677 research outputs found
Origin of large magnetocurrent in three-terminal double-barrier magnetic tunnel junctions
Copyright © 2005 American Institute of PhysicsDouble-barrier magnetic tunnel junctions (DBMTJs) of composition Co/AlOx/Co/AlOx/Ni81Fe19 have been fabricated by magnetron sputtering through shadow masks. Two terminal measurements made upon the individual tunnel barriers revealed nonlinear I–V curves and significant room-temperature tunnel magnetoresistance (TMR) in all cases. Measurements were also performed with connections made to all three electrodes. The TMR of a particular tunnel barrier within the DBMTJ can be strongly modified by applying a bias voltage to the other barrier, while the TMR measured across the two barriers in series decreases more slowly with increasing bias voltage than for a single barrier. With zero bias applied between the central Co base electrode and the Co collector electrode, the collector current was measured as electrons were injected from the Ni81Fe19 electrode. For structures grown on Si/SiO2 substrates, the collector current showed a nonmonotonic dependence upon the emitter-base bias voltage, and collector magnetocurrent values in excess of 100% were observed at nonzero emitter-base bias values. For structures grown on quartz the collector current increased while the magnetocurrent decreased with increasing emitter-base voltage. We suggest that the enhanced TMR and magnetocurrent effects can be explained by substrate leakage and geometrical artifacts rather than by transport of spin-polarized hot electrons across the base layer
Evidence for hot electron magnetocurrent in a double barrier tunnel junction device
Copyright © 2005 American Institute of PhysicsHot electron transport has been studied in three terminal Ta/TaOx/Co/AlOx/Ni81Fe19 structures fabricated by magnetron sputtering through shadow masks. With the Co base and Ta collector connected together via a small resistor, the collector current contains contributions first from hot electrons injected from the Ni81Fe19 emitter, and second from a geometrical artifact that leads to tunneling from the Fermi level in the base. Both sources of collector current lead to a room temperature magnetocurrent effect. The hot electron contribution begins to dominate as the emitter-base voltage −Veb exceeds 0.3 V
Super-harmonic injection locking of nano-contact spin-torque vortex oscillators
Super-harmonic injection locking of single nano-contact (NC) spin-torque
vortex oscillators (STVOs) subject to a small microwave current has been
explored. Frequency locking was observed up to the fourth harmonic of the STVO
fundamental frequency in microwave magneto-electronic measurements. The
large frequency tunability of the STVO with respect to allowed the
device to be locked to multiple sub-harmonics of the microwave frequency
, or to the same sub-harmonic over a wide range of by tuning
the DC current. In general, analysis of the locking range, linewidth, and
amplitude showed that the locking efficiency decreased as the harmonic number
increased, as expected for harmonic synchronization of a non-linear oscillator.
Time-resolved scanning Kerr microscopy (TRSKM) revealed significant differences
in the spatial character of the magnetization dynamics of states locked to the
fundamental and harmonic frequencies, suggesting significant differences in the
core trajectories within the same device. Super-harmonic injection locking of a
NC-STVO may open up possibilities for devices such as nanoscale frequency
dividers, while differences in the core trajectory may allow mutual
synchronisation to be achieved in multi-oscillator networks by tuning the
spatial character of the dynamics within shared magnetic layers.Comment: 21 pages, 8 figure
Direct observation of magnetization dynamics generated by nano-contact spin-torque vortex oscillators
Time-resolved scanning Kerr microscopy has been used to directly image the
magnetization dynamics of nano-contact (NC) spin-torque vortex oscillators
(STVOs) when phase-locked to an injected microwave (RF) current. The Kerr
images reveal free layer magnetization dynamics that extend outside the NC
footprint, where they cannot be detected electrically, but which are crucial to
phase-lock STVOs that share common magnetic layers. For a single NC, dynamics
were observed not only when the STVO frequency was fully locked to that of the
RF current, but also for a partially locked state characterized by periodic
changes in the core trajectory at the RF frequency. For a pair of NCs, images
reveal the spatial character of dynamics that electrical measurements show to
have enhanced amplitude and reduced linewidth. Insight gained from these images
may improve understanding of the conditions required for mutual phase-locking
of multiple STVOs, and hence enhanced microwave power emission.Comment: 10 pages, 3 figure
Spin polarization and barrier oxidation effects at the Co/alumina interface in magnetic tunnel junctions
Copyright © 2004 American Institute of PhysicsThe electronic structure and polarization in magnetic tunnel junctions prepared with varying degrees of barrier-layer oxidation have been studied using x-ray absorption spectroscopy across the Co L2,3 absorption edges. It was found that the Co electronic structure near the Co∕alumina interface tended to that of cobalt oxide as the barrier oxidation time was increased. However, the net Co 3d spin polarization, determined from x-ray magnetic circular dichroism, increased for moderate oxidation times compared to that obtained for an under-oxidized Co∕Al interface. It is proposed that the expected dilution of the measured polarization due to the formation of (room temperature) paramagnetic cobalt oxide, is offset by an increase in the Co 3d spin-polarization of the interface layer as the interface bonding changes from Co–Al to Co–O with increasing oxidation times
Time-resolved investigation of magnetization dynamics of arrays of non-ellipsoidal nanomagnets with a non-uniform ground state
We have performed time-resolved scanning Kerr microscopy (TRSKM) measurements
upon arrays of square ferromagnetic nano-elements of different size and for a
range of bias fields. The experimental results were compared to micromagnetic
simulations of model arrays in order to understand the non-uniform precessional
dynamics within the elements. In the experimental spectra two branches of
excited modes were observed to co-exist above a particular bias field. Below
the so-called crossover field, the higher frequency branch was observed to
vanish. Micromagnetic simulations and Fourier imaging revealed that modes from
the higher frequency branch had large amplitude at the center of the element
where the effective field was parallel to the bias field and the static
magnetization. Modes from the lower frequency branch had large amplitude near
the edges of the element perpendicular to the bias field. The simulations
revealed significant canting of the static magnetization and the effective
field away from the direction of the bias field in the edge regions. For the
smallest element sizes and/or at low bias field values the effective field was
found to become anti-parallel to the static magnetization. The simulations
revealed that the majority of the modes were de-localized with finite amplitude
throughout the element, while the spatial character of a mode was found to be
correlated with the spatial variation of the total effective field and the
static magnetization state. The simulations also revealed that the frequencies
of the edge modes are strongly affected by the spatial distribution of the
static magnetization state both within an element and within its nearest
neighbors
Hubble Residuals of Nearby Type Ia Supernovae Are Correlated with Host Galaxy Masses
From Sloan Digital Sky Survey u'g'r'i'z' imaging, we estimate the stellar
masses of the host galaxies of 70 low redshift SN Ia (0.015 < z < 0.08) from
the hosts' absolute luminosities and mass-to-light ratios. These nearby SN were
discovered largely by searches targeting luminous galaxies, and we find that
their host galaxies are substantially more massive than the hosts of SN
discovered by the flux-limited Supernova Legacy Survey. Testing four separate
light curve fitters, we detect ~2.5{\sigma} correlations of Hubble residuals
with both host galaxy size and stellar mass, such that SN Ia occurring in
physically larger, more massive hosts are ~10% brighter after light curve
correction. The Hubble residual is the deviation of the inferred distance
modulus to the SN, calculated from its apparent luminosity and light curve
properties, away from the expected value at the SN redshift. Marginalizing over
linear trends in Hubble residuals with light curve parameters shows that the
correlations cannot be attributed to a light curve-dependent calibration error.
Combining 180 higher-redshift ESSENCE, SNLS, and HigherZ SN with 30 nearby SN
whose host masses are less than 10^10.8 solar masses in a cosmology fit yields
1+w=0.22 +0.152/-0.143, while a combination where the 30 nearby SN instead have
host masses greater than 10^10.8 solar masses yields 1+w=-0.03 +0.217/-0.108.
Progenitor metallicity, stellar population age, and dust extinction correlate
with galaxy mass and may be responsible for these systematic effects. Host
galaxy measurements will yield improved distances to SN Ia.Comment: 16 pages, 6 figures, published in ApJ, minor change
Heavily loaded ferrite-polymer composites to produce high refractive index materials at centimetre wavelengths
A cold-pressing technique has been developed for fabricating composites composed of a polytetrafluoroethylene-polymer matrix and a wide range of volume-fractions of MnZn-ferrite filler (0%–80%). The electromagnetic properties at centimetre wavelengths of all prepared composites exhibited good reproducibility, with the most heavily loaded composites possessing simultaneously high permittivity (180 ± 10) and permeability (23±2). The natural logarithm of both the relative complex permittivity and permeability shows an approximately linear dependence with the volume fraction of ferrite. Thus, this simple method allows for the manufacture of bespoke materials required in the design and construction of devices based on the principles of transformation optics
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