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Evaluating the Impact of Charter Schools on Student Achievement: A Longitudinal Look at the Great Lakes States
This study looks at student achievement in math and reading in charter and traditional public schools over a five-year period in Illinois, Indiana, Michigan, Minnesota, Ohio, and Wisconsin. The primary finding is that student achievement in charter schools in these six states is lower than in traditional public schools. The study also finds, however, that student achievement in charter schools is improving over time
Reversal and Termination of Current-Induced Domain Wall Motion via Magnonic Spin-Transfer Torque
We investigate the domain wall dynamics of a ferromagnetic wire under the
combined influence of a spin-polarized current and magnonic spin-transfer
torque generated by an external field, taking also into account Rashba
spin-orbit coupling interactions. It is demonstrated that current-induced
motion of the domain wall may be completely reversed in an oscillatory fashion
by applying a magnonic spin-transfer torque as long as the spin-wave velocity
is sufficiently high. Moreover, we show that the motion of the domain wall may
be fully terminated by means of the generation of spin-waves, suggesting the
possibility to pin the domain-walls to predetermined locations. We also discuss
how strong spin-orbit interactions modify these results.Comment: Accepted for publication in Phys. Rev.
Lagrange-Fedosov Nonholonomic Manifolds
We outline an unified approach to geometrization of Lagrange mechanics,
Finsler geometry and geometric methods of constructing exact solutions with
generic off-diagonal terms and nonholonomic variables in gravity theories. Such
geometries with induced almost symplectic structure are modelled on
nonholonomic manifolds provided with nonintegrable distributions defining
nonlinear connections. We introduce the concept of Lagrange-Fedosov spaces and
Fedosov nonholonomic manifolds provided with almost symplectic connection
adapted to the nonlinear connection structure.
We investigate the main properties of generalized Fedosov nonholonomic
manifolds and analyze exact solutions defining almost symplectic Einstein
spaces.Comment: latex2e, v3, published variant, with new S.V. affiliatio
Current induced magnetization reversal on the surface of a topological insulator
We study dynamics of the magnetization coupled to the surface Dirac fermions
of a three di- mensional topological insulator. By solving the
Landau-Lifshitz-Gilbert equation in the presence of charge current, we find
current induced magnetization dynamics and discuss the possibility of mag-
netization reversal. The torque from the current injection depends on the
transmission probability through the ferromagnet and shows nontrivial
dependence on the exchange coupling. The mag- netization dynamics is a direct
manifestation of the inverse spin-galvanic effect and hence another ferromagnet
is unnecessary to induce spin transfer torque in contrast to the conventional
setup.Comment: 4 pages, 4 figure
Dynamics of magnetization on the topological surface
We investigate theoretically the dynamics of magnetization coupled to the
surface Dirac fermions of a three dimensional topological insulator, by
deriving the Landau-Lifshitz-Gilbert (LLG) equation in the presence of charge
current. Both the inverse spin-Galvanic effect and the Gilbert damping
coefficient are related to the two-dimensional diagonal conductivity
of the Dirac fermion, while the Berry phase of the ferromagnetic
moment to the Hall conductivity . The spin transfer torque and the
so-called -terms are shown to be negligibly small. Anomalous behaviors
in various phenomena including the ferromagnetic resonance are predicted in
terms of this LLG equation.Comment: 4+ pages, 1 figur
Phenomenology of chiral damping in noncentrosymmetric magnets
A phenomenology of magnetic chiral damping is proposed in the context of
magnetic materials lacking inversion symmetry breaking. We show that the
magnetic damping tensor adopts a general form that accounts for a component
linear in magnetization gradient in the form of Lifshitz invariants. We propose
different microscopic mechanisms that can produce such a damping in
ferromagnetic metals, among which spin pumping in the presence of anomalous
Hall effect and an effective "-" Dzyaloshinskii-Moriya antisymmetric
exchange. The implication of this chiral damping in terms of domain wall motion
is investigated in the flow and creep regimes. These predictions have major
importance in the context of field- and current-driven texture motion in
noncentrosymmetric (ferro-, ferri-, antiferro-)magnets, not limited to metals.Comment: 5 pages, 2 figure
Diffusive spin dynamics in ferromagnetic thin films with a Rashba interaction
In a ferromagnetic metal layer, the coupled charge and spin diffusion
equations are obtained in the presence of both Rashba spin-orbit interaction
and magnetism. The mis-alignment between the magnetization and the
non-equilibrium spin density induced by the Rashba field gives rise to Rashba
spin torque acting on the ferromagnetic order parameter. In a general form, we
find that the Rashba torque consists of both in-plane and out-of-plane
components, ie .
Numerical simulations on a two dimensional nano-wire discuss the impact of
diffusion on the Rashba torque, which reveals a large enhancement to the ratio
for thin wires. Our theory provides an explanation to
the mechanism that drives the magnetization switching in a single ferromagnet
as observed in the recent experiments.Comment: 5 pages and 3 figure
Random-Field Blume-Capel Model: Mean-Field Theory
The global phase diagram of the Blume-Capel model in a random field obeying the bimodal symmetric distribution is determined by using the mean-field method. The phase diagram includes an isolated ordered critical end point and two lines of tricritical points. A new phase emerges for strong enough random fields: the ferromagnetic-nonmagnetic phase. It is argued that such a phase occurs in three dimensions
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