Fluctuation effects in ultralow temperature easy-plane magnets with dipole interaction

High-temperature expansion of free energy is obtained by taking into account the spin fluctuations for an ultralow temperature easy-plane magnet with comparable values of exchange, dipole and one-ion interactions. The dependence of the thermodynamic properties, including Curie temperature, on system parameters is analyzed. It is shown that spin fluctuations may inhibit or even completely prevent the long-range magnetic ordering predicted by the molecular field theory.

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