K.H. Chow

Spin dynamics in conducting polymers studied by μSR

We report studies of spin dynamics in the conducting polymers polyaniline and polypyrrole using both μ+SR and μ-SR techniques. These measurements reveal characteristic field dependences and cutoff frequencies for the muon spin relaxation which can be related to the spin diffusion process. Clear evidence is seen for increased spin localisation at low temperatures where a crossover occurs from two or three dimensional spin diffusion to a one dimensional diffusion regime.

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.

Spin dynamics in the organic spin - Peierls system MEM(TCNQ) studied using muon-spin relaxation

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.

DIAMAGNETIC MUONIUM STATES IN INP

Transverse and zero‐field muon spin relaxation reveal several diamagnetic muonium states in InP characterized by their static linewidths and diffusion properties. We tentatively associate low‐temperature diamagnetic states with Mu+ in the BC and TP interstitial sites and a missing fraction with Mu0 rapidly diffusing through TIn interstices. Trapping peaks above 250 K imply static centers which depend on doping type, consistent with Mu- at TIn for n‐type samples and Mu coupled with a dopant or other defect for p‐type.

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.

Organic magnetism in nitronyl nitroxides studied by μSR

Ferromagnetism has been observed in a family of organic molecular crystals based on the nitronyl nitroxide radical. We present the results of μSR experiments on a number of nitronyl nitroxide compounds. The zero‐field spin precession of muons implanted in diamagnetic states can be used to follow the magnetic order parameter as a function of temperature. Five of the materials studied show magnetic transitions, although the transition temperature and the nature of the magnetic ground state in each case are quite different. μSR can be used to study these ground states and thus help to relate the observed magnetic properties to the crystal structure of each material.

Muon studies of spin dynamics in polyaniline

The muon is a valuable probe of spin excitations in polymers, as the muon implantation process itself generates a test excitation whose dynamical properties can be studied via the evolution of the muon spin polarisation. We report here studies on undoped polyaniline in its emeraldine base form (PANI:EB). Characteristic field dependences and cutoff frequencies for the muon spin relaxation are observed which are related to the spin diffusion. One-dimensional diffusion is seen at low temperatures and at short probe times. The on chain diffusion is observed to have a weak metallic temperature dependence whereas the interchain diffusion is strongly activated and phenyl ring rotations are seen to have an important effect on the diffusion processes.

Interplay of magnetism and superconductivity in thulium and lutetium nickel-borocarbides

Abstract Implanted muon spectroscopy shows that magnetic order coexists with superconductivity in TmNi2B2C. The muon response indicates that large amplitude fluctuations of the internal field are superimposed on a relatively small static component. The static field exhibits an unusual temperature dependence which shows no interruption at the superconducting transition and may represent evolution of a staggered or spiral arrangement of Tm moments. The dynamic component changes its spectral density at the superconducting transition and the question arises as to whether this is associated with Ni moment formation and fluctuation. In LuNi2B2C, where the rare earth ion is nonmagnetic, no signature of static order is seen. An intriguing change in the muon response does occur below about 4 K in this material; this seems suggestive of Ni moment fluctuation, although the evidence is not entirely conclusive.

Modelling hydrogen in the group-III nitrides by its pseudo-isotope, muonium

Abstract Muon and muonium states in the wide-bandgap semiconductors BN, AlN, and GaN are characterised by various types of μSR measurement on polycrystalline samples. The muonium fractions range from 80% in hexagonal BN to zero in GaN. The hyperfine constants estimated from repolarization curves are 80% of the free muonium value in BN and 95% in AlN, with superhyperfine interactions to the host nuclei is evident. The electronically diamagnetic states show strong level-crossing resonances in AlN and GaN (although none is detectable in BN). These have the signature of cross-relaxation to 14 N in AlN and to 69 Ga and 71 Ga in GaN, suggesting that the diamagnetic states are Mu + and Mu − in these naturally p- and n-type materials, respectively. Mu − diffusion in GaN sets is only above 600 K, with an activation energy of 1 eV.

Zero field μSR and QLCR in the molecular metal system (DMe-DCNQI)2Cu

We have carried out implanted positive muon studies on the molecular metal system dn- (DMe-DCNQI)2 Cu in order to understand better its novel magnetic properties. Examples of these salts at different levels of deuteration were studied. The fully deuterated ( d8) salt shows a metal–insulator (MI) transition around 80 K and a magnetic transition around 7 K. The muon spin relaxation rate is enhanced below the MI transition, reflecting the localisation of spins along the Cu columns, however, the increase in muon spin relaxation rate occurs well above the metal–insulator (MI) transition and suggests a slowdown of the spin fluctuations around 120 K. At temperatures below 7 K a zero field precession signal was observed as a result of the 3D magnetic ordering of the Cu spins. For a muon site associated with the ring of the DCNQI molecule, the local field distribution was found to be consistent with the previously proposed magnetic structure. A sharp nuclear quadrupolar level crossing resonance (QLCR) was observed at 50 G which was assigned to resonance with the imine nitrogen on the DCNQI molecule.