Muons and magnets

Abstract:

Radical cation and anion salts of the neutral organic radicals, 2-imidazolyl nitronyl nitroxide (2-IMNN) and 2-benzimidazolyl nitronyl nitroxide (2-BIMNN), have been prepared and their magnetic properties studied by SQUID magnetometry. The radical salts exhibit one-dimensional (1-d) antiferromagnetic (AFM) intermolecular interactions with the exchange coupling J/k between -0.8 and -6.3 K, which are significantly reduced from those observed in the two neutral radicals, while 2-IMNN shows an AFM interaction with J/k= -88 K within the molecular dimers and 2-BIMNN has quasi 1-d ferromagnetic (AFM) intermolecular interactions with J/k = +22 K (intrachain) and zJ'/k = +0.24 K (interchain). The magnetic properties of the nitronyl nitroxide and iminonitroxide derivatives having molecular structure related to 2-IMNN have also been investigated. In 2-benzimidazolyl iminonitroxide (2-BIMIN), the FM interaction observed in 2-BIMNN is replaced by strong 1-d AFM interaction with J/k = -11.7 K. (C) 2003 Elsevier Science Ltd. All rights reserved.

Abstract:

Muon spin rotation and relaxation measurements have been made on the molecular magnetic superconductors kappa-BETS2FeCl4 and kappa-BETS2FeBr4 and the non-magnetic molecular metals kappa-BETS2GaCl4 and lambda-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 lambda 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 muSR, a striking correlation was observed between T-c and lambda(-3). (C) 2003 Elsevier Science Ltd. All rights reserved.

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.