Thermodynamical properties of nuclear spin systems with dipolar interaction in solids. Low-field case

The properties of spin systems subjected to dipolar interactions in thermodynamic equilibrium are considered for arbitrary and zero magnetic field. It has been shown that by converting spin operators to boson annihilation and creation operators and taking into account the complete dipolar Hamiltonian, it is possible to find a partition function and density operator in non-exponential form. The obtained results are valid in a wide region of spin temperatures without restriction to the high temperatures approximation. The exact properties of two spin systems in arbitrary spin temperatures are considered. (author)