Muons and magnets

Abstract:

We present the results of zero field muon-spin relaxation (mu+SR) studies of the molecular magnets Mn(dca)(2)(pyz) and Fe(NCS)(2)(pyz)(2) (where dca = [N(CN)(2)](-) and pyz = pyrazine). Both of these materials possess low-dimensional structural motifs resulting in magnetic behaviour consistent with this reduced dimensionality. Mn(dca)(2)(pyz) is formed from Mn-pyz-Mn chains, linked with dca anions to form two interpenetrating three-dimensional (3D) networks. Antiferromagnetic ordering in this compound is seen as a rapid change in the measured asymmetry around T-N = 2.5 K. Fe(NCS)(2)(pyz)(2) is a two-dimensional (2D) sheet-like polymer with the sheets consisting of Fe atoms bridged by pyz ligands. mu+SR measurements display both the 3D and 2D nature of the material in the measured relaxation spectra. We observe the transition into the LRO state below T-N along with an excitation of E = 1.09(5) meV above T-N associated with fluctuations within the correlated 2D layers. (c) 2005 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 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.