404 research outputs found
Antiferromagnetically coupled CoFeB/Ru/CoFeB trilayers
This work reports on the magnetic interlayer coupling between two amorphous
CoFeB layers, separated by a thin Ru spacer. We observe an antiferromagnetic
coupling which oscillates as a function of the Ru thickness x, with the second
antiferromagnetic maximum found for x=1.0 to 1.1 nm. We have studied the
switching of a CoFeB/Ru/CoFeB trilayer for a Ru thickness of 1.1 nm and found
that the coercivity depends on the net magnetic moment, i.e. the thickness
difference of the two CoFeB layers. The antiferromagnetic coupling is almost
independent on the annealing temperatures up to 300 degree C while an annealing
at 350 degree C reduces the coupling and increases the coercivity, indicating
the onset of crystallization. Used as a soft electrode in a magnetic tunnel
junction, a high tunneling magnetoresistance of about 50%, a well defined
plateau and a rectangular switching behavior is achieved.Comment: 3 pages, 3 figure
Spin-Polarized Current Induced Torque in Magnetic Tunnel Junctions
We present tight-binding calculations of the spin torque in non-collinear
magnetic tunnel junctions based on the non-equilibrium Green functions
approach. We have calculated the spin torque via the effective local magnetic
moment approach and the divergence of the spin current. We show that both
methods are equivalent, i.e. the absorption of the spin current at the
interface is equivalent to the exchange interaction between the electron spins
and the local magnetization. The transverse components of the spin torque
parallel and perpendicular to the interface oscillate with different phase and
decay in the ferromagnetic layer (FM) as a function of the distance from the
interface. The period of oscillations is inversely proportional to the
difference between the Fermi-momentum of the majority and minority electrons.
The phase difference between the two transverse components of the spin torque
is due to the precession of the electron spins around the exchange field in the
FM layer. In absence of applied bias and for a relatively thin barrier the
perpendicular component of the spin torque to the interface is non-zero due to
the exchange coupling between the FM layers across the barrier.Comment: 6 pages, 3 figure
Morphologies of three-dimensional shear bands in granular media
We present numerical results on spontaneous symmetry breaking strain
localization in axisymmetric triaxial shear tests of granular materials. We
simulated shear band formation using three-dimensional Distinct Element Method
with spherical particles. We demonstrate that the local shear intensity, the
angular velocity of the grains, the coordination number, and the local void
ratio are correlated and any of them can be used to identify shear bands,
however the latter two are less sensitive. The calculated shear band
morphologies are in good agreement with those found experimentally. We show
that boundary conditions play an important role. We discuss the formation
mechanism of shear bands in the light of our observations and compare the
results with experiments. At large strains, with enforced symmetry, we found
strain hardening.Comment: 6 pages 5 figures, low resolution figures
Vertical current induced domain wall motion in MgO-based magnetic tunnel junction with low current densities
Shifting electrically a magnetic domain wall (DW) by the spin transfer
mechanism is one of the future ways foreseen for the switching of spintronic
memories or registers. The classical geometries where the current is injected
in the plane of the magnetic layers suffer from a poor efficiency of the
intrinsic torques acting on the DWs. A way to circumvent this problem is to use
vertical current injection. In that case, theoretical calculations attribute
the microscopic origin of DW displacements to the out-of-plane (field-like)
spin transfer torque. Here we report experiments in which we controllably
displace a DW in the planar electrode of a magnetic tunnel junction by vertical
current injection. Our measurements confirm the major role of the out-of-plane
spin torque for DW motion, and allow to quantify this term precisely. The
involved current densities are about 100 times smaller than the one commonly
observed with in-plane currents. Step by step resistance switching of the
magnetic tunnel junction opens a new way for the realization of spintronic
memristive devices
Critical packing in granular shear bands
In a realistic three-dimensional setup, we simulate the slow deformation of
idealized granular media composed of spheres undergoing an axisymmetric
triaxial shear test. We follow the self-organization of the spontaneous strain
localization process leading to a shear band and demonstrate the existence of a
critical packing density inside this failure zone. The asymptotic criticality
arising from the dynamic equilibrium of dilation and compaction is found to be
restricted to the shear band, while the density outside of it keeps the memory
of the initial packing. The critical density of the shear band depends on
friction (and grain geometry) and in the limit of infinite friction it defines
a specific packing state, namely the \emph{dynamic random loose packing}
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