719 research outputs found
Current-driven Magnetization Reversal in a Ferromagnetic Semiconductor (Ga,Mn)As/GaAs/(Ga,Mn)As Tunnel Junction
Current-driven magnetization reversal in a ferromagnetic semiconductor based
(Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junction is demonstrated at 30 K.
Magnetoresistance measurements combined with current pulse application on a
rectangular 1.5 x 0.3 um^2 device revealed that magnetization switching occurs
at low critical current densities of 1.1 - 2.2 x 10^5 A/cm^2 despite the
presence of spin-orbit interaction in the p-type semiconductor system. Possible
mechanisms responsible for the effect are discussed.Comment: 16 pages, 4 figure
CoFeB Thickness Dependence of Thermal Stability Factor in CoFeB/MgO Perpendicular Magnetic Tunnel Junctions
Thermal stability factor (delta) of recording layer was studied in
perpendicular anisotropy CoFeB/MgO magnetic tunnel junctions (p-MTJs) with
various CoFeB recording layer thicknesses and junction sizes. In all series of
p-MTJs with different thicknesses, delta is virtually independent of the
junction sizes of 48-81 nm in diameter. The values of delta increase linearly
with increasing the recording layer thickness. The slope of the linear fit is
explained well by a model based on nucleation type magnetization reversal.Comment: 12 pages, 5 figure
MgO barrier-perpendicular magnetic tunnel junctions with CoFe/Pd multilayers and ferromagnetic insertion layers
The authors studied an effect of ferromagnetic (Co20Fe60B20 or Fe) layer
insertion on tunnel magnetoresistance (TMR) properties of MgO-barrier magnetic
tunnel junctions (MTJs) with CoFe/Pd multilayer electrodes. TMR ratio in MTJs
with CoFeB/MgO/Fe stack reached 67% at an-nealing temperature (Ta) of 200
degree C and then decreased rapidly at Ta over 250 degree C. The degradation of
the TMR ratio may be related to crystallization of CoFe(B) into fcc(111) or
bcc(011) texture result-ing from diffusion of B into Pd layers. MTJs which were
in-situ annealed at 350oC just after depo-siting bottom CoFe/Pd multilayer
showed TMR ratio of 78% by post annealing at Ta =200 degree C.Comment: 12 pages, 4 figure
Reorientation Transition in Single-Domain (Ga,Mn)As
We demonstrate that the interplay of in-plane biaxial and uniaxial anisotropy
fields in (Ga,Mn)As results in a magnetization reorientation transition and an
anisotropic AC susceptibility which is fully consistent with a simple single
domain model. The uniaxial and biaxial anisotropy constants vary respectively
as the square and fourth power of the spontaneous magnetization across the
whole temperature range up to T_C. The weakening of the anisotropy at the
transition may be of technological importance for applications involving
thermally-assisted magnetization switching.Comment: 4 pages, 4 figure
Articular cartilage degradation induced by extensive treadmill exercise is greatly exacerbated by estrogen depletion in mice
On the nature of ferromagnetism in diluted magnetic semiconductors: GaAs:Mn, GaP:Mn
A microscopic Hamiltonian for interacting manganese impurities in diluted
magnetic semiconductors (DMS) is derived. It is shown that in p -type III-V DMS
the indirect exchange between Mn impurities has similarities with the Zener
mechanism in transition metal oxides. Here the mobile holes and localized
states near the top of the valence band play the role of unoccupied oxygen
orbitals which induce ferromagnetism. The Curie temperature estimated from the
proposed kinematic exchange agrees with recent experiments on GaAs:Mn. The
model is also applicable to the GaP:Mn system.Comment: 10 pages, 3 figures. Submitted to Europhysics Letters, June 25, 200
Positional Disorder, Spin-Orbit Coupling and Frustration in GaMnAs
We study the magnetic properties of metallic GaMnAs. We calculate the
effective RKKY interaction between Mn spins using several realistic models for
the valence band structure of GaAs. We also study the effect of positional
disorder of the Mn on the magnetic properties. We find that the interaction
between two Mn spins is anisotropic due to spin-orbit coupling within both the
so-called spherical approximation and in the more realistic six band model. The
spherical approximation strongly overestimates this anistropy, especially for
short distances between Mn ions. Using the obtained effective Hamiltonian we
carry out Monte Carlo simulations of finite and zero temperature magnetization
and find that, due to orientational frustration of the spins, non-collinear
states appear in both valence band approximations for disordered, uncorrelated
Mn impurities in the small concentration regime. Introducing correlations among
the substitutional Mn positions or increasing the Mn concentration leads to an
increase in the remnant magnetization at zero temperature and an almost fully
polarized ferromagnetic state.Comment: 17 Pages, 13 Figure
Mn incorporation in as-grown and annealed (Ga,Mn)As layers studied by x-ray diffraction and standing-wave uorescence
A combination of high-resolution x-ray diffraction and a new technique of
x-ray standing wave uorescence at grazing incidence is employed to study the
structure of (Ga,Mn)As diluted magnetic semiconductor and its changes during
post-growth annealing steps. We find that the film is formed by a uniform,
single crystallographic phase epilayer covered by a thin surface layer with
enhanced Mn concentration due to Mn atoms at random non-crystallographic
positions. In the epilayer, Mn incorporated at interstitial position has a
dominant effect on lattice expansion as compared to substitutional Mn. The
expansion coeffcient of interstitial Mn estimated from our data is consistent
with theory predictions. The concentration of interstitial Mn and the
corresponding lattice expansion of the epilayer are reduced by annealing,
accompanied by an increase of the density of randomly distributed Mn atoms in
the disordered surface layer. Substitutional Mn atoms remain stable during the
low-temperature annealing.Comment: 9 pages, 9 figure
Disorder, spin-orbit, and interaction effects in dilute
We derive an effective Hamiltonian for in
the dilute limit, where can be described in
terms of spin polarons hopping between the {\rm Mn} sites and coupled
to the local {\rm Mn} spins. We determine the parameters of our model from
microscopic calculations using both a variational method and an exact
diagonalization within the so-called spherical approximation. Our approach
treats the extremely large Coulomb interaction in a non-perturbative way, and
captures the effects of strong spin-orbit coupling and Mn positional disorder.
We study the effective Hamiltonian in a mean field and variational calculation,
including the effects of interactions between the holes at both zero and finite
temperature. We study the resulting magnetic properties, such as the
magnetization and spin disorder manifest in the generically non-collinear
magnetic state. We find a well formed impurity band fairly well separated from
the valence band up to for which finite size
scaling studies of the participation ratios indicate a localization transition,
even in the presence of strong on-site interactions, where is the fraction of magnetically active Mn. We study the
localization transition as a function of hole concentration, Mn positional
disorder, and interaction strength between the holes.Comment: 15 pages, 12 figure
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