49,774 research outputs found
Correlation between tunneling magnetoresistance and magnetization in dipolar coupled nanoparticle arrays
The tunneling magnetoresistance (TMR) of a hexagonal array of dipolar coupled
anisotropic magnetic nanoparticles is studied using a resistor network model
and a realistic micromagnetic configuration obtained by Monte Carlo
simulations. Analysis of the field-dependent TMR and the corresponding
magnetization curve shows that dipolar interactions suppress the maximum TMR
effect, increase or decrease the field-sensitivity depending on the direction
of applied field and introduce strong dependence of the TMR on the direction of
the applied magnetic field. For off-plane magnetic fields, maximum values in
the TMR signal are associated with the critical field for irreversible rotation
of the magnetization. This behavior is more pronounced in strongly interacting
systems (magnetically soft), while for weakly interacting systems (magnetically
hard) the maximum of TMR (Hmax) occurs below the coercive field (Hc), in
contrast to the situation for non-interacting nanoparticles or in-plane fields
(Hmax=Hc). The relation of our simulations to recent TMR measurements in
self-assembled Co nanoparticle arrays is discussed.Comment: 21 pages, 8 figures, submitted to Physical Review
The circumstellar environment of the YSO TMR-1 and a revisit to the candidate very low-mass object TMR-1C
TMR-1 (IRAS~04361+2547) is a class~I proto-stellar source located in the
nearby Taurus star-forming region. Its circumstellar environment is
characterized by extended dust emission with complex structures and conspicuous
filaments. A faint companion, called TMR-1C, located near the proto-star had
been detected in previous studies, but its nature as a very young substellar
object remained inconclusive. To improve the constraints on the nature of
TMR-1C, and to investigate the process of very low-mass star formation in the
TMR-1 system we use very sensitive infrared imaging observations as well as NIR
spectroscopy. We construct the SED of TMR-1C over a much larger wavelength
range as had been possible in previous work and compare it with models of
extincted background stars, young sub-stellar objects, and very low-mass stars
with circumstellar disk and envelope emission. We also search for additional
low-luminosity objects in the immediate environment of the TMR-1, study the
surrounding NIR dust morphology, and analyse the emission line spectrum of a
filamentary structure in the physical context of a bow-shock model. We find
that the observed SED of TMR-1C is inconsistent with an extincted background
star, nor can be fitted with available models for a young extremely low-mass
(<12M_Jup) object. Our near-IR spectrum indicates an effective temperature of
at least ~3000K. Based on a good match of TMR-1C's SED with radiation transfer
models of young stellar objects with circumstellar disks, we propose that
TMR-1C is most likely a very low-mass star with M~0.1-0.2M_sun surrounded by a
circumstellar disk with high inclination, i>80deg. Moreover, we detect an
additional very faint source, which we call TMR-1D, and that shows a quite
striking symmetry in position with TMR-1C. TMR-1C and TMR-1D may have been
formed from a common triggered star-formation event, caused by... (abstract
abridged)Comment: 15 pages, 11 figures, accepted for publication in A&
The benefits of targeted memory reactivation for consolidation in sleep are contingent on memory accuracy and direct cue-memory associations
Objectives: To investigate how the effects of targeted memory reactivation (TMR) are influenced by memory accuracy prior to sleep and the presence or absence of direct cue-memory associations. Methods: 30 participants associated each of 50 pictures with an unrelated word and then with a screen location in two separate tasks. During picture-location training, each picture was also presented with a semantically related sound. The sounds were therefore directly associated with the picture locations but indirectly associated with the words. During a subsequent nap, half of the sounds were replayed in slow wave sleep (SWS) (TMR). The effect of TMR on memory for the picture locations (direct cue-memory associations) and picture-word pairs (indirect cue-memory associations) was then examined. Results: TMR reduced overall memory decay for recall of picture locations. Further analyses revealed a benefit of TMR for picture locations recalled with a low degree of accuracy prior to sleep, but not those recalled with a high degree of accuracy. The benefit of TMR for low accuracy memories was predicted by time spent in SWS. There was no benefit of TMR for memory of the picture-word pairs, irrespective of memory accuracy prior to sleep. Conclusions: TMR provides the greatest benefit to memories recalled with a low degree of accuracy prior to sleep. The memory benefits of TMR may also be contingent on direct cue-memory associations
Dependence of tunnel magnetoresistance on ferromagnetic electrode materials in MgO-barrier magnetic tunnel junctions
We investigated the relationship between the tunnel magnetoresistance (TMR)
ratio and the electrode structure in MgO-barrier magnetic tunnel junctions
(MTJs). The TMR ratio in a MTJ with Co40Fe40B20 reference and free layers
reached 355% at the post-deposition annealing temperature of Ta=400 degree C.
When Co50Fe50 or Co90Fe10 is used for the reference layer material, no high TMR
ratio was observed. The key to have high TMR ratio is to have highly oriented
(001) MgO barrier/CoFeB crystalline electrodes. The highest TMR ratio obtained
so far is 450% at Ta = 450 degree C in a pseudo spin-valve MTJ.Comment: 6 pages, 2 figures, 1 table. to be published in J. Magn. Magn. Mate
Improved tunneling magnetoresistance at low temperature in manganite junctions grown by molecular beam epitaxy
We report resistance versus magnetic field measurements for a
La0.65Sr0.35MnO3/SrTiO3/La0.65Sr0.35MnO3 tunnel junction grown by
molecular-beam epitaxy, that show a large field window of extremely high
tunneling magnetoresistance (TMR) at low temperature. Scanning the in-plane
applied field orientation through 360^/circ, the TMR shows 4-fold symmetry,
i.e. biaxial anisotropy, aligned with the crystalline axes but not the junction
geometrical long axis. The TMR reaches ~ 1900% at 4K, corresponding to an
interfacial spin polarization of > 95% assuming identical interfaces. These
results show that uniaxial anisotropy is not necessary for large TMR, and lay
the groundwork for future improvements in TMR in manganite junctions.Comment: 6 pages, 7 figures; accepted in Applied Physics Letter
Mechanisms of memory retrieval in slow-wave sleep : memory retrieval in slow-wave sleep
Study Objectives: Memories are strengthened during sleep. The benefits of sleep for memory can be enhanced by re-exposing the sleeping brain to auditory cues; a technique known as targeted memory reactivation (TMR). Prior studies have not assessed the nature of the retrieval mechanisms underpinning TMR: the matching process between auditory stimuli encountered during sleep and previously encoded memories. We carried out two experiments to address this issue. Methods: In Experiment 1, participants associated words with verbal and non-verbal auditory stimuli before an overnight interval in which subsets of these stimuli were replayed in slow-wave sleep. We repeated this paradigm in Experiment 2 with the single difference that the gender of the verbal auditory stimuli was switched between learning and sleep. Results: In Experiment 1, forgetting of cued (vs. non-cued) associations was reduced by TMR with verbal and non-verbal cues to similar extents. In Experiment 2, TMR with identical non-verbal cues reduced forgetting of cued (vs. non-cued) associations, replicating Experiment 1. However, TMR with non-identical verbal cues reduced forgetting of both cued and non-cued associations. Conclusions: These experiments suggest that the memory effects of TMR are influenced by the acoustic overlap between stimuli delivered at training and sleep. Our findings hint at the existence of two processing routes for memory retrieval during sleep. Whereas TMR with acoustically identical cues may reactivate individual associations via simple episodic matching, TMR with non-identical verbal cues may utilise linguistic decoding mechanisms, resulting in widespread reactivation across a broad category of memories
Giant tunnel magnetoresistance and high annealing stability in CoFeB/MgO/CoFeB magnetic tunnel junctions with synthetic pinned layer
We investigated the relationship between tunnel magnetoresistance (TMR) ratio
and the crystallization of CoFeB layers through annealing in magnetic tunnel
junctions (MTJs) with MgO barriers that had CoFe/Ru/CoFeB synthetic ferrimagnet
pinned layers with varying Ru spacer thickness (tRu). The TMR ratio increased
with increasing annealing temperature (Ta) and tRu, reaching 361% at Ta = 425C,
whereas the TMR ratio of the MTJs with pinned layers without Ru spacers
decreased at Ta over 325C. Ruthenium spacers play an important role in forming
an (001)-oriented bcc CoFeB pinned layer, resulting in a high TMR ratio through
annealing at high temperatures.Comment: 10 pages, 5 figures, submitted to Applied Physics Letter
A near-infrared variability campaign of TMR-1: New light on the nature of the candidate protoplanet TMR-1C
(abridged) We present a near-infrared (NIR) photometric variability study of
the candidate protoplanet, TMR-1C, located at a separation of about 10" (~1000
AU) from the Class I protobinary TMR-1AB in the Taurus molecular cloud. Our
campaign was conducted between October, 2011, and January, 2012. We were able
to obtain 44 epochs of observations in each of the H and Ks filters. Based on
the final accuracy of our observations, we do not find any strong evidence of
short-term NIR variability at amplitudes of >0.15-0.2 mag for TMR-1C or
TMR-1AB. Our present observations, however, have reconfirmed the
large-amplitude long-term variations in the NIR emission for TMR-1C, which were
earlier observed between 1998 and 2002, and have also shown that no particular
correlation exists between the brightness and the color changes. TMR-1C became
brighter in the H-band by ~1.8 mag between 1998 and 2002, and then fainter
again by ~0.7 mag between 2002 and 2011. In contrast, it has persistently
become brighter in the Ks-band in the period between 1998 and 2011. The (H-Ks)
color for TMR-1C shows large variations, from a red value of 1.3+/-0.07 and
1.6+/-0.05 mag in 1998 and 2000, to a much bluer color of -0.1+/-0.5 mag in
2002, and then again a red color of 1.1+/-0.08 mag in 2011. The observed
variability from 1998 to 2011 suggests that TMR-1C becomes fainter when it gets
redder, as expected from variable extinction, while the brightening observed in
the Ks-band could be due to physical variations in its inner disk structure.
The NIR colors for TMR-1C obtained using the high precision photometry from
1998, 2000, and 2011 observations are similar to the protostars in Taurus,
suggesting that it could be a faint dusty Class I source. Our study has also
revealed two new variable sources in the vicinity of TMR-1AB, which show
long-term variations of ~1-2 mag in the NIR colors between 2002 and 2011.Comment: Accepted in A&
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