No abstract

We use zero-and longitudinal-field muon spin rotation and relaxation to study the muon states in the organic molecular ferromagnet: j3-phase para-nitrophenyl nitronyl nitroxide (p-NPNN). Below Tc = 0.65 K, oscillations in the relaxation function are observed in zero applied field which follow the magnetic order parameter, implying that the muon is bonded in an electronic spin-singlet state. We also study the behaviour of the oscillations in an applied longitudinal field and interpret the results by considering the magnetisation process and the demagnetising field. A significant field-induced repolarisation is also observed which is ascribed to the presence of spin-triplet states formed by a fraction of muons.Recently, there has been much interest in the search for a purely organic molecular ferromagnet which contains only light elements (like carbon, nitrogen, hydrogen, and oxygen) [l]. The frst such material to be found, the j3 crystal phase of para-nitrophenyl nitronyl nitroxide (p-NPNN, see fig. lb)), was reported to have a Curie temperature (T,) of 0.60K, a discovery which was made using heat capacity and magnetisation measurements [2,3]. The unpaired spin in this nitronyl nitroxide is associated with the N-0 groups. The role of the rest of the molecule is to ensure the appropriate overlap of the correct orbitals on neighbouring molecules to produce 3D ferromagnetism. A whole series of materials which incorporate this nitronyl nitroxide group are currently being synthesized in order to provide a systematic study of the effects of the variation of the rest of the molecule on the molecular packing and overlap and hence on the magnetic properties.Zero-field (ZF) muon spin rotation/relaxation (pSR) studies on aligned single crystals [4]and polycrystalline [51 samples have also demonstrated magnetic order with Tc -0.65 K; in these experiments, the muon spin precession rate directly yields the value of the internal magnetic field at the muon site. In a pSR experiment, a beam of spin-polarized muons is stopped in a target specimen. Each muon subsequently decays (with a mean lifetime of z p = = 2.2 ps), emitting a positron preferentially along the muon spin direction because of the parity violation of the weak interaction [6]. The time histograms N * (t) of positrons counted in the 8 Les Editions de Physique

No abstract

We report the first muon-spin relaxation study of an organic spin - Peierls system, the linear-chain compound MEM(TCNQ). Our results show a crossover from a Gaussian relaxation to an exponential relaxation as the temperature is lowered below the spin - Peierls transition. We associate this behaviour with the slowing down of the electronic fluctuations resulting from the opening of a gap in the magnetic excitation spectrum.

No abstract

No abstract

No abstract

No abstract