LOW-TEMPERATURE MAGNETIC-PROPERTIES OF THE FERROMAGNETIC ORGANIC RADICAL, P-NITROPHENYL NITRONYL NITROXIDE

Low-temperature magnetic properties of the beta and gamma phases of p-nitrophenyl nitronyl nitroxide (p-NPNN) were determined by measuring the specific heat, the magnetic susceptibility, and the hysteresis curve of magnetization above He-3 temperature in external magnetic fields. The beta-phase crystal undergoes a bulk ferromagnetic transition at 0.60 K, which was confirmed by the magnetic entropy of R ln2 due to one unpaired electron on the radical molecule and the hysteresis curve. The gamma phase, on the other hand, revealed an antiferromagnetic transition at 0.65 K and one-dimensional ferromagnetic fluctuations above it. The specific-heat data of the gamma phase in external fields were analyzed by a mean-field theory incorporated in the one-dimensional Heisenberg model. The details of sample characterization of each phase based on thermal analysis are also given.