Muons and magnets

Abstract:

Muon-spin rotation and relaxation (muSR) 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-C-60 have also been studied with muSR. Experiments on a disc-shaped molecular complex containing Fe-19 (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 muSR technique. (C) 2003 Elsevier Science Ltd. All rights reserved.

Abstract:

The temperature dependence of the internal field in the one-dimensional S = 1 chain compound LiVGe2O6 has been measured using muon-spin rotation. Two muon-spin-precession frequencies are observed, corresponding to two distinct stopping sites. The results demonstrate that LiVGe2O6 undergoes a second-order phase transition to long-range magnetic order at TN = 24.5 K. In the paramagnetic regime, we find exponential muon-spin relaxation which is dominated by fluctuations of the ⁵¹V nuclear moments. © 2003 The American Physical Society.

Abstract:

A polycrystalline sample of Sr1.2Nd0.8Mn0.6Rh0.4O4 has been synthesised and characterised by neutron diffraction, dc magnetometry, muon spectroscopy and magnetotransport measurements. Ir is an n = 1 member of the Ruddlesden-Popper (RP) An + 1BnO3n + 1 family, having tetragonal (I4/mmm) symmetry [a = 3.86418(6), c = 12.4414(2) Å]. Rh and Mn cations are disordered over the six-coordinate sites within the perovskite layers of the RP structure and the A sites are occupied by a disordered distribution of Nd and Sr. In contrast to the analogous compound Sr1.2La 0.8Mn0.6Rh0.4O4, which showed long range ferromagnetic ordering in an applied field ar 2 K, a spin-glass transition was observed ar 16 K for the Nd-containing compound. No long range magnetic order was observed ar 2 K. A maximum magnetoresistance of ∼ 30% in a 140 kOe field was found at 125 K.

Abstract:

Zero field muon spin relaxation measurements have been made on the charge transfer salt (tetrakisdimethylaminoethylene)C-60 [TDAE-C-60]. The effects of high temperature annealing on the dynamics and local magnetic properties of the compound are investigated. With the sample in the ferromagnetic alpha-phase, a signal characteristic of magnetic order is observed below 16 K. The initial asymmetry shows a discontinuity around 200 K, while the relaxation rate shows temperature activated behaviour above around 120 K. Annealing at 383 K is found to destroy the ferromagnetic signal and results in a relaxation rate peaked at 10 K, which may be attributable to the freezing out of a dynamic Jahn-Teller distortion.

Abstract:

Organic molecular FETs provide an experimental framework for studying the band-filling dependent properties of two-dimensional metals. We propose experiments to investigate the band-filling dependent cyclotron resonance and the effect of van Hove singularities. (C) 2002 Elsevier Science B.V. All rights reserved.