Muons and magnets

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.

Abstract:

We present the results of recent muon-spin rotation (muSR) 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 muSR 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. (C) 2002 Elsevier Science B.V. All rights reserved.