Series expansion for theJ1-J2Heisenberg antiferromagnet on a square lattice

We have developed series expansions about the Ising limit for the ground state energy, magnetization, susceptibility, and energy gap of the frustrated ${\mathit{J}}_1$-${\mathit{J}}_2$ antiferromagnet. We find that the Néel order vanishes at ${\mathit{J}}_2$/${\mathit{J}}_1$≃0.4 and collinear order sets in around ${\mathit{J}}_2$/${\mathit{J}}_1$≃0.6, in broad agreement with other recent work. We also explore the nature of the phase diagram for the spin-anisotropic case. © 1996 The American Physical Society.