LOW-TEMPERATURE SERIES EXPANSIONS FOR THE SQUARE-LATTICE ISING-MODEL WITH FIRST AND 2ND NEIGHBOR INTERACTIONS

We have calculated the low-temperature series expansions of the sponta neous magnetization and susceptibility of the quare-lattice Ising mode l with first and second neighbour interactions to the 24th and 20th or der respectively. We use the Pade approximants to estimate the critica l exponents and find that 0.122 < beta < 0.128 and 1.70 < gamma' < 1.8 5. Our results are consistent with the universality hypothesis which p redicts that square-lattice Ising models with interactions beyond firs t neighbours have the same critical exponents beta = 0.125 and gamma' = 1.75.