Professor Stephen Blundell

Abstract:

We report a study of the magnetic and electrical properties of the ferromagnetic insulator La1.5Sr0.5MnRhO6. A significant magnetoresistance is found in this system which is largest at the Curie temperature (Tc) even though there is no metal-insulator transition. The electrical transport is found to be activated above Tc and described by a variable range hopping law below Tc. Above Tc the carriers are magnetic polarons with a size which increases as the temperature approaches the magnetic transition. Rh substitution preserves ferromagnetic ordering, in contrast with the effect of other dopants on the B sites, but modifies the electrostatic potential leading to carrier localization. We attribute the peak in the magnetoresistance at Tc to the field-induced suppression of critical spin fluctuations which modulate the energetic barriers seen by the carriers.

Abstract:

The magnetic and conducting properties of many transition metal oxides may be drastically altered by doping. This study concerns a family of compounds with compositions intermediate between the itinerant helical antiferromagnet SrFeO3 and the ferromagnetic metal SrCoO3. We have measured the temperature-dependent muon-spin relaxation in five compounds in the family SrFe1-xCoxO3 where Co substitution varies from x = 0 to 0.2 and investigated the relation between the relaxation rate and the magnetic susceptibility above the ordering temperature. These materials show large magnetoresistance which may be associated with the presence of competing interactions and non-equilibrium behaviour due to cluster formation. We find a quasistatic local field at the muon site below the magnetic transition temperature indicative of magnetic order in samples with x = 0.15 and 0.2 but not in the sample with x = 0. (C) 2000 Elsevier Science B.V. All rights reserved.

Abstract:

Muon-spin rotation (mu SR) measurements have been used to study the superconducting vortex properties of the organic superconductors kappa-(BEDT-TTTF)(2) Cu(SCN)(2), alpha-(BEDT-TTF)(2) NH4Hg(SCN)(4) and beta-(BEDT-TTF)(2)IBr2. These materials all have highly anisotropic structures consisting of metallic layers of BEDT-TTF molecules alternating with less well conducting anion layers. Varying the anion gives rise to a change in the anisotropy of the superconductivity and also to changes in the superconducting transition temperature. We have used both transverse and longitudinal magnetic fields to study the three-dimensional flux line lattice that is present at low temperatures and fields and to study also the loss of flux lattice order that occurs on increasing the temperature and field. (C) 2000 Elsevier Science B.V. All rights reserved.

Abstract:

We have investigated the molecular dynamics in the nematic liquid crystal 5CB using the ALC mu SR technique. Our measurements are consistent with a change in the amplitude of director fluctuations at the nematic-isotropic transition and we develop a model to describe this. (C) 2000 Elsevier Science B.V. All rights reserved.

Abstract:

We have measured the muon-spin rt laxation (mu SR) in La0.6Sr0.4MnO3 and in its layered sister system La1.2Sr1.8Mn2O7 to study the influence of dimensionality on their magnetic properties. Both compounds show ferromagnetic ordering with decreasing temperature. Above their Curie temperatures we followed the spin fluctuations. While the electron spin fluctuations in the three-dimensional La0.6Sr0.4MnO3 slow down critically as the transition temperature is approached they remain finite in the layered material La1.2Sr1.8Mn2O7. We can fit them to 2D critical behaviour with a lower T-C. In addition, we observe a spin precession signal in the ferromagnetically ordered state in La1.2Sr1.8Mn2O7; the temperature dependence of the muon precession frequency is in agreement with a three-dimensional character of the ordering. (C) 2000 Published by Elsevier Science B.V. Ail rights reserved.