Field-induced X Y and Ising ground states in a quasi-two-dimensional S = 1 2 Heisenberg antiferromagnet

High field specific heat up to 35 T, ${C}_{p}$, and magnetic susceptibility, \ensuremath{\chi}, measurements were performed on the quasi-two-dimensional (2D) Heisenberg antiferromagnet [Cu(pyz)${}_{2}$(pyO)${}_{2}$](PF${}_{6}$)${}_{2}$. While no ${C}_{p}$ anomaly is observed down to 0.5 K in zero magnetic field, the application of field parallel to the crystallographic $ab$-plane induces a lambda-like anomaly in ${C}_{p}$, suggesting Ising-type magnetic order. On the other hand when the field is parallel to the $c$-axis, ${C}_{p}$ and \ensuremath{\chi} show evidence of $XY$-type antiferromagnetism. This dependence upon the field orientation occurs because the extreme two-dimensionality allows the intrinsic (zero field) spin anisotropy to dominate the interlayer coupling, which has hitherto masked such effects in other materials.