A Husmann

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.

μSR in polymers

μSR can be applied to the study of various dynamical processes in polymers. These processes may relate to carrier motion, as in studies of conducting polymers which make use of muon generated polarons to measure carrier diffusion rates. Alternatively the processes of interest may be related to the structural dynamics of the polymer, which can show dramatic changes around the glass transition temperature. We report here examples of the use of μSR to study the muon states and muon mobility in the polymers polyethylene and polytetrafluoroethylene, where coherent FμF precession signals have been observed. In the case of polystyrene, muon radical states formed on the phenyl ring have been used to make a detailed study of the dynamical freezing and onset of static disorder that accompanies the glass transition. Finally, we report a study of polaron diffusion in two polyphenylenevinylene conducting polymers.

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.

Muon-spin relaxation study of anisotropic charge carrier motion in polyphenylene vinylene-based polymers

Muon-spin relaxation (μ SR) experiments on the conducting polymers poly(2, 3-dibutoxy-1, 4-phenylene vinylene) and poly(2, 5-bis(dimethyloctylsilyl)-1, 4-phenylene vinylene) probe the dynamics of the highly mobile polarons created by the muon-implantation process in which muonium reacts with the polymer forming a radical state. The fluctuating spin density induced by the electronic spin defect rapidly diffusing up and down the chain leads to a characteristic relaxation, the temperature and field dependences of which permit the extraction of intrachain and interchain diffusion rates. The intrachain diffusion rate decreases with temperature and can be fitted to a model of phonon-limited transport. The interchain diffusion rate increases with temperature and can be fitted to an activated temperature dependence.

AμSR study of the CDW in TTF-TCNQ

The muon spin rotation (pSR) technique is primarily a powerful probe of the magnetic properties of materials. but it is also possible to measure charge-density related effects using muon quadrupolar level crossing resonance (QLCR). We present the first study of a charge density wave (CDW) using pSR in TTF-TCNQ. The CDW develops below the metal-insulator transition at 54 K and we study the evolution of the CDW as a function of temperature using the QLCR resonances of the nitrogen atoms on the TCNQ molecules as the probe.

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.

Muon-spin relaxation study of charge carrier dynamics in the conducting polymer PPV

Muon-spin relaxation (μSR) experiments on the conducting polymer poly(2,3-dibutoxy-1,4-phenylene vinylene) (DB-PPV) probe the dynamics of the highly mobile polarons created by the muon-implantation process in which muonium reacts with the polymer forming a radical state The fluctuating spin density induced by the electronic spin defect rapidly diffusing up and down the chain leads to a characteristic relaxation, the temperature and field dependence of which permit the extraction of on-chain and inter-chain diffusion rates. These results provide information concerning the charge-transport mechanism in DB-PPV.

BEDT-TTF superconductors studied by μSR

Muon-spin rotation (μSR) measurements have been used to study the superconducting vortex properties of the organic superconductors κ-(BEDT-TTF)2Cu(SCN)2, α-(BEDT-TTF)2NH4Hg(SCN)4 and β-(BEDT-TTF)2IBr2. These materials all have highly anisotropic structures consisting of metallic layers of BEDT-TTF molecules alternating with less well conducting anion layers. Varying the anion gives rise to a change in the anisotropy of the superconductivity and also to changes in the superconducting transition temperature. We have used both transverse and longitudinal magnetic fields to study the three-dimensional flux line lattice that is present at low temperatures and fields and to study also the loss of flux lattice order that occurs on increasing the temperature and field.

Effect of dimensionality on the magnetic properties of Ruddlesden–Popper manganites

Abstract We have measured the muon-spin relaxation (μSR) in La0.6Sr0.4MnO3 and in its layered sister system La1.2Sr1.8Mn2O7 to study the influence of dimensionality on their magnetic properties. Both compounds show ferromagnetic ordering with decreasing temperature. Above their Curie temperatures we followed the spin fluctuations. While the electron spin fluctuations in the three-dimensional La0.6Sr0.4MnO3 slow down critically as the transition temperature is approached they remain finite in the layered material La1.2Sr1.8Mn2O7. We can fit them to 2D critical behaviour with a lower TC. In addition, we observe a spin precession signal in the ferromagnetically ordered state in La1.2Sr1.8Mn2O7; the temperature dependence of the muon precession frequency is in agreement with a three-dimensional character of the ordering.

A μSR study of high oxidation state iron oxides displaying large magnetoresistance

The magnetic and conducting properties of many transition metal oxides may be drastically altered by doping. This study concerns a family of compounds with compositions intermediate between the itinerant helical antiferromagnet SrFeO3 and the ferromagnetic metal SrCoO3. We have measured the temperature-dependent muon-spin relaxation in five compounds in the family SrFe1−xCoxO3 where Co substitution varies from x=0 to 0.2 and investigated the relation between the relaxation rate and the magnetic susceptibility above the ordering temperature. These materials show large magnetoresistance which may be associated with the presence of competing interactions and non-equilibrium behaviour due to cluster formation. We find a quasistatic local field at the muon site below the magnetic transition temperature indicative of magnetic order in samples with x=0.15 and 0.2 but not in the sample with x=0.