Exchange interactions in the genuine organic ferromagnet accompanying pressure-induced ferro- to antiferromagnetic transition

The continuous variation of magnetic intermolecular interactions has been s tudied in the pressurized genuine organic ferromagnet, beta-phase para-nitr ophenyl nitronyl nitroxide (T-c = 0.61 K), in which a pressure-induced ferr o- to antiferromagnetic transition was observed under pressure around 6.5 k bar at 0.45 K. The rotation of the radical group C6H4NO2 around its symmetr ical axis in crystallized molecules is pointed out to be essential to chang e magnitude and a sign of the dominant exchange interactions, rather than t he shrinkage of the lattice parameters under pressure. (C) 1999 Elsevier Sc ience B.V. All rights reserved.