I.M. Marshall

μSR studies of organic and molecular magnets

Muon-spin rotation and relaxation (μSR) experiments have been performed on a variety of novel organic and molecular magnetic systems. In these experiments, implanted muons are used to study the magnitude, distribution and dynamics of the local field at the muon site. Calculations of the spatial dependence of the dipole-field inside the unit cell are used to interpret the data and determine the muon site in certain cases. We describe and review muon experiments on nitronyl nitroxide organic ferromagnets and antiferromagnets. We discuss a muon study of the spin crossover phenomenon which has been studied in Fe(PM-PEA)2(NCS)2, and which shows Gaussian and root-exponential muon relaxation in the high-spin and low-spin phases, respectively. The effects of high temperature annealing on TDAE-C60 have also been studied with μSR. Experiments on a disc-shaped molecular complex containing Fe19 (with spin 31/2) reveal the effects of fluctuations of magnetization and allow an estimate of the fluctuation rate. These experiments demonstrate the wide range of problems which can be tackled using the μSR technique.

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.

μSR study of organic systems: ferromagnetism, antiferromagnetism, the spin-crossover effect, and fluctuations in magnetic nanodiscs

Abstract We present the results of recent μSR experiments on a variety of novel organic and molecular magnetic systems. Muons are sensitive to local static fields and magnetic fluctuations, but can probe much more than just the onset of long-range magnetic order. We review our work on nitronyl nitroxide organic ferromagnets and antiferromagnets. We describe a muon study of the spin-crossover phenomenon which has been studied in Fe(PM-PEA)2(NCS)2, and which shows Gaussian and root-exponential muon relaxation in the high-spin and low-spin phases, respectively. Experiments on a disc-shaped molecular complex containing Fe19 (with spin 31 2 ) reveal the effects of quantum tunneling of magnetization and allow an estimate of the quantum tunneling rate.

μSR studies of magnetic superconductors based on the BETS molecule

Muon spin rotation and relaxation measurements have been made on the molecular magnetic superconductors κ-BETS2FeCl4 and κ-BETS2FeBr4 and the non-magnetic molecular metals κ-BETS2GaCl4 and λ-BETS2GaCl4. In the magnetic materials, zero field muon spin relaxation signals show the formation of static antiferromagnetically ordered states and multiple precession frequencies are observed, corresponding to muon sites situated both within the anion layers and within the BETS layers. Studies of the relaxation behavior in the FeCl4 salt have previously shown significant changes around its superconducting transition and the FeBr4 salt reveals similar changes in the region of its superconducting transition, whose onset is around 1.5 K. In the non-magnetic GaCl4 salts the formation of a superconducting vortex lattice was observed and the penetration depth λ was derived from the transverse field muon relaxation. When the BETS superconductors are compared with the ET-based superconductors that we have already studied using μSR, a striking correlation was observed between Tc and λ−3.

Magnetism in oxide chains bridged with the hydride anion: LaSrCoO3H0.7 studied using muon-spin rotation

Abstract The transition metal oxide hydride, LaSrCoO 3 H 0.7 , adopts an unprecedented structure in which oxide chains are bridged by hydride anions to form a two-dimensional extended network in which magnetic ordering is found up to at least 350 K . Muon-spin rotation has been used to demonstrate that this material is uniformly magnetically ordered throughout its bulk. Our results are compared with those on Sr 2 CuO 3 and Ca 2 CuO 3 which adopt a similar oxide chain structure and we demonstrate the crucial role of the bridging hydride ions.

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 study of molecular magnets, spin crossover and magnetic nanodiscs

Abstract We present the results of recent muon-spin rotation ( μ SR) experiments on a variety of novel molecular magnetic systems. Muons are sensitive to local static fields and magnetic fluctuations, but can probe much more than just the onset of long-range magnetic order. The spin-crossover phenomenon has been studied in Fe(PM-PEA) 2 (NCS) 2 , which shows Gaussian and root-exponential muon relaxation in the high-spin and low-spin phases, respectively. Dy spin fluctuations have been studied with μ SR in the ferrimagnetic chain Dy(hfac) 3 (NITEt). Experiments on a disc-shaped molecular complex containing Fe 19 (with spin 31 2 ) reveal the effects of quantum tunneling of magnetization and allow an estimate of the quantum tunneling rate.

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.

Angle-dependent vortex structure in a high anisotropy superconductor

Angle-dependent muon spin rotation measurements have been made on the organic superconductor kðBEDT-TTFÞ2CuðSCNÞ2: Oscillations are observed in the width of the internal field distribution, which are periodic in the perpendicular component of the applied magnetic field, Bz ¼ B cos y; with a uniform period over a range of angles and fields. These oscillations are superimposed on the standard cos y scaling expected for the width in a highly anisotropic superconductor. The oscillation period is of order 2 mT and the amplitude is particularly strong at fields comparable with the period. The origin of this novel phenomenon is discussed in terms of the low field instabilities of tilted vortices in this highly anisotropic superconductor. r 2002 Elsevier Science B.V. All rights reserved.

A μSR study of the spin dynamics in Ir-diluted layered manganites

Abstract Dilution on the B-site with Ir or Rh can lead to the enhancement of ferromagnetic couplings in layered manganites. We report a μSR study of the spin dynamics of the layered manganites La 0.25 Sr 1.75 Mn p Ir 1−p O 4 (p=0.5,0.75) as a function of the dilution. We have characterized the static magnetic behavior using SQUID magnetic measurements and used muon spin relaxation spectroscopy to provide evidence for short-range ferromagnetic correlations. Furthermore, the effect of dilution on the critical dynamics in this two-dimensional spin system is reported and related to the dynamics above and below the critical percolation threshold.