Effect of local field fluctuations on orientational ordering in random-site dipole systems |
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Authors: | B E Vugmeister H Rabitz |
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Institution: | (1) Department of Chemistry, Princeton University, 08544 Princeton, New Jersey |
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Abstract: | Some peculiarities of dipole ordering in systems with uniaxial or cubic anisotropy with an arbitrary degree of dilution are
analyzed in terms of random local field theory. The approach takes into account the effect of thermal and spatial fluctuations
of the local fields acting on each particle from its neighbors with an accuracy corresponding to that of the Bethe-Paierls
pair clusters approach. We show that ferromagnetic (ferroelectric) structure for uniaxial Ising dipoles distributed on a simple
cubic lattice is intrinsically unstable against the fluctuations of the local fields for any concentration of the dipoles.
This result is quite different from the prediction of the mean-field theory which implies the possibility of ferromagnetic
ordering as a metastable state in field-cooled experiments. The local field fluctuations do not exclude, however, antiferromagnetic
ordering above a certain critical concentration. Ferromagnetic ordering is possible for other types of lattice geometries
and for an amorphous-like dipole distribution above a certain critical concentration. A simple physical explanation of such
behavior is given based on the specific angular dependence of the dipole-dipole interaction that results in a relatively high
value of the local field second moment for simple cubic lattice. |
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Keywords: | Oreintational ordering dipole-dipole interactions soft dipole spheres simple cubic lattice local field long-range order fluctuations |
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