We explore the phase diagram of Ising spins on one-dimensional chains which criss-cross in two perpendicular directions and are connected by interchain couplings. This system is of interest as a simpler, classical analog of a quantum Hamiltonian which has been proposed as a model of magnetic behavior in Nb
12O
29 and also, conceptually, as a geometry which is intermediate between one and two dimensions. Using mean field theory as well as Metropolis Monte Carlo and Wang-Landau simulations, we locate quantitatively the boundaries of four ordered phases. Each becomes an effective Ising model with unique effective couplings at large interchain coupling. Away from this limit we demonstrate non-trivial critical behavior, including tricritical points which separate first and second order phase transitions. Finally, we find that the crossover of the magnetization critical exponent from the Ising to tricritical Ising value shows an unusual non-monotonic behavior.
Monte Carlo calculations have been used to study the behavior of a simple cubic spin-\textonehalf{} Ising model with spatially anisotropic nearest-neighbor coupling. The interaction in the
x and
y directions, which is equal to (${J}_{\ensuremath{\perp}}$), was systematically decreased compared to that in the
z direction (${J}_{\ensuremath{\parallel}}$), and results were obtained for values of $\ensuremath{\Delta}=\frac{{J}_{\ensuremath{\perp}}}{{J}_{\ensuremath{\parallel}}}$ as small as 0.003. For small $\ensuremath{\Delta}$ the system behaves like a one-dimensional Ising chain at high temperature before undergoing a low-temperature transition to a three-dimensionally ordered state. We find that the asymptotic ($\ensuremath{\Delta}\ensuremath{\rightarrow}0$) variation of
Tc is accurate over a surprisingly wide range of $\ensuremath{\Delta}$.