Correlation function and susceptibility of site and bond random Heisenberg paramagnets

The wave‐vector dependent susceptibility and the correlation function for a Heisenberg paramagnet in a Bravais lattice with site or nearest neighbor exchange bond randomness is studied by high temperature series expansion technique. The first five coefficients for the spin correlation function 〈S_k S_−k〉 and susceptibility, χ (k), are calculated for arbitrary k, the sign of the exchange and the spin magnitude. [2, 1] Pade for χ (0) is used to calculate an approximate value for the magnetic transition temperature T_c (random). A correlation length ξ (T) is defined through the use of Ornstein‐Zernike representation for the correlation function for ka≫1. The behavior of ξ (T) and χ (k) is analyzed in terms of a reduced temperature ε=[T−T_c (random)]/T_c (random). It is noted that when ε→0, the range ξ (T) diverges and χ (k) reaches a maximum. The increase in these quantities is the more pronounced the larger the system randomness.