Measurements of magnetic susceptibility of a genuine organic bulk-ferromagnet at high pressures in a diamond anvil cell

We studied the effect of pressure P on a genuine organic bulk-ferromagnet, 2,5-difluorophenyl-alpha -nitronyl nitroxide (2,5-DFPNN), in the region up to 9.2 GPa, by measuring the ac magnetic susceptibility chi (ac) in a diamo nd anvil cell (DAC). The transition temperature T-c of 2,5-DFPNN at ambient pressure is 0.45 K. T-c is enhanced almost linearly against the pressure u p to P less than or equal to2.6 GPa, but that enhancement tends to saturate at P greater than or equal to 2.6 GPa. T-c finally increases up to 1.36 K at P = 3.7 GPa. On the other hand, the magnitude of chi (ac) is suppressed by the pressure, and the ferromagnelic signal finally disappears at P great er than or equal to 4.6 GPa. We consider that the enhancement of T-c and th e decrease of the magnitude of chi (ac) are due to the increase of the two- dimensional ferromagnetic intermolecular interaction on a plane including t he a-axis and the decrease of the ferromagnetic interaction along a third a xis. (C) 2001 Elsevier Science Ltd. All rights reserved.