132 research outputs found
Magneto-optical imaging of magnetic deflagration in Mn12-Acetate
For the first time, the morphology and dynamics of spin avalanches in
Mn12-Acetate crystals using magneto-optical imaging has been explored. We
observe an inhomogeneous relaxation of the magnetization, the spins reversing
first at one edge of the crystal and a few milliseconds later at the other end.
Our data fit well with the theory of magnetic deflagration, demonstrating that
very slow deflagration rates can be obtained, which makes new types of
experiments possible.Comment: 5 two-column pages, 3 figures, EPL styl
Switching of Magnetic Moments of Nanoparticles by Surface Acoustic Waves
We report evidence of the magnetization reversal in nanoparticles by surface
acoustic waves (SAWs). The experimental system consists of isolated magnetite
nanoparticles dispersed on a piezoelectric substrate. Magnetic relaxation from
a saturated state becomes significantly enhanced in the presence of the SAW at
a constant temperature of the substrate. The dependence of the relaxation on
SAW power and frequency has been investigated. The effect is explained by the
effective ac magnetic field generated by the SAW in the nanoparticles.Comment: Accepted in Europhysics Letter
Field induced magnetic transition and metastability in Co substituted
A detailed investigation of first order ferrimagnetic (FRI) to
antiferromagnetic (AFM) transition in Co (15%) doped is carried out.
These measurements demonstrate anomalous thermomagnetic irreversibility and
glass-like frozen FRI phase at low temperatures. The irreversibility arising
between the supercooling and superheating spinodals is distinguised in an
ingenious way from the irreversibility arising due to kinetic arrest. Field
annealing measurements shows reentrant FRI-AFM-FRI transition with increasing
temperature. These measurements also show that kinetic arrest band and
supercooling band are anitcorrelated i.e regions which are kinetically arrested
at higher temperature have lower supercooling temperature and vice versa.Comment: 10 pages, 8 figure
Quantum Magnetic Deflagration in Mn12 Acetate
We report controlled ignition of magnetization reversal avalanches by surface
acoustic waves in a single crystal of Mn12 acetate. Our data show that the
speed of the avalanche exhibits maxima on the magnetic field at the tunneling
resonances of Mn12. Combined with the evidence of magnetic deflagration in Mn12
acetate (Suzuki et al., cond-mat/0506569) this suggests a novel physical
phenomenon: deflagration assisted by quantum tunneling.Comment: 4 figure
Low temperature microwave emission from molecular clusters
We investigate the experimental detection of the electromagnetic radiation
generated in the fast magnetization reversal in Mn12-acetate at low
temperatures. In our experiments we used large single crystals and assemblies
of several small single crystals of Mn12-acetate placed inside a cylindrical
stainless steel waveguide in which an InSb hot electron device was also placed
to detect the radiation. All this was set inside a SQUID magnetometer that
allowed to change the magnetic field and measure the magnetic moment and the
temperature of the sample as the InSb detected simultaneously the radiation
emitted from the molecular magnets. Our data show a sequential process in which
the fast inversion of the magnetic moment first occurs, then the radiation is
detected by the InSb device, and finally the temperature of the sample
increases during 15 ms to subsequently recover its original value in several
hundreds of milliseconds.Comment: changed conten
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
The role of temperature in the magnetic irreversibility of type-I Pb superconductors
Evidence of how temperature takes part in the magnetic irreversibility in the
intermediate state of a cylinder and various disks of pure type-I
superconducting lead is presented. Isothermal measurements of first
magnetization curves and magnetic hysteresis cycles are analyzed in a reduced
representation that defines an equilibrium state for flux penetration in all
the samples and reveals that flux expulsion depends on temperature in the disks
but not in the cylinder. The magnetic field at which irreversibility sets in
along the descending branch of the hysteresis cycle and the remnant
magnetization at zero field are found to decrease with temperature in the
disks. The contributions to irreversibility of the geometrical barrier and the
energy minima associated to stress defects that act as pinning centers on
normal-superconductor interfaces are discussed. The differences observed among
the disks are ascribed to the diverse nature of the stress defects in each
sample. The pinning barriers are suggested to decrease with the magnetic field
to account for these results
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