21 research outputs found
Large microwave generation from d.c. driven magnetic vortex oscillators in magnetic tunnel junctions
Spin polarized current can excite the magnetization of a ferromagnet through
the transfer of spin angular momentum to the local spin system. This pure
spin-related transport phenomena leads to alluring possibilities for the
achievement of a nanometer scale, CMOS compatible and tunable microwave
generator operating at low bias for future wireless communications. Microwave
emission generated by the persitent motion of magnetic vortices induced by spin
transfer effect seems to be a unique manner to reach appropriate spectral
linewidth. However, in metallic systems, where such vortex oscillations have
been observed, the resulting microwave power is much too small. Here we present
experimental evidences of spin-transfer induced core vortex precessions in
MgO-based magnetic tunnel junctions with similar good spectral quality but an
emitted power at least one order of magnitude stronger. More importantly,
unlike to others spin transfer excitations, the thorough comparison between
experimental results and models provide a clear textbook illustration of the
mechanisms of vortex precessions induced by spin transfer
Tailoring the magnetic properties of Fe asymmetric nanodots
Asymmetric dots as a function of their geometry have been investigated using
three-dimensional (3D) object oriented micromagnetic framework (OOMMF) code.
The effect of shape asymmetry of the disk on coercivity and remanence is
studied. Angular dependence of the remanence and coercivity is also addressed.
Asymmetric dots are found to reverse their magnetization by nucleation and
propagation of a vortex, when the field is applied parallel to the direction of
asymmetry. However, complex reversal modes appear when the angle at which the
external field is applied is varied, leading to a non monotonic behavior of the
coercivity and remanence.Comment: 5 pages, 7 figure
Polarons and confinement of electronic motion to two dimensions in a layered transition metal oxide
A very remarkable feature of the layered transition metal oxides (TMOs),
whose most famous members are the high-temperature superconductors (HTSs), is
that even though they are prepared as bulk three-dimensional single crystals,
they display hugely anisotropic electrical and optical properties, seeming to
be insulating perpendicular to the layers and metallic within them. This is the
phenomenon of confinement, a concept at odds with the conventional theory of
solids and recognized as due to magnetic and electron-lattice interactions in
the layers which must be overcome at a substantial energy cost if electrons are
to be transferred between layers. The associated energy gap or 'pseudogap' is
particularly obvious in experiments where charge is moved perpendicular to the
planes, most notably scanning tunneling microscopy (STM) and polarized infrared
spectroscopy. Here, using the same experimental tools, we show that there is a
second family of TMOs - the layered manganites La2-2xSr1+2xMn2O7 (LSMO) - with
even more extreme confinement and pseudogap effects. The data, which are the
first to resolve atoms in any metallic manganite, demonstrate quantitatively
that because they are attached to polarons - lattice and spin textures within
the planes -, it is equally difficult to remove carriers from the planes via
vacuum tunneling into a conventional metallic tip, as it is for them to move
between Mn-rich layers within the material itself
Large voltage-controlled magnetic anisotropy effect in magnetic tunnel junctions prepared by deposition at cryogenic temperatures
We investigated the influence of the buffer material and a cryogenic temperature deposition process on the voltage-controlled magnetic anisotropy (VCMA) effect for an ultrathin CoFeB layer in bottom-free type MgO-based magnetic tunnel junctions prepared by a mass production sputtering process. We used Ta and TaB buffers and compared the differences between them. The TaB buffer enabled us to form a flat and less-contaminated CoFeB/MgO interface by suppressing the diffusion of Ta with maintaining a stable amorphous phase. Furthermore, the introduction of cryogenic temperature deposition for the ultrathin CoFeB layer on the TaB buffer improved the efficiency of the VCMA effect and its annealing tolerance. Combining this with interface engineering employing an Ir layer for doping and a CoFe termination layer, a large VCMA coefficient of â138 ± 3 fJ/Vm was achieved. The developed techniques for the growth of ultrathin ferromagnet and oxide thin films using cryogenic temperature deposition will contribute to the development of high-performance spintronic devices, such as voltage-controlled magnetoresistive random access memories