128 research outputs found
Oscillation modes of dc microdischarges with parallel-plate geometry
Two different oscillation modes in microdischarge with parallel-plate
geometry has been observed: relaxation oscillations with frequency range
between 1.23 and 2.1 kHz and free-running oscillations with 7 kHz frequency.
The oscillation modes are induced by increasing power supply voltage or
discharge current. For a given power supply voltage, there is a spontaneous
transition from one to other oscillation mode and vice versa. Before the
transition from relaxation to free-running oscillations, the spontaneous
increase of oscillation frequency of relaxation oscillations form 1.3 kHz to
2.1 kHz is measured. Fourier Transform Spectra of relaxation oscillations
reveal chaotic behaviour of microdischarge. Volt-Ampere characteristics
associated with relaxation oscillations describes periodical transition between
low current, diffuse discharge and normal glow. However, free-running
oscillations appear in subnormal glow only.Comment: Submitted to: New Journal of Physic
Enhancement of thermovoltage and tunnel magneto-Seebeck effect in CoFeB based magnetic tunnel junctions by variation of the MgAlO and MgO barrier thickness
We investigate the influence of the barrier thickness of
CoFeB based magnetic tunnel junctions on the laser-induced
tunnel magneto-Seebeck effect. Varying the barrier thickness from 1nm to 3nm,
we find a distinct maximum in the tunnel magneto-Seebeck effect for 2.6nm
barrier thickness. This maximum is independently measured for two barrier
materials, namely MgAlO and MgO. Additionally, samples with an
MgAlO barrier exhibit a high thermovoltage of more than 350V in
comparison to 90V for the MTJs with MgO barrier when heated with the
maximum laser power of 150mW. Our results allow for the fabrication of improved
stacks when dealing with temperature differences across magnetic tunnel
junctions for future applications in spin caloritronics, the emerging research
field that combines spintronics and themoelectrics
Robust cell culture performance facilitated by comprehensive equipment characterization – the importance of hydrodynamic stress
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Electronic and magnetic structure of epitaxial NiO/FeO(001) heterostructures grown on MgO(001) and Nb-doped SrTiO(001)
We study the underlying chemical, electronic and magnetic properties of a
number of magnetite based thin films. The main focus is placed onto
NiO/FeO(001) bilayers grown on MgO(001) and Nb-SrTiO(001)
substrates. We compare the results with those obtained on pure FeO(001)
thin films. It is found that the magnetite layers are oxidized and Fe
dominates at the surfaces due to maghemite (-FeO) formation,
which decreases with increasing magnetite layer thickness. From a layer
thickness of around 20 nm on the cationic distribution is close to that of
stoichiometric FeO. At the interface between NiO and FeO we
find the Ni to be in a divalent valence state, with unambiguous spectral
features in the Ni 2p core level x-ray photoelectron spectra typical for NiO.
The formation of a significant NiFeO interlayer can be excluded by
means of XMCD. Magneto optical Kerr effect measurements reveal significant
higher coercive fields compared to magnetite thin films grown on MgO(001), and
a 45 rotated magnetic easy axis. We discuss the spin magnetic moments
of the magnetite layers and find that the moment increases with increasing thin
film thickness. At low thickness the NiO/FeO films grown on
Nb-SrTiO exhibits a significantly decreased spin magnetic moments. A
thickness of 20 nm or above leads to spin magnetic moments close to that of
bulk magnetite
Large magneto-Seebeck effect in magnetic tunnel junctions with half-metallic Heusler electrodes
Spin caloritronics studies the interplay between charge-, heat- and
spin-currents, which are initiated by temperature gradients in magnetic
nanostructures. A plethora of new phenomena has been discovered that promises,
e.g., to make wasted heat in electronic devices useable or to provide new
read-out mechanisms for information. However, only few materials have been
studied so far with Seebeck voltages of only some {\mu}V, which hampers
applications. Here, we demonstrate that half-metallic Heusler compounds are hot
candidates for enhancing spin-dependent thermoelectric effects. This becomes
evident when considering the asymmetry of the spin-split density of electronic
states around the Fermi level that determines the spin-dependent thermoelectric
transport in magnetic tunnel junctions. We identify CoFeAl and CoFeSi
Heusler compounds as ideal due to their energy gaps in the minority density of
states, and demonstrate devices with substantially larger Seebeck voltages and
tunnel magneto-Seebeck effect ratios than the commonly used Co-Fe-B based
junctions.Comment: 9 pages, 4 figure
Sign change in the tunnel magnetoresistance of Fe3O4/MgO/Co-Fe-B magnetic tunnel junctions depending on the annealing temperature and the interface treatment
Magnetite (Fe3O4) is an eligible candidate for magnetic tunnel junctions
(MTJs) since it shows a high spin polarization at the Fermi level as well as a
high Curie temperature of 585{\deg}C. In this study, Fe3O4/MgO/Co-Fe-B MTJs
were manufactured. A sign change in the TMR is observed after annealing the
MTJs at temperatures between 200{\deg}C and 280{\deg}C. Our findings suggest an
Mg interdiffusion from the MgO barrier into the Fe3O4 as the reason for the
change of the TMR. Additionally, different treatments of the magnetite
interface (argon bombardment, annealing at 200{\deg}C in oxygen atmosphere)
during the preparation of the MTJs have been studied regarding their effect on
the performance of the MTJs. A maximum TMR of up to -12% could be observed
using both argon bombardment and annealing in oxygen atmosphere, despite
exposing the magnetite surface to atmospheric conditions before the deposition
of the MgO barrier.Comment: 5 pages, 5 figures, 2 table
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