Physical review. B, Solid state
High-Temperature-Series Study of Models Displaying Tricritical Behavior. II. A Nearest-Neighbor Ising Antiferromagnet with Next-Nearest-Neighbor Ferromagnetic Interactions
Fredric Harbus, Eugene Stanley
Abstract
The nnn model is an Ising model with nearest-neighbor antiferromagnetic interactions (${J}_{1}<0$) but also next-nearest-neighbor ferromagnetic exchange (${J}_{2}>0$). This model is analyzed in external magnetic field using the same techniques as applied to the meta model of Paper I. Again, the staggered susceptibility ${\ensuremath{\chi}}_{\mathrm{st}}$ appears to diverge along the critical line in the $H\ensuremath{-}T$ plane with a constant exponent ${\ensuremath{\gamma}}_{\mathrm{st}}=\frac{5}{4}$, consistent with the universality hypothesis. However, in contrast to the meta model, it is found that the direct susceptibility $\ensuremath{\chi}$ diverges at the tricritical point with an exponent $\overline{\ensuremath{\gamma}}\ensuremath{\simeq}\frac{1}{4}$. Implications of the scaling hypothesis at the tricritical point are discussed and the results for both the meta and nnn models are utilized to obtain the scaling power ${\overline{a}}_{2}$ corresponding to the "weak" direction (in the sense of Griffiths and Wheeler). Included in this discussion is the double-power-law form, predicted to hold within the crossover region by tricritical-point scaling.