Professor Stephen Blundell

Abstract:

We report muon-spin rotation, heat capacity, magnetization, and AC magnetic susceptibility measurements of the magnetic properties of the layered spin-1/2 antiferromagnet NaNiO2. These show the onset of long-range magnetic order below T-N = 19.5K. The temperature dependence of the muon precession frequency suggests 2D XY magnetic ordering. Rapid muon depolarization persisting to about 5 K above TN is consistent with the presence of short-range magnetic order. Our AC susceptibility measurements suggest that magnetic clusters persist above 25 K, with their volume fraction decreasing with increasing temperature, and that there is a slowing of spin fluctuations at T-sf = 3 K. A partial magnetic phase diagram has been deduced. (c) 2006 Elsevier B.V. All rights reserved.

Abstract:

We report muon-spin rotation, heat capacity, magnetization, and AC magnetic susceptibility measurements of the magnetic properties of the layered spin-1/2 antiferromagnet NaNiO2. These show the onset of long-range magnetic order below TN=19.5K. The temperature dependence of the muon precession frequency suggests 2D XY magnetic ordering. Rapid muon depolarization persisting to about 5K above TN is consistent with the presence of short-range magnetic order. Our AC susceptibility measurements suggest that magnetic clusters persist above 25K, with their volume fraction decreasing with increasing temperature, and that there is a slowing of spin fluctuations at Tsf=3K. A partial magnetic phase diagram has been deduced. © 2006 Elsevier B.V. All rights reserved.

Abstract:

We report the results of transverse field (TF) and zero-field (ZF) mu SR measurements on the hexaboride system EuxCa1-xB6. EuB6 is a semimetallic ferromagnet that magnetically orders via two transitions at T-m approximate to 15 K and T-c approximate to 12 K, with colossal magnetoresistance accompanying the higher temperature transition. New TF mu SR measurements on EuB6 allow us to follow the temperature evolution of the local magnetic field distribution through the two magnetic transitions. Substitution of Ca for Eu dilutes the magnetic sublattice, causing a substantial suppression of the transition temperature, with the transition completely removed when the doping level approaches the three-dimensional site percolation limit, x(p) = 0.31. ZF experiments on doped samples, 0.35 <= x <= 1, enable a sensitive local-probe exploration of the order and dynamics across the phase diagram. (c) 2005 Elsevier B.V. All rights reserved.

Abstract:

TmAgGe is an antiferromagnet based on the ZrNiAl structure. At low temperatures the spins are confined to distorted kagome-like planes, wherein the magnetisation is strongly anisotropic. A previous study has shown that a series of stepped magnetic transitions are apparent in low, in-plane magnetic fields and can be explained using a three-fold Ising-like model. Here we present high-magnetic-field magnetisation experiments showing that further stepped transitions are observed when the field is directed out of the kagome planes. Angledependent measurements in fields of up to 55 T show that there are at least two distinct and separate energy scales present in this system; the weak exchange interactions and the strong crystalline electric field interactions. Simulations of the magnetisation using a three-dimensional, free-energy minimisation technique allow us to suggest the nature and hierarchy of the forces acting on the Tm3+ moments.