PRESSURE-DEPENDENCE OF INTERMOLECULAR INTERACTIONS IN THE GENUINE ORGANIC BETA-PHASE P-NITROPHENYL NITRONYL NITROXIDE CRYSTAL ACCOMPANYING A FERROMAGNETIC TO ANTIFERROMAGNETIC TRANSITION

The ferromagnetic to antiferromagnetic transition in genuine organic f erromagnets has been observed for the first time in the pressurized be ta-phase p-nitrophenyl nitronyl nitroxide crystal at the pressure arou nd p(c) = 6.5 +/- 0.5 kbar below 0.5 K, through the measurements of ma gnetic susceptibility and magnetization. The pressure dependence of th e lattice parameters of its F2dd crystal structure, examined up to 12. 6 kbar at room temperature, shows an anomaly near p(c). At ambient pre ssure, the effective intermolecular interaction is estimated to be (zJ ) over tilde/k(B) = 2.3 K by the analysis of the magnetic heat capacit y in the external field of 10 and 30 kOe. The Curie temperature in the ferromagnetic state at p < p(c) decreases with increasing pressure. T n the antiferromagnetic state at p > p(c), however, the transition tem perature starts to increase. The variation of intermolecular interacti ons under pressure, including their signs, is discussed on the basis o f the charge-transfer mechanism and the recent ab initio method for th e exchange interactions.