Field dependent ordering temperature in copper pyrazine perchlorate, Cu(pz)$_{2}$(ClO$_{4}$)$_{2}$

Copper pyrazine perchlorate is a molecular-based 2D S=1/2 Heisenberg antiferromagnet (QHAF) with a moderate exchange constant (J/k = 17.5 K) and a saturation field of 60 T. The zero-field ordering temperature, as recently determined by muon spin relaxation experiments [1], is 4.3 K corresponding to excellent isolation (J$'$/J $\approx $ 8x10$^{-4})$ between magnetic layers [2]. Recent studies of Cu(pz)$_{2}$(ClO$_{4})_{2}$ in applied fields (specific heat and ESR) show the ordering transition to increase with field by as much as 30{\%} in a field of nine tesla. This effect will be discussed in terms of a field-induced anisotropy crossover model [3]. 1. T. Lancaster, S. J. Blundell et al, submitted for publication. 2. P. Sengupta, A. W. Sandvik, and R. R. P. Singh, Phys. Rev. B \textbf{68}, 094423 (2003). 3. A. Cuccoli et al, Phys. Rev. B \textbf{68}, 060402 (2003).