261 research outputs found
Observation of a Distribution of Internal Transverse Magnetic Fields in a Mn12-Based Single Molecule Magnet
A distribution of internal transverse magnetic fields has been observed in
single molecule magnet (SMM) Mn12-BrAc in the pure magnetic quantum tunneling
(MQT) regime. Magnetic relaxation experiments at 0.4 K are used to produce a
hole in the distribution of transverse fields whose angle and depth depend on
the orientation and amplitude of an applied transverse ``digging field.'' The
presence of such transverse magnetic fields can explain the main features of
resonant MQT in this material, including the tunneling rates, the form of the
relaxation and the absence of tunneling selection rules. We propose a model in
which the transverse fields originate from a distribution of tilts of the
molecular magnetic easy axes.Comment: 4 page
Millikelvin magnetic relaxation measurements of alpha-Fe2O3 antiferromagnetic particles
In this paper we report magnetic relaxation data for antiferromagnetic
alpha-Fe2O3 particles of 5 nm mean diameter in the temperature range 0.1 K to
25 K. The average spin value of these particles S=124 and the uniaxial
anisotropy constant D=1.6x10^-2 K have been estimated from the experimental
values of the blocking temperature and anisotropy field. The observed plateau
in the magnetic viscosity from 3 K down to 100 mK agrees with the occurrence of
spin tunneling from the ground state Sz = S. However, the scaling M vs Tln(nu
t) is broken below 5 K, suggesting the occurrence of tunneling from excited
states below this temperature.Comment: 4 pages (two columns), 4 figure
Asymmetric Berry-Phase Interference Patterns in a Single-Molecule Magnet
A Mn4 single-molecule magnet displays asymmetric Berry-phase interference
patterns in the transverse-field (HT) dependence of the magnetization tunneling
probability when a longitudinal field (HL) is present, contrary to symmetric
patterns observed for HL=0. Reversal of HL results in a reflection of the
transverse-field asymmetry about HT=0, as expected on the basis of the
time-reversal invariance of the spin-orbit Hamiltonian which is responsible for
the tunneling oscillations. A fascinating motion of Berry-phase minima within
the transverse-field magnitude-direction phase space results from a competition
between noncollinear magnetoanisotropy tensors at the two distinct Mn sites.Comment: 4 double-column page
Linear viscoelastic properties of high reclaimed asphalt content mixes with biobinders
The use of high Reclaimed Asphalt (RA) content mixtures together with binders produced from renewable resources (biobinders) is one of the current challenges in pavement engineering research. On one hand, RA has been used for decades but there are still some concerns about its performance, especially when high contents are used (>30%). On the other hand, biobinders are relatively new materials which have to be deeply characterised and studied in order to develop good-practices for their use. In this paper, linear viscoelastic properties of biobinders and bio-mixtures manufactured with high-RA content and biobinders are analysed and discussed. High-modulus mixtures with 50% RA were selected for the mix design. Binders and mixtures were tested over a wide range of asphalt service temperatures and frequencies by means of DSR and two-point bending tests respectively. Results show that biobinders have an important effect on mixtures behaviour. However, no direct links between their linear viscoelastic properties were found. Bio-asphalt mixtures still need further development for commercial exploitation; however the take-away fact of this investigation is that it is possible to manufacture asphalt-like mixtures with acceptable viscoelastic properties while being composed only of RA and non-petroleum based binders
Electron Paramagnetic Resonance Linewidths and Lineshapes for the Molecular Magnets Fe8 and Mn12
We study theoretically Electron Paramagentic Resonance (EPR) linewidths for
single crystals of the molecular magnets Fe and Mn as functions of
energy eigenstates , frequency, and temperature when a magnetic field
along the easy axis is swept at fixed excitation frequency. This work was
motivated by recent EPR experiments. To calculate the linewidths, we use
density-matrix equations, including dipolar interactions and distributions of
the uniaxial anisotropy parameter and the Land\'{e} factor. Our
calculated linewidths agree well with the experimental data. We also examine
the lineshapes of the EPR spectra due to local rotations of the magnetic
anisotropy axes caused by defects in samples. Our preliminary results predict
that this effect leads to asymmetry in the EPR spectra.Comment: 2001 MMM conferenc
Relaxation and Landau-Zener experiments down to 100 mK in ferritin
Temperature-independent magnetic viscosity in ferritin has been observed from
2 K down to 100 mK, proving that quantum tunneling plays the main role in these
particles at low temperature. Magnetic relaxation has also been studied using
the Landau-Zener method making the system crossing zero resonant field at
different rates, alpha=dH/dt, ranging from 10^{-5} to 10^{-3} T/s, and at
different temperatures, from 150 mK up to the blocking temperature. We propose
a new Tln(Delta H_{eff}/tau_0 alpha) scaling law for the Landau-Zener
probability in a system distributed in volumes, where Delta H_{eff} is the
effective width of the zero field resonance.Comment: 13 pages, 4 postscript figure
Definitive spectroscopic determination of the transverse interactions responsible for the magnetic quantum tunneling in Mn12-acetate
We present detailed angle-dependent single crystal electron paramagnetic
resonance (EPR) data for field rotations in the hard plane of the S=10 single
molecule magnet Mn12-acetate. A clear four-fold variation in the resonance
positions may be attributed to an intrinsic fourth order transverse anisotropy
(O44). Meanwhile, a four-fold variation of the EPR lineshapes confirms a
recently proposed model wherein disorder associated with the acetic acid of
crystallization induces a locally varying quadratic (rhombic) transverse
anisotropy (O22). These findings explain most aspects of the magnetic quantum
tunneling observed in Mn12-acetate.Comment: 7 pages, including figures, accepted for publication in Phys. Rev.
Let
Intrinsic adhesive and cohesive assessment of the moisture sensitivity of bio-rejuvenated recycled asphalt binders
Alternative binders not derived from fossil fuels, known as biobinders, are opening new paths for multiple applications in road infrastructure. Biobinders, usually produced from bio-oils obtained from the processing of biomass and industry by-products, are tuneable materials whose properties can be adjusted to meet specific targets. For this reason, an interesting approach is to couple biobinders with Reclaimed Asphalt (RA) by taking advantage of their rejuvenating properties to design bio-asphalt mixtures with high-content RA and no additional virgin bitumen. Recent research has proven the feasibility of this approach through validation at full-scale (BioRePavation project). However certain aspects related to the durability of bio-asphalt mixtures still require further research, one of these being their resistance to moisture damage. This study aims at filling some of these current gaps by conducting an initial investigation of the moisture sensitivity of selected biobinders and bio-rejuvenated asphalt binders. In order to do this, the intrinsic adhesion and cohesion properties of an extracted RA binder, two biobinders, their blends and two types of aggregates were characterised by means of Surface Free Energy (SFE), individually and as a system. The binders/blends-aggregate systems were further tested by means of the Pneumatic Adhesion Tensile Test Instrument (PATTI) to determine their pull-off tensile strength (POTS). The results show that the bio-rejuvenated asphalt binders present equivalent cohesive and adhesive properties to a conventional bitumen and superior performance when compared to the RA binder. Hence, the combination of biobinders and RA has great potential to guarantee resistance to moisture damage of bio-recycled asphalt mixtures with high-content RA and no additional bitumen
Incommensurate Transverse Anisotropy Induced by Disorder and Spin-Orbit-Vibron Coupling in Mn12-acetate
It has been shown within density-functional theory that in Mn-acetate
there are effects due to disorder by solvent molecules and a coupling between
vibrational and electronic degrees of freedom. We calculate the in-plane
principal axes of the second-order anisotropy caused by the second effect and
compare them with those of the fourth-order anisotropy due to the first effect.
We find that the two types of the principal axes are not commensurate with each
other, which results in a complete quenching of the tunnel-splitting
oscillation as a function of an applied transverse field.Comment: Will be presented at MMM conference 200
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