28 research outputs found
Quadratic transverse anisotropy term due to dislocations in Mn12-Ac directly obtained by EPR spectroscopy
High-Sensitivity Electron Paramagnetic Resonance experiments have been
carried out in fresh and stressed Mn12-Acetate single crystals for frequencies
ranging from 40 GHz up to 110 GHz. The high number of crystal dislocations
formed in the stressing process introduces a E(S_x^2-S_y^2) transverse
anisotropy term in the spin hamiltonian. From the behaviour of the resonant
absorptions on the applied transverse magnetic field we have obtained an
average value for E = 22 mK, corresponding to a concentration of dislocations
per unit cell of c = 10^-3.Comment: 13 pages and 4 figure
Behavior of bulk high-temperature superconductors of finite thickness subjected to crossed magnetic fields
Crossed magnetic field effects on bulk high-temperature superconductors have
been studied both experimentally and numerically. The sample geometry
investigated involves finite-size effects along both (crossed) magnetic field
directions. The experiments were carried out on bulk melt-processed Y-Ba-Cu-O
(YBCO) single domains that had been pre-magnetized with the applied field
parallel to their shortest direction (i.e. the c-axis) and then subjected to
several cycles of the application of a transverse magnetic field parallel to
the sample ab plane. The magnetic properties were measured using orthogonal
pick-up coils, a Hall probe placed against the sample surface and
Magneto-Optical Imaging (MOI). We show that all principal features of the
experimental data can be reproduced qualitatively using a two-dimensional
finite-element numerical model based on an E-J power law and in which the
current density flows perpendicularly to the plane within which the two
components of magnetic field are varied. The results of this study suggest that
the suppression of the magnetic moment under the action of a transverse field
can be predicted successfully by ignoring the existence of flux-free
configurations or flux-cutting effects. These investigations show that the
observed decay in magnetization results from the intricate modification of
current distribution within the sample cross-section. It is also shown that the
model does not predict any saturation of the magnetic induction, even after a
large number (~ 100) of transverse field cycles. These features are shown to be
consistent with the experimental data.Comment: 41 pages, 9 figures, accepted in Phys. Rev. B Changes : 8 references
added, a few precisions added, some typos correcte
High frequency resonant experiments in Fe molecular clusters
Precise resonant experiments on Fe magnetic clusters have been
conducted down to 1.2 K at various tranverse magnetic fields, using a
cylindrical resonator cavity with 40 different frequencies between 37 GHz and
110 GHz. All the observed resonances for both single crystal and oriented
powder, have been fitted by the eigenstates of the hamiltonian . We have identified the
resonances corresponding to the coherent quantum oscillations for different
orientations of spin S = 10.Comment: to appear in Phys.Rev. B (August 2000
High field magnetic resonant properties of beta'-(ET)2SF5CF2SO3
A systematic electron spin resonance (ESR) investigation of the low
temperature regime for the (ET)2SF5CF2SO3 system was performed in the frequency
range of ~200-700 GHz, using backward wave oscillator sources, and at fields up
to 25 T. Newly acquired access to the high frequency and fields shows
experimental ESR results in agreement with the nuclear magnetic resonance (NMR)
investigation, revealing evidence that the transition seen at 20 K is not of
conventional spin-Peierls order. A significant change of the spin resonance
spectrum in beta'-(ET)2SF5CF2SO3 at low temperatures, indicates a transition
into a three-dimensional-antiferromagnetic (3D AFM) phase.Comment: 4 pages, 7 figures, minor grammatical change
High Field Alloy, Thermoelectric, and mm Wave Studies of the Field Induced Superconducting State in λ-(BETS)
Alloy studies in the π -d organic conductor λ-(BETS) 2 Fe x Ga 1−x Cl 4 have given new insight into the nature of field induced superconductivity (FISC), since the mechanism of the FISC involves cancellation of the π − d exchange field by the external field. Alloying on the Fe x Ga 1−x site allows tuning of the exchange field, thereby influencing the FISC phase boundary. A brief review of the low temperature phases are given, and new high magnetic field thermoelectric and mm wave results that probe the low temperature ground state are presented. The discovery of magnetic field induced superconductivity in λ-(BETS) 2 FeCl 4 [1] (BETS= bis(ethylenedithio)tetraselenafulvalene) has drawn attention to the π−d electron spin exchange mechanisms At higher magnetic fields , field induced superconductivity (FISC) is stabilized in λ-(BETS) 2 FeCl 4 below 5 K between 18 and 45