Magnetic ordering in some organic molecular magnets

Summary form only given. In order to elucidate correlation between magnetic ordering and a crystal structure of organic molecular magnets, we have investigated muon spin rotation (vSR), electron spin resonance (ESR) and a.c. susceptibility of the organic neutral radicals, 3-quinolyl nitronyl nitroxide (3-QNNN) andp-pyridyl nitron´l nitroxide (p-PYNN). The onset of magnetic order is indicated in the JASR measurements by the appearance of a rotation signal in zero applied field for temperatures below 0.21 K for 3-QNNN and 0.10 K forp-PYNN. The temperature dependence of internal field of 3-QNNN, derived from the muon spin rotation and relaxation signals, is consistent with the S = 1/2 Weiss molecular field model. T´here is further evidence for the magnetic order in the temperature dependence of a.c. susceptibility as a small dip at 0.21 K for 3-QNNN and a sharp peak at 0.12 K forp-PYNN. The nature of the magnetic order is discussed in terms of a canted ferromagnetic spin structure as well as spin glass behavior. Short-range ferromagnetic order is manifested for 3-QNNN andp-PYNN in the ESR measurements above the ordering temperature through a significant shift of resonance field compared with that near room temperature. The shift is highly anisotropic and should be related to the molecular orientations in the crystal. It is suggested that preferred spin direction is approximately perpendicular to the O-N-C N-0 atomic chain of nitronyl nitroxide molecular moiety and lies on its molecular plane.