R.M. Valladares

Organic ferromagnetism in the nitronyl nitroxides p-NPNN and 3-QNNN: MUSR, EPR and a.c. 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-quinoyl 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 ∼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 = 12 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.

Observation of a magnetic transition in para-pyridyl nitronyl nitroxide using zero-field μSR

We report the observation of a magnetic transition at around 90 mK in the nitronyl nitroxide radical system para-pyridyl nitronyl nitroxide (p-PYNN) in zero applied field using the muon-spin rotation (μSR) technique.

Muon-spin-rotation and magnetization study of metal-organic magnets based on the dicyanamide anion

We report the results of a study of the metal-organic magnets MII[N(CN)2]2, where MII = Ni, Co and Mn, using bulk magnetization and muon-spin relaxation (µSR). Implanted muons are sensitive to the onset of long-range magnetic order in each of these materials and strong muon-spin relaxation is observed in the paramagnetic state due to low-frequency fluctuations of the electronic moments in the 109-1010 Hz range. The size of the muon-spin relaxation in the paramagnetic state can be related to the magnitude of the transition-metal-ion moment. Very strongly damped oscillations are observed below the magnetic transition temperature in Co[N(CN)2]2.

Magnetism in the nitronyl nitroxide isomers 1-NAPNN and 2-NAPNN studied by

We report the results of muon-spin-rotation/relaxation () experiments on two recently discovered nitronyl nitroxides, 1- and 2-naphthyl nitronyl nitroxide (1-NAPNN and 2-NAPNN). These two compounds are chemical isomers but, because of their slightly different molecular shapes, they have quite different crystal packing. A clear magnetic transition in the zero-field muon-spin relaxation is observed in 1-NAPNN below 100 mK while no such transition is seen in 2-NAPNN. These results support the postulate that the crystal structure strongly influences the nature of the magnetic ground states in these materials.

μSR studies of magnetism in the organic systems p-NPNN and 3-QNNN

Abstract Recently, ferromagnetism has been observed in a family of organic molecular crystals based on the nitronyl nitroxide radical group. We report zero-field μSR 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).

μSR studies of spin excitations in poylaniline

Abstract Muon techniques have been used to study the spin excitations in polyaniline. The relaxation rate provides information about the spin dynamics: ID spin diffusion is observed at low temperature in non-metallic materials with a crossover to 2 or 3D behaviour at higher temperature. In the metallic polyaniline, in contrast to the undoped cases, the relaxation is rapidly quenched by field at all temperatures. Static and dynamic terms in the relaxation can be separated. The dynamic term shows a high field cutoff which occurs around 25 G at 20 K. This relaxation behaviour indicates a broad spectrum of spin fluctuations concentrated at frequencies in the region below 0.3 MHz. This is a frequency region that is somewhat lower than previously studied by other probes of spin dynamics.

Studies of implanted muons in organic radicals

We have performed semi-empirical molecular dynamics calculations of the electronic and molecular structure of muonium incorporated into the organic radicals 3-quinolyl nitronyl nitroxide (3-QNNN), para-pyridyl nitronyl nitroxide (p-PYNN), phenyl nitronyl nitroxide (PNN) and para-nitrophenyl nitronyl nitroxide (p-NPNN). These materials are of interest because they show ferromagnetic order at very low temperatures and can be effectively studied using implanted positive muons. Our calculations give evidence for a wide variety of possible muonium binding sites in the conjugated ring systems of the materials as well as in the nitronyl nitroxide group and suggest the formation of local spin triplet as well as singlet states near the muon, a result which is supported by recent experiments.

Defect electronic states in beta -carotene and lower homologues

The authors present semi-empirical calculations of the atomic geometries and electronic charge distributions of beta -carotene homologues of different chain lengths. They find defects in charged and photoexcited chains that are similar to the defects found in the degenerate polymer transpolyacetylene, and show how confinement affects these defects as the chains are shortened. The results exhibit a generalized form of charge-conjugation symmetry in which the properties of a negatively charged defect are related to those of a positive one and vice versa.

/spl mu/SR studies of nitronyl nitroxide organic magnets

Summary form only given. The field of organic magnetism has opened up recently following the discovery of bulk ferromagnet Ism in a nitronyl nitroxide compound at -0.6 K. A whole series of magnetic compounds may be synthesised based on the nitronyl nitroxide radical group with its single unpaired spin, thus allowing a study of the interplay between the molecular and the crystal properties. We have studied three members of this series of compounds using /spl mu/SR and magnetic susceptibility to characterise the spontaneous magnetic-ordering. We report here /spl mu/SR measurements on the following nitronyl nitroxides: /spl beta/ phase p-nitrophenyl (p NPNN), 3-quinolyl (3-QNNN) and p-pyridyl (p-PYNN). The magnetic transition has rather different character in each system. Our data for p-NPNN is consistent with previous reports suggesting a 3D Heisenberg ferromagnet (T/sub c/-670mK). In contrast, p-PYNN shows behaviour indicative of a spin-glass transition around 110mK. In 3-QNNN there is also a strongly defined transition to a state of magnetic order at 210mK, but the complex /spl mu/SR signal indicates that this is not simply ferromagnetic nor spin-glass like and suggests that some form of mixed behaviour occurs in 3-QNNN below 210mK.

MUSR studies of spin excitations in polyaniline

Summary form only given. Muon techniques have been used to study muon-generated and dopant-generated spin excitations in polyaniline. For non-metallic emeraldine (EB) and pernigraniline (PPG), it is found that both mobile and bound spins are present. The hyperfine coupling between the muon and the unpaired electron is extracted from the field dependent longitudinal muon polarisation. It is independent of temperature for (EB), but falls by a factor of /spl ap/2 between 300K and 15K for PPG. This indicates an increased spin delocalisation for PPG which may be associated with the mobile solitons predicted for PPG. Relaxation rate studies provide information about the spin dynamics: 1D spin diffusion is observed at low temperature for both materials with a crossover to 2 or 3D behaviour at higher temperature. In metallic polyaniline, in contrast to the undoped cases, the relaxation is rapidly quenched by field at all temperatures. At low temperatures a marked difference between low and high field behaviour is seen, with a crossover point around 40G. This critical field gives a cutoff frequency which is related to the on-chain diffusion rate in the doped material. The cutoff frequency is found to be quite constant between I 00 K and 20 K at /spl sim/-0. 5 MHz.