ORGANIC FERROMAGNETISM IN THE NITRONYL NITROXIDES P-NPNN AND 3-QNNN - MUSR, EPR AND AC SUSCEPTIBILITY STUDIES

Following the discovery of organic ferromagnetism in p-NPNN, we report the observation of ferromagnetic ordering in a second nitronyl nitroxide radical system, 3-quinolyl nitronyl nitroxide (3-QNNN), using the muon spin rotation (MUSR) technique. The onset of ferromagnetic order is indicated in the MUSR experiment by the appearance of a rotation signal in zero applied field for temperatures below approximately 0.2 K. This signal corresponds to the precession of the muon spin in the internal field of the ferromagnetic state. We have measured the muon spin rotation and relaxation signals as a function of temperature, giving a measure of the magnetization and spin dynamical properties. The temperature dependence of the internal field is consistent with the S = 1/2 Weiss molecular field model with a Curie temperature of 0.21 K and a saturation internal field of 60 G. We also report additional studies of the magnetic properties of 3-QNNN and p-NPNN using EPR and a.c. susceptibility techniques. We find evidence for a canted ferromagnetic spin structure in 3-QNNN and a simpler uniform magnetic lattice in p-NPNN.