Muons and magnets

Abstract:

A ferromagnetic system which has an external magnetization that is intrinsically zero is an interesting and unusual type of magnet. The rare earth ion Sm3+ provides a basis for building such a system, having the property that its spin and orbital moments are both close to 4 mu(B). Antiparallel coupling of these moments by spin-orbit interaction leads to a very small resultant moment that is either parallel or antiparallel to the spin moment of the ion, depending on the exact balance. By doping Gd into the ferromagnet SmAl2 (T-c = 125 K) an experimental realization of a zero magnetization ferromagnet can be produced, with exactly zero magnetization occurring at temperature T-comp. Zero-field muon spin relaxation measurements have been made on single crystal and polycrystalline samples of the Sm1-xGdxAl2 system and for all the compositions studied an extremely sharp transition into the ferromagnetic state was observed. Muon spin precession was seen in the magnetically ordered state of the Sm compounds and anomalies in the relaxation were observed around T-comp, reflecting the unusual nature of this magnetic state. (c) 2005 Elsevier B.V. All rights reserved.

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.