96 research outputs found
Transmission electron microscopy and ferromagnetic resonance investigations of tunnel magnetic junctions using Co2MnGe Heusler alloy as magnetic electrodes
HRTEM, nano-beam electronic diffraction, energy dispersive X-rays scanning
spectroscopy, Vibrating Sample Magnetometry (VSM) and FerroMagnetic Resonance
(FMR) techniques are used in view of comparing (static and dynamic) magnetic
and structural properties of Co2MnGe (13 nm)/Al2O3 (3 nm)/Co (13 nm) tunnel
magnetic junctions (TMJ), deposited on various single crystalline substrates
(a-plane sapphire, MgO(100) and Si(111)). They allow for providing a
correlation between these magnetic properties and the fine structure
investigated at atomic scale. The Al2O3 tunnel barrier is always amorphous and
contains a large concentration of Co atoms, which, however, is significantly
reduced when using a sapphire substrate. The Co layer is polycrystalline and
shows larger grains for films grown on a sapphire substrate. The VSM
investigation reveals in-plane anisotropy only for samples grown on a sapphire
substrate. The FMR spectra of the TMJs are compared to the obtained ones with a
single Co and Co2MnGe films of identical thickness deposited on a sapphire
substrate. As expected, two distinct modes are detected in the TMJs while only
one mode is observed in each single film. For the TMJ grown on a sapphire
substrate the FMR behavior does not significantly differ from the superposition
of the individual spectra of the single films, allowing for concluding that the
exchange coupling between the two magnetic layers is too small to give rise to
observable shifts. For TMJs grown on a Si or on a MgO substrate the resonance
spectra reveal one mode which is nearly identical to the obtained one in the
single Co film, while the other observed resonance shows a considerably smaller
intensity and cannot be described using the magnetic parameters appropriate to
the single Co2MnGe film.Comment: 11 pages, 10 figures, Thin Solid Film
Magnetization dynamics in Co2O3$/Co tunnel junctions grown on different substrates
We study static and dynamic magnetic properties of Co2MnGe (13 nm)/Al2O3 (3
nm)/Co (13 nm) tunnel magnetic junctions (TMJ), deposited on various single
crystalline substrates (a-plane sapphire, MgO(100), Si(111)). The results are
compared to the magnetic properties of Co and of CoMnGe single films
lying on sapphire substrates. X-rays diffraction always shows a (110)
orientation of the CoMnGe films. Structural observations obtained by high
resolution transmission electron microscopy confirmed the high quality of the
TMJ grown on sapphire. Our vibrating sample magnetometry measurements reveal
in-plane anisotropy only in samples grown on a sapphire substrate. Depending on
the substrate, the ferromagnetic resonance spectra of the TMJs, studied by the
microstrip technique, show one or two pseudo-uniform modes. In the case of MgO
and of Si substrates only one mode is observed: it is described by magnetic
parameters (g-factor, effective magnetization, in-plane magnetic anisotropy)
derived in the frame of a simple expression of the magnetic energy density;
these parameters are practically identical to those obtained for the Co single
film. With a sapphire substrate two modes are present: one of them does not
appreciably differ from the observed mode in the Co single film while the other
one is similar to the mode appearing in the CoMnGe single film: their
magnetic parameters can thus be determined independently, using a classical
model for the energy density in the absence of interlayer exchange coupling.Comment: 5 pages, 6 figure
Probing the Dzyaloshinskii-Moriya interaction in CoFeB ultrathin films using domain wall creep and Brillouin light spectroscopy
We have characterized the strength of the interfacial Dyzaloshinskii-Moriya
interaction (DMI) in ultrathin perpendicularly magnetized CoFeB/MgO films,
grown on different underlayers of W, TaN, and Hf, using two experimental
methods. First, we determined the effective DMI field from measurements of
field-driven domain wall motion in the creep regime, where applied in-plane
magnetic fields induce an anisotropy in the wall propagation that is correlated
with the DMI strength. Second, Brillouin light spectroscopy was employed to
quantify the frequency non-reciprocity of spin waves in the CoFeB layers, which
yielded an independent measurement of the DMI. By combining these results, we
show that DMI estimates from the different techniques only yield qualitative
agreement, which suggests that open questions remain on the underlying models
used to interpret these results.Comment: 8 page
Current-induced nucleation and dynamics of skyrmions in a Co-based Heusler alloy
We demonstrate room-temperature stabilization of dipolar magnetic skyrmions
with diameters in the range of nm in a single ultrathin layer of the
Heusler alloy CoFeAl (CFA) under moderate magnetic fields. Current-induced
skyrmion dynamics in microwires is studied with a scanning Nitrogen-Vacancy
magnetometer operating in the photoluminescence quenching mode. We first
demonstrate skyrmion nucleation by spin-orbit torque and show that its
efficiency can be significantly improved using tilted magnetic fields, an
effect which is not specific to Heusler alloys and could be advantageous for
future skyrmion-based devices. We then show that current-induced skyrmion
motion remains limited by strong pinning effects, even though CFA is a magnetic
material with a low magnetic damping parameter.Comment: 5 pages, 4 figure
Room temperature chiral magnetic skyrmion in ultrathin magnetic nanostructures
Magnetic skyrmions are chiral spin structures with a whirling configuration.
Their topological properties, nanometer size and the fact that they can be
moved by small current densities have opened a new paradigm for the
manipulation of magnetisation at the nanoscale. To date, chiral skyrmion
structures have been experimentally demonstrated only in bulk materials and in
epitaxial ultrathin films and under external magnetic field or at low
temperature. Here, we report on the observation of stable skyrmions in
sputtered ultrathin Pt/Co/MgO nanostructures, at room temperature and zero
applied magnetic field. We use high lateral resolution X-ray magnetic circular
dichroism microscopy to image their chiral N\'eel internal structure which we
explain as due to the large strength of the Dzyaloshinskii-Moriya interaction
as revealed by spin wave spectroscopy measurements. Our results are
substantiated by micromagnetic simulations and numerical models, which allow
the identification of the physical mechanisms governing the size and stability
of the skyrmions.Comment: Submitted version. Extended version to appear in Nature
Nanotechnolog
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