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

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

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

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