High Frequency ESR study of the two-dimensional Heisenberg antiferromagnet copper pyrazine perchlorate.

Copper pyrazine perchlorate, Cu(Pyrazine)$_{2}$(ClO$_{4})_{2}$, is thought to be an almost prefect 2D Heisenberg antiferromagnet. We have performed single crystal electron magnetic resonance experiments at 120, 240, and 336 GHz on this system. Both the line-width and resonance shift are field-dependant and indicate that the 3D antiferromagnetic ordering temperature increases from 4.2 K at zero field to about 7 K at 12 Tesla. This can be interpreted by a the field-induced XY-behavior, as has been predicted[1]. A. Cuccoli et al., \textit{Phys. Rev. B} \textbf{68}, 060402 (2003).