5,106 research outputs found
On spinodal decomposition in alnico---a transmission electron microscopy and atom probe tomography study
Alnico is a prime example of a finely tuned nanostructure whose magnetic
properties are intimately connected to magnetic annealing (MA) during spinodal
transformation and subsequent lower temperature annealing (draw) cycles. Using
a combination of transmission electron microscopy and atom probe tomography, we
show how these critical processing steps affect the local composition and
nanostructure evolution with impact on magnetic properties. The nearly 2-fold
increase of intrinsic coercivity () during the draw cycle is not
adequately explained by chemical refinement of the spinodal phases. Instead,
increased Fe-Co phase () isolation, development of Cu-rich
spheres/rods/blades and additional rod precipitation that occurs
during the MA and draw, likely play a key role in enhancement.
Chemical ordering of the Al-Ni-phase () and formation of Ni-rich
() may also contribute. Unraveling of the subtle effect of these
nano-scaled features is crucial to understanding on how to improve shape
anisotropy in alnico magnets
Unlocking CO Depletion in Protoplanetary Disks II. Primordial C/H Predictions Inside the CO Snowline
CO is thought to be the main reservoir of volatile carbon in protoplanetary
disks, and thus the primary initial source of carbon in the atmospheres of
forming giant planets. However, recent observations of protoplanetary disks
point towards low volatile carbon abundances in many systems, including at
radii interior to the CO snowline. One potential explanation is that gas phase
carbon is chemically reprocessed into less volatile species, which are frozen
on dust grain surfaces as ice. This mechanism has the potential to change the
primordial C/H ratio in the gas. However, current observations primarily probe
the upper layers of the disk. It is not clear if the low volatile carbon
abundances extend to the midplane, where planets form. We have run a grid of
198 chemical models, exploring how the chemical reprocessing of CO depends on
disk mass, dust grain size distribution, temperature, cosmic ray and X-ray
ionization rate, and initial water abundance. Building on our previous work
focusing on the warm molecular layer, here we analyze the results for our grid
of models in the disk midplane at 12 au. We find that either an ISM level
cosmic ray ionization rate or the presence of UV photons due to a low dust
surface density are needed to chemically reduce the midplane CO gas abundance
by at least an order of magnitude within 1 Myr. In the majority of our models
CO does not undergo substantial reprocessing by in situ chemistry and there is
little change in the gas phase C/H and C/O ratios over the lifetime of the
typical disk. However, in the small sub-set of disks where the disk midplane is
subject to a source of ionization or photolysis, the gas phase C/O ratio
increases by up to nearly 9 orders of magnitude due to conversion of CO into
volatile hydrocarbons.Comment: Accepted for publication in ApJ, 15 pages, 10 figures, 3 table
Exchange interactions and temperature dependence of the magnetization in half--metallic Heusler alloys
We study the exchange interactions in half-metallic Heusler alloys using
first-principles calculations in conjunction with the frozen-magnon
approximation. The Curie temperature is estimated within both mean-field (MF)
and random-phase-approximation (RPA) approaches. For the half-Heusler alloys
NiMnSb and CoMnSb the dominant interaction is between the nearest Mn atoms. In
this case the MF and RPA estimations differ strongly. The RPA approach provides
better agreement with experiment. The exchange interactions are more complex in
the case of full-Heusler alloys CoMnSi and CoCrAl where the dominant
effects are the inter-sublattice interactions between the Mn(Cr) and Co atoms
and between Co atoms at different sublattices. For these compounds we find that
both MF and RPA give very close values of the Curie temperature slightly
underestimating experimental quantities. We study the influence of the lattice
compression on the magnetic properties. The temperature dependence of the
magnetization is calculated using the RPA method within both quantum mechanical
and classical approaches.Comment: New figures and discussio
Simulation of alnico coercivity
Micromagnetic simulations of alnico show substantial deviations from
Stoner-Wohlfarth behavior due to the unique size and spatial distribution of
the rod-like Fe-Co phase formed during spinodal decomposition in an external
magnetic field. The maximum coercivity is limited by single-rod effects,
especially deviations from ellipsoidal shape, and by interactions between the
rods. Both the exchange interaction between connected rods and magnetostatic
interaction between rods are considered, and the results of our calculations
show good agreement with recent experiments. Unlike systems dominated by
magnetocrystalline anisotropy, coercivity in alnico is highly dependent on
size, shape, and geometric distribution of the Fe-Co phase, all factors that
can be tuned with appropriate chemistry and thermal-magnetic annealing
Cooperative and noncooperative magnetization reversal in alnicos
It is investigated how magnetostatic interactions affect the coercivity of alnicotype magnets. Starting from exact micromagnetic relations, we analyze two limits, namely cooperative reversal processes operative on short lengths scales and noncooperative reversal processes on long length scales. In alnicos, intrawire interactions are predominantly cooperative, whereas interwire effects are typically noncooperative. However, the transition between the regimes depends on feature size and hysteresis-loop shape, and interwire cooperative effects are largest for nearly rectangular loops. Our analysis revises the common shape-anisotropy interpretation of alnicos
Probing the Gas Content of Late-stage Protoplanetary Disks with N_2H^+
The lifetime of gas in circumstellar disks is a fundamental quantity that informs our understanding of planet formation. Studying disk gas evolution requires measurements of disk masses around stars of various ages. Because H_2 gas is unobservable under most disk conditions, total disk masses are based on indirect tracers such as sub-mm dust and CO emission. The uncertainty in the relation between these tracers and the disk mass increases as the disk evolves. In a few well-studied disks, CO exhibits depletions of up to 100Ă below the assumed interstellar value. Thus, additional tracers are required to accurately determine the total gas mass. The relative lack of nitrogen found in solid solar system bodies may indicate that it persists in volatile form, making nitrogen-bearing species more robust tracers of gas in more evolved disks. Here we present Atacama Large Millimeter/submillimeter Array detections of N_2H^+ in two mature, ~5â11 Myr old disks in the Upper Scorpius OB Association. Such detections imply the presence of H_2-rich gas and sources of ionization, both required for N_2H^+ formation. The Upper Sco disks also show elevated N_2H^+/CO flux ratios when compared to previously observed disks with âł10Ă higher CO fluxes. Based on line ratio predictions from a grid of thermochemical disk models, a significantly reduced CO/H_2 abundance of <10^(â6) for a gas-to-dust ratio of âł100 is required to produce the observed N_2H^+ fluxes. These systems appear to maintain H_2 gas reservoirs and indicate that carbon- and nitrogen-bearing species follow distinct physical or chemical pathways as disks evolve
Microstructure and coercivity in alnico 9
Magnetic property enhancement of alnico, a rare-earth free permanent magnet, is highly dependent on both the initial microstructure and the evolution of the spinodal decomposition (SD) inside each grain during the heat treatment process. The size, shape and distribution of the magnetic FeCo-rich (α1) phase embedded in continuous non-magnetic AlNi-rich (α2) phase as well as a minor Cu-enriched phase residing in between are shown to be crucial in controlling coercivity. Phase and magnetic domain morphology in a commercial alnico 9 alloy was studied using a combination of structural characterization techniques, including scanning electron microscopy, electron backscatter diffraction, aberration-corrected scanning transmission electron microscopy and Lorentz microscopy. Our results showed that casting created structural nonuniformity and defects, such as porosity, TiS2 precipitates and grain misorientation, are heterogeneously distributed, with the center section having the best crystallographic orientation and minimal defects. The optimal spinodal is a âmosaic structureâ, composed of rod-shape α1 phase with {1â1â0} or {1â0â0} planar faceting and diameter of âŒ30â45âŻnm. There is also a Cu-enriched phase residing at the corners of two ă1â1â0ă facets of the α1 phase. It was observed that grain boundary phase reverse magnetization direction at lower external magnetic field compared to the SD region inside the grain. Improving grain orientation uniformity, reducing detrimental grain boundary phase volume fraction, and the branching of the α1 rods, as well as their diameter, are promising routes to improve energy product of alnico
New times, new politics: history and memory during the final years of the CPGB
This article examines the relationship between collective memory, historical interpretation and political identity. It focuses on the dissolution of the Communist Party of Great Britain (CPGB) as constructed through collective narrative memory, and on Marxist interpretations of history. The divisions within the party and the wider Marxist community, stretching from 1956 until 1991, were often framed around questions of historical interpretation. The events of 1989â1991 created an historical and mnemonic crisis for CPGB members who struggled to reconcile their past identities with their present situation. Unlike the outward-facing revisionism of other political parties, this was an intensely personal affair. The solution for many was to emphasise the need to find new ways to progress socialist aims, without relying on a discredited grand narrative. In contrast, other Communist parties, such as the Communist Party of Britain, which had been established (or âre-establishedâ) in 1988, fared rather better. By adhering to the international party line of renewal and continued struggle, the party was able to hold its narrative together, condemning the excesses of totalitarian regimes, while reaffirming the need for international class struggle
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