Muons and magnets

Abstract:

We report a series of muSR measurements performed on single crystals of La1-x,SrxMnO3, (0 less than or equal to x less than or equal to 0.125) and on a polycrystalline bilayer manganite La2-2x Sr1+2xMn2O7 (v = 0.52). We find that the temperature dependence of the spinlattice relaxation rate is strongl, dependent on the hole doping and the associated changes in the magnetic structure. Y The systems have competing, anisotropic interactions, leading to a complex interplay of charge, spin and orbital order. This has a significant influence on the spin dynamics, both for perovskite and layered manganites. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract:

The transition metal oxide hydride, LaSrCO3H0.7, adopts an unprecedented structure in which oxide chains are bridged by hydride anion's 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 Sr2CuO3 and Ca2CuO3 which adopt a similar oxide chain structure and we demonstrate the crucial role of the bridging hydride ions. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract:

The ISIS European Muon Facility provides surface muons for condensed matter investigations in a wide range of subject areas. The Facility has been in its present form since 1994, and consists of three experimental areas fed simultaneously using two muon pulses generated every 20ms by the ISIS proton beam. The current spectrometers enable count rates of 20-25 million events per hour; the maximum applied field presently available is 0.45 T. Plans are being developed to provide a new instrument at ISIS to extend the Facility's capabilities. This new spectrometer would have a highly segmented detector array, significantly increasing the available data rate, and would also be equipped with higher longitudinal fields. Such an instrument would offer the possibility of collecting time-differential data more rapidly than is currently possible on either pulsed or continuous sources; this, together with the improved field, would provide a unique facility enabling new science areas to be explored using muSR. The present plan would also lead to an improved time resolution on the existing ISIS instruments through the time slicing of the muon pulses that feed them. We describe the current development ideas and the scientific and technical considerations underlying them, including the effect of the magnetic field on the muon and positron trajectories for particular spectrometer geometries. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract:

muSR 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 muSR to study the muon states and muon mobility in the polymers polyethylene and polytetrafluoroethylene, where coherent FmuF 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. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract:

We present the results of recent muSR 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 anti ferromagnets. 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 Fe-19 (with spin 31/2) reveal the effects of quantum tunneling of magnetization and allow an estimate of the quantum tunneling rate. (C) 2002 Elsevier Science B.V. All rights reserved.