Professor Stephen Blundell

Abstract:

The spin-crossover phenomenon is a cooperative low-spin to high-spin transition which can be initiated using temperature or light-irradiation. We have used muon-spin relaxation (μSR) to study this effect in Fe(PM-PEA) 2(NCS)2 and Fe(PMAzA)2(NCS)2. We find Gaussian or exponential muon relaxation in the high-spin phase for the two compounds, reflecting differences in their intermolecular interactions. For both compounds, the low-spin phase gives rise to root-exponential relaxation which we associate with a dilute distribution of fluctuating moments resulting from incomplete spin crossover. © 2003 Elsevier Ltd. All rights reserved.

Abstract:

We present zero-field muon spin relaxation (μ+SR) measurements on the rare earth antiferromagnet PrO2. Oscillations in the time dependence of the muon polarization, characteristic of a quasistatic magnetic field at two distinct muon sites, are observed below TN, along with relaxation dominated by the magnitude of the local field. Candidate muon sites in the material are suggested on the strength of dipole field calculations. Relaxation above TN allows a prediction of the quasielastic neutron scattering linewidth, which is discussed in terms of a simple model of magnetoelastic relaxation involving a crystal field state at an energy Δ above the ground level. The obtained value of Δ may be taken as an approximate estimate of the splitting of the Γ8 ground state of Pr4+ in the distorted fluorite structure.

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.