Magnetic-field-driven quantum criticality of the Ising-class square lattice Cr(dien) (O2)2·H2O and the orientation depend

Abstract

The magnetic phase diagram of Cr(dien)(O 2 ) 2 • H 2 O (dien = C 4 H 13 N 3 ), a quasi-2D spin-1 antiferromagnet known to exhibit quantum phase behavior, is reported. Specific heat, torque magnetometry, and magnetocalorimetry were employed to obtain the phase diagram down to 200 mK. Near the T → 0 limit, the antiferromagnetic phase defers to a field-induced ferromagnetic phase at a critical field, HC . Analysis of the phase boundary through fitting with (H − HC ) = βT α yielded HC and the critical exponent α of the spin system. HC and α were found to be 15.2 ±0.02 T and 2.05 ± 0.09 respectively when H is parallel to the easy magnetic axis, and 12.4 ± 0.01 T and 1.91 ± 0.06 when perpendicular to it. These critical exponents are indicative of an Ising-type spin system, which was unanticipated result. Further, torque magnetometry detected a spin-flop transition of unknown origin over a wide orientation range of nearly 70°in the ab plane. Mean field theory yielded an estimate of the long sought-after single-ion anisotropy for this compound, D ≈ 0.25 K.