324 research outputs found
Dipolar ordering in crystals of Mn12 Ac
Ordering in realistic elongated box-shape crystals of the molecular magnet
Mn_12 Ac is investigated with the site-resolved mean-field approximation that
does not assume a uniform ordering. It is shown that ferromagnetic ordering
should not occur in crystals with the aspect ratio up to 12. Instead, for the
aspect ratio about 6, the inner and outer regions of the crystal order in
different directions, uniformly along its length. Finding ordering temperature
by extrapolating the inverse susceptibility curve does not provide a correct
T_C.Comment: 4 Phys. Rev. pages, 3 figures, submitted to PR
Turbulent fronts of quantum detonation in molecular magnets
Dipolar-controlled quantum deflagration going over into quantum detonation in
the elongated Mn_12 Ac molecular magnet in a strong transverse field has been
considered within the full 3d model. It is shown that within the dipolar window
around tunneling resonances the deflagration front is non-flat. With increasing
bias, dipolar instability makes the front turbulent, while its speed reaches
sonic values, that is a signature of detonation.Comment: 4 PR pages, 5 figure
Susceptibilities and Correlation Functions of the Anisotropic Spherical Model
The static transverse and longitudinal correlation functions (CF) of a
3-dimensional ferromagnet are calculated for the exactly solvable anisotropic
spherical model (ASM) determined as the limit D \to \infty of the classical
D-component vector model. The results are nonequivalent to those for the
standard spherical model of Berlin and Kac even in the isotropic case. Whereas
the transverse CF has the usual Ornstein-Zernike form for small wave vectors,
the longitudinal CF shows a nontrivial behavior in the ordered region caused by
spin-wave fluctuations. In particular, in the isotropic case below T_c one has
S_{zz}(k) \propto 1/k (the result of the spin-wave theory) for k \lsim \kappa_m
\propto T_c-T.Comment: 6 pages, 4 figure
The new integral relaxation time for thermal activation of spins
The integral relaxation time for the difference of populations of the two
potential wells for double-well classical spin systems is introduced. For the
uniaxial symmetry, it is given by a quadrature. Unlike the previously
introduced integral relaxation time for the magnetization, the former at low
temperatures describes the rate of crossing the barrier under all conditions,
including the strongly biased case. In the high-barrier case, the new integral
relaxation time approaches the inverse of the lowest eigenvalue of the
Fokker-Planck equation. It can be more conveniently found by numerical methods
than the latter.Comment: 5 pages, 1 figure. Final published versio
Extended Debye Model for Molecular Magnets
Heat capacity data on Mn12 are fitted within the extended Debye model that
takes into account a continuum of optical modes as well as three different
speeds of sound.Comment: 4 Phys. Rev pages, 4 figures, submitted to Physical Review B as a
Brief Report. In version 3 speeds of sound correcte
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