Th. Jestädt

μSR of conducting and non-conducting polymers

Abstract μSR has been used to study a variety of polymers with very different electronic properties. In conducting polymers, the muon-generated radical states take the form of highly mobile polarons. Muon spin relaxation has been used to study the mobility of these polarons and to measure the temperature dependence of their intra-chain and inter-chain diffusion rates. It is found that the transport properties are strongly influenced by the librational ring modes of the phenylene rings in these polymers. In contrast, the muon-generated radical states in non-conducting polymers such as polybutadiene remain localised near the site of the muon. High field muon spin rotation, avoided level crossing resonance and longitudinal relaxation studies have been made, using the muon radical state as a probe of the dynamical properties of the polymer. Dramatic changes in the μSR signals are seen on going through the glass–rubber transition, as various dynamical degrees of freedom become frozen out. Additional information about the stability of the muon radical states on the microsecond timescale has also been obtained using RF muon spin rotation techniques. Using time-delayed RF resonance of the diamagnetic state at the RIKEN-RAL muon facility, the transition rate between paramagnetic and diamagnetic states could be studied as a function of temperature.

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.

Physical properties of the n =3 Ruddlesden - Popper compound

We present the results of a combined magnetization, muon-spin rotation, transport and magnetotransport study of the n = 3 Ruddlesden - Popper (RP) compound . This compound adopts a layered structure in which groups of three perovskite layers alternate with single rock-salt layers. The muon-spin rotation data show that there is a sharp magnetic phase transition at 115 K. The resistance and magnetoresistance of the sample show no particular features at this temperature, but the transition affects the energy barriers associated with hopping transport. The magnetoresistance is proportional to the square of the magnetization, and is largest at low temperatures; a 40% drop in resistivity is observed in a magnetic field of 14 T at 61 K, much smaller than that measured in the related RP (perovskite) manganites which exhibit colossal magnetoresistance (CMR).

Director fluctuations in a nematic liquid crystal probed using ALC spectroscopy

Abstract We have investigated the molecular dynamics in the nematic liquid crystal 5CB using the ALC μSR technique. Our measurements are consistent with a change in the amplitude of director fluctuations at the nematic–isotropic transition and we develop a model to describe this.

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.

Muon studies of molecular magnetism

Abstract We present the results of μSR experiments on a variety of molecular magnetic materials, either purely organic or combinations of transition metal ions and organic groups, which have been recently prepared. In a purely organic metamagnet, tanol suberate, we have observed a spin precession signal with a temperature dependence which has provided evidence of the two-dimensional nature of the antiferromagnetic ground state. In a family of dicyanamide-based molecular magnets with ordering temperatures of up to 21 K, and in a ferrimagnetic cobalt hydroxide, μSR has been used to study the temperature dependence of the spin fluctuations.

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.

Muon radical states in some electron donor and acceptor molecules

Muon radical states were studied in the electron donor molecules TTF and BEDT‐TTF and in the electron acceptor molecule TCNQ. Muonium addition to double bond carbon sites results in hyperfine coupling contants ranging from 350 MHz in TTF down to 80 MHz for TCNQ. In TTF and BEDT‐TTF additional radical states with extremely low hyperfine coupling constants in the 5 MHz region are observed; these are assigned to muon addition at sulphur sites accompanied by C — S bond cleavage. Copyright

Very low-temperature muon relaxation in an organic spin-Peierls compound

We have observed strong muon-spin relaxation (μSR) in the spin-Peierls compound MEM(TCNQ)2 at temperatures down to 39 mK. We attribute this relaxation to the creation of defect spins by the muon. Furthermore, we observe a slowing down of spin fluctuations as the spin-Peierls energy gap opens, and we relate this effect to the size of the energy gap.

Magnetic properties of DCNQI salts studied using μSR

Abstract Examples from the (R 1 ,R 2 -DCNQI) 2 X family of molecular conductors have been studied using μSR in order to provide information about the microscopic magnetic properties of the various phases. For the fully deuterated dimethyl Cu salt (d 8 -DMe-Cu) there is a metalinsulator (MI) transition around 80 K and a magnetic transition around 7 K. The muon spin relaxation rate becomes enhanced in the region of the MI transition and below, reflecting the quenching of valence fluctuations and the appearance of localised spins on the Cu sites. A zero field precession signal develops below 7 K as a result of the 3D magnetic ordering of the Cu spins; the field distribution derived from the precession frequency is consistent with the proposed magnetic structure. In addition to the zero field studies, nuclear quadrupolar level crossing resonance between the muon and the imine nitrogen of the DCNQI has been used to study the temperature dependence of the electronic state of the molecular conductor.