MU-SR STUDIES OF MAGNETISM IN THE ORGANIC-SYSTEMS P-NPNN AND 3-QNNN

Recently, ferromagnetism has been observed in a family of organic molecular crystals based on the nitronyl nitroxide radical group. We report zero-field mu SR measurements on compressed powder samples of the organic magnets p-NPNN and 3-QNNN which have been used to directly probe the temperature dependence of the spontaneous field and the relaxation rate in each system. In p-NPNN, we observe a temperature dependence of the spontaneous field which is very similar to that seen in an aligned single crystal experiment and yield the same Curie temperature (T-c=670 mK). However, the oscillations in the measured signal are very strongly damped, particularly so just below the transition temperature. In 3-QNNN, the oscillations indicate a lower spontaneous field and a smaller Curie temperature (T-c=210 mK).