J2000 reference frame (equatorial {\alpha} = 52 deg., {\delta} = +12 deg.), and determine an improved sidereal period of 8.168 270 \pm 0.000 001 h. This pole solution implies the southern hemisphere of Lutetia will be in "seasonal" shadow at the time of the flyby. The apparent cross-section of Lutetia is triangular as seen "pole-on" and more rectangular as seen "equator-on". The best-fit model suggests the presence of several concavities. The largest of these is close to the north pole and may be associated with large impacts.Comment: 17 pages, 5 figures, 3 tables, submitted to Astronomy and Astrophysic

The Effects of Home Computers on Educational Outcomes: Evidence from a Field Experiment with Schoolchildren

Are home computers are an important input in the educational production function? To address this question, we conduct a field experiment involving the provision of free computers to schoolchildren for home use. Low-income children attending middle and high schools in 15 schools in California were randomly selected to receive free computers and followed over the school year. The results indicate that the experiment substantially increased computer ownership and total computer use among the schoolchildren with no substitution away from use at school or other locations outside the home. We find no evidence that the home computers improved educational outcomes for the treatment group. From detailed administrative data provided by the schools and a follow-up survey, we find no evidence of positive effects on a comprehensive set of outcomes such as grades, test scores, credits, attendance, school enrollment, computer skills, and college aspirations. The estimates also do not indicate that the effects of home computers on educational outcomes are instead negative. Our estimates are precise enough to rule out even modestly-sized positive or negative impacts. The lack of a positive net effect on educational outcomes may be due to displacement from non-educational uses such as for games, social networking, and entertainment. We find evidence that total hours of computer use for games and social networking increases substantially with having a home computer, and increases more than total hours of computer use for schoolwork

The DECam Ecliptic Exploration Project (DEEP) IV: Constraints on the shape distribution of bright TNOs

We present the methods and results from the discovery and photometric measurement of 26 bright (VR $>$ 24 trans-Neptunian objects (TNOs) during the first year (2019-20) of the DECam Ecliptic Exploration Project (DEEP). The DEEP survey is an observational TNO survey with wide sky coverage, high sensitivity, and a fast photometric cadence. We apply a computer vision technique known as a progressive probabilistic Hough transform to identify linearly-moving transient sources within DEEP photometric catalogs. After subsequent visual vetting, we provide a photometric and astrometric catalog of our TNOs. By modeling the partial lightcurve amplitude distribution of the DEEP TNOs using Monte Carlo techniques, we find our data to be most consistent with an average TNO axis ratio b/a $<$ 0.5, implying a population dominated by non-spherical objects. Based on ellipsoidal gravitational stability arguments, we find our data to be consistent with a TNO population containing a high fraction of contact binaries or other extremely non-spherical objects. We also discuss our data as evidence that the expected binarity fraction of TNOs may be size-dependent