High Field Calorimetry Study on Magnetic Phase Diagram of Quasi-2D QHAF compound Cu(pz)$_2$(ClO$_4$)$_2$

The ordering temperature of the quasi-2D quantum Heisenberg antiferromagnetic compound Cu(pz)$_2$(ClO$_4$)$_2$ has been found to be strongly influenced by the external field [1]. The ordering temperature of a single crystal of Cu(pz)$_2$(ClO$_4$)$_2$ (less than 1~mg) was determined by calorimetry measurement in applied DC field up to 33T at NHMFL. The magnetic phase diagram shows a round maximum at 18 T (about 1/3 of its saturation field) at which the ordering temperature was raised by 30\%. The experimental result is consistent with a recent prediction for the ideal 2D QHAF system [2], indicating a field induced Heisenberg to XY crossover behavior. Similar crossover behavior has recently been observed between uniaxial and isotropic ordering in a related compound [3]. [1] N. Tsyrulin, F. Xiao et al, submitted to {\it Phys. Rev. B},2009. [2] A. Cuccoli, T. Roscilde, R. Vaia, and P. Verrucchi, {\it Phys. Rev. B}, {\bf 68}, 060402 (2003). [3]P. Sengupta et al, {\it Phys. Rev. B}, {\bf 79},060409(R) (2009).