19,630 research outputs found
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
Josephson Vortex States in Intermediate Fields
Motivated by recent resistance data in high superconductors in fields
{\it parallel} to the CuO layers, we address two issues on the Josephson-vortex
phase diagram, the appearances of structural transitions on the observed first
order transition (FOT) curve in intermediate fields and of a lower critical
point of the FOT line. It is found that some rotated pinned solids are more
stable than the ordinary rhombic pinned solids with vacant interlayer spacings
and that, due to the vertical portion in higher fields of the FOT line, the FOT
tends to be destroyed by creating a lower critical point.Comment: 12 pages, 3 figures. To appear in J.Phys.Soc.Jpn. 71, No.2 (February,
2002
Theoretical Description of Resistive Behavior near a Quantum Vortex-Glass Transition
Resistive behaviors at nonzero temperatures (T > 0) reflecting a quantum
vortex-glass (VG) transition (the so-called field-tuned
superconductor-insulator transition at T=0) are studied based on a quantum
Ginzburg-Landau (GL) action for a s-wave pairing case containing microscopic
details. The ordinary dissipative dynamics of the pair-field is assumed on the
basis of a consistency between the fluctuation conductance terms excluded from
GL approach and an observed negative magnetoresistance. It is shown that the VG
contribution, G_{vg}(B=B_{vg}, T \to 0),to 2D fluctuation conductance at the VG
transition field B_{vg} depends on the strength of a repulsive-interaction
between electrons and takes a universal value only in the ordinary dirty limit
neglecting the electron-repulsion. Available resistivity data near B_{vg} are
discussed based on our results, and extensions to the cases of a d-wave pairing
and of 3D systems are briefly commented on.Comment: Explanation of data in strongly disordered case, as well as Fig.2 and
3, was renewed, and comments on recent publications were added. To appear in
J.Phys.Soc. Jp
Universal Irreversibility of Normal Quantum Diffusion
Time-reversibility measured by the deviation of the perturbed time-reversed
motion from the unperturbed one is examined for normal quantum diffusion
exhibited by four classes of quantum maps with contrastive physical nature.
Irrespective of the systems, there exist a universal minimal quantum threshold
above which the system completely loses the past memory, and the time-reversed
dynamics as well as the time-reversal characteristics asymptotically trace
universal curves independent of the details of the systems.Comment: 4 pages, 4 figure
Nonlinear behavior of geometric phases induced by photon pairs
In this study, we observe the nonlinear behavior of the two-photon geometric
phase for polarization states using time-correlated photons pairs. This phase
manifests as a shift of two-photon interference fringes. Under certain
arrangements, the geometric phase can vary nonlinearly and become very
sensitive to a change in the polarization state. Moreover, it is known that the
geometric phase for identically polarized photons is times larger than
that for one photon. Thus, the geometric phase for two photons can become two
times more sensitive to a state change. This high sensitivity to a change in
the polarization can be exploited for precision measurement of small
polarization variation. We evaluate the signal-to-noise ratio of the
measurement scheme using the nonlinear behavior of the geometric phase under
technical noise and highlight the practical advantages of this scheme.Comment: 10 pages, 10 figure
Evidence for realignment of the charge density wave state in ErTe and TmTe under uniaxial stress via elastocaloric and elastoresistivity measurements
We report the evolution of a charge density wave (CDW) state in the quasi-2D
rare-earth tritellurides (Te for =Er,Tm) as a function of in-plane
uniaxial stress. Measurements of the elastocaloric effect, resistivity, and
elastoresistivity allow us to demonstrate the importance of in-plane
antisymmetric strain on the CDW and to establish a phase diagram. We show that
modest tensile stress parallel to the in-plane -axis can reversibly switch
the direction of the ordering wavevector between the two in-plane directions.
This work establishes Te as a promising model system for the study of
strain-CDW interactions in a quasi-2D square lattice.Comment: 18 pages, 12 figure
Uniaxial-Pressure induced Ferromagnetism of Enhanced Paramagnetic Sr3Ru2O7
We report a uniaxial pressure-dependence of magnetism in layered perovskite
strontium ruthenate Sr3Ru2O7. By applying a relatively small uniaxial pressure,
greater than 0.1 GPa normal to the RuO2 layer, ferromagnetic ordering manifests
below 80 K from the enhanced-paramagnet. Magnetization at 1 kOe and 2 K becomes
100 times larger than that under ambient condition. Uniaxial pressure
dependence of Curie temperature T_C suggests the first order magnetic
transition. Origin of this uniaxial-pressure induced ferromagnetism is
discussed in terms of the rotation of RuO6 octahedra within the RuO2 plane.Comment: 8 pages, 3 figures. to be published in Journal of the Physical
Society of Japan, vol.73, No.5 (2004
EFFECT OF SOLVENT ON PROTON TUNNELING IN THE REACTIONS OF 4-NITROPHENYLNITROMETHANE WITH 1-ETHYLPIPERIDINE AND 1,8-DIAZABICYCLO [5.4.0] UNDEC-7-ENE
A kinetic study has been carried out on the proton-transfer reactions from 4-nitrophenylnitromethane (4NPNM) to 1-ethylpiperidine (EP) and to 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in acetonitrile and dichloromethane by means of a stopped-flow method, the attention being focused to solvent effect on kinetic isotope effect (k_f^H/k_f^D) and degree of tunneling. For the 4NPNM-EP system, k_f^H/k_f^D at 25℃ was 17.2 and the activation energy difference, E_af^D-E_af^H, was as large as 10.8 kJmol^-, which suggested that the degree of proton-tunneling was significant. On the contrary, for the 4NPNM-DBU system the degree of proton-tunneling in both acetonitrile and dichloromethane was not so significant as 4NPNM-EP reaction. The semi-empirical analysis on the basis of the Bell\u27s treatment showed that the barrier width for 4NPNM-EP was larger and barrier height became much higher in comparison to the reaction of 4NPNM with the bases such as DBU or tetramethylguanidine (TMG). Those results were considered from the viewpoints of solvation effect on the transition state
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