IRREDUCIBLE GREEN-FUNCTION THEORY FOR FERROMAGNETS WITH FIRST-NEIGHBOR AND 2ND-NEIGHBOR EXCHANGE

A conceptually new version of the irreducible Green function (IRG) for malism is applied to study a Heisenberg ferromagnet with first-neighbo ur exchange (J(1)) and second-neighbour exchange (J(2)) The most impor tant quantity which appears in IRG formalism is the commutator average lambda = ([I-kk' S-q'(-)) (where k, q' and k' refer to the incoming, outgoing and internal momentum lines, respectively), which is found to be not only non-zero but also independent of k' in a natural way. Thi s replaces the old irreducibility condition lambda = 0 and enables one to recast the equation of motion into the exact Dyson form. For the e stimation of self-energy the reducible operators are mapped onto irred ucible observables, which introduces a parameter xi into the self-ener gy operator. The values of xi for various ratios J(2)/J(1) and for sev eral spins have been found from the results of exact high-temperature series. From the least-squares fit of these values of xi against 1/S, we obtain an estimate of xi for S = infinity. This is then used to eva luate the Curie temperature for S = infinity and a FCC lattice. The re sult is found to be in good agreement with the series result. The theo ry is then applied to EuS which corresponds to a S = 7/2 FCC lattice. Using the experimental values of T-C the exchange parameters J(1) and J(2) have been computed. The results agree very well with the series r esults and with those obtained from spin-wave analysis,