121,552 research outputs found
Mediating exchange bias by Verwey transition in CoO/Fe3O4 thin film
We report the tunability of the exchange bias effect by the first-order
metal-insulator transition (known as the Verwey transition) of Fe3O4 in CoO (5
nm)/Fe3O4 (40 nm)/MgO (001) thin film. In the vicinity of the Verwey
transition, the exchange bias field is substantially enhanced because of a
sharp increase in magnetocrystalline anisotropy constant from high-temperature
cubic to lowtemperature monoclinic structure. Moreover, with respect to the
Fe3O4 (40 nm)/MgO (001) thin film, the coercivity field of the CoO (5 nm)/Fe3O4
(40 nm)/MgO (001) bilayer is greatly increased for all the temperature range,
which would be due to the coupling between Co spins and Fe spins across the
interface
Mass retention efficiencies of He accretion onto carbon-oxygen white dwarfs and type Ia supernovae
Type Ia supernovae (SNe Ia) play a crucial role in studying cosmology and
galactic chemical evolution. They are thought to be thermonuclear explosions of
carbon-oxygen white dwarfs (CO WDs) when their masses reach the Chandrasekar
mass limit in binaries. Previous studies have suggested that He novae may be
progenitor candidates of SNe Ia. However, the mass retention efficiencies
during He nova outbursts are still uncertain. In this article, we aim to study
the mass retention efficiencies of He nova outbursts and to investigate whether
SNe Ia can be produced through He nova outbursts. Using the stellar evolution
code Modules for Experiments in Stellar Astrophysics, we simulated a series of
multicycle He-layer flashes, in which the initial WD masses range from 0.7 to
1.35 Msun with various accretion rates. We obtained the mass retention
efficiencies of He nova outbursts for various initial WD masses, which can be
used in the binary population synthesis studies. In our simulations, He nova
outbursts can increase the mass of the WD to the Chandrasekar mass limit and
the explosive carbon burning can be triggered in the center of the WD; this
suggests that He nova outbursts can produce SNe Ia. Meanwhile, the mass
retention efficiencies in the present work are lower than those of previous
studies, which leads to a lower birthrates of SNe Ia through the WD + He star
channel. Furthermore, we obtained the elemental abundances distribution at the
moment of explosive carbon burning, which can be used as the initial input
parameters in studying explosion models of SNe Ia.Comment: 8 pages, 12 figures, 2 tables, published in Astronomy & Astrophysics
(A&A 604, A31, 2017
Asymptotic properties of eigenmatrices of a large sample covariance matrix
Let where is a matrix
with i.i.d. complex standardized entries having finite fourth moments. Let
in which
and where
is the Mar\v{c}enko--Pastur law with parameter ; which
converges to a positive constant as , and and are unit vectors in ,
having indices and , ranging in a compact subset
of a finite-dimensional Euclidean space. In this paper, we prove that the
sequence converges weakly to a
-dimensional Gaussian process. This result provides further evidence in
support of the conjecture that the distribution of the eigenmatrix of is
asymptotically close to that of a Haar-distributed unitary matrix.Comment: Published in at http://dx.doi.org/10.1214/10-AAP748 the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Effect of spin relaxations on the spin mixing conductances for a bilayer structure
The spin current can result in a spin-transfer torque in the
normal-metal(NM)|ferromagnetic-insulator(FMI) or
normal-metal(NM)|ferromagnetic-metal(FMM) bilayer. In the earlier study on this
issue, the spin relaxations were ignored or introduced phenomenologically. In
this paper, considering the FMM or FMI with spin relaxations described by a
non-Hermitian Hamiltonian, we derive an effective spin-transfer torque and an
effective spin mixing conductance in the non-Hermitian bilayer. The dependence
of the effective spin mixing conductance on the system parameters (such as
insulating gap, \textit{s-d} coupling, and layer thickness) as well as the
relations between the real part and the imaginary part of the effective spin
mixing conductance are given and discussed. We find that the effective spin
mixing conductance can be enhanced in the non-Hermitian system. This provides
us with the possibility to enhance the spin mixing conductance
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