Muons and magnets

Abstract:

A new μ SR spectrometer (HiFi) is being constructed at the ISIS pulsed muon source. The spectrometer is intended for measurements in longitudinal magnetic fields up to 5 T. Here we discuss various aspects of the design and future operation of the HiFi spectrometer. Detailed analysis of the detector design is presented, and results from tests on detector prototypes are compared to simulations. Potential applications of this spectrometer in the fields of condensed matter and chemistry are also discussed. © 2008 Elsevier B.V. All rights reserved.

Abstract:

X-ray powder diffraction and magnetic susceptibility measurements show that Ag(pyz)(2)(S(2)O(8)) consists of 2D square nets of Ag(2+) ions resulting from the corner-sharing of axially elongated AgN(4)O(2) octahedra and exhibits characteristic 2D antiferromagnetism. Nevertheless, mu(+)SR measurements indicate that Ag(pyz)(2)(S(2)O(8)) undergoes 3D magnetic ordering below 7.8(3) K.

Abstract:

The magnetic properties of a family of molecular-based quasi-two- dimensional S=1/2 Heisenberg antiferromagnets are reported. Three compounds, [Cu (pz) 2 (ClO4) 2, Cu (pz) 2 (BF4) 2, and [Cu (pz) 2 (NO3)] (PF6)] contain similar planes of Cu2+ ions linked into magnetically square lattices by bridging pyrazine molecules (pz= C4 H4 N2). The anions provide charge balance as well as isolation between the layers. Single crystal measurements of susceptibility and magnetization, as well as muon-spin-relaxation studies, reveal low ratios of Néel temperatures to exchange strengths (4.25/17.5=0.243, 3.80/15.3=0.248, and 3.05/10.8=0.282, respectively) while the ratio of the anisotropy fields HA (kOe) to the saturation field HSAT (kOe) are small (2.6/490=5.3× 10-3, 2.4/430=5.5× 10-3, and 0.07/300=2.3× 10-4, respectively), demonstrating close approximations to a two-dimensional Heisenberg model. The susceptibilities of ClO4 and BF4 show evidence of an exchange-anisotropy crossover (Heisenberg to XY) at low temperatures; their ordering transitions are primarily driven by the XY behavior with the ultimate three-dimensional transition appearing parasitically. The PF6 compound remains Heisenberg type at all temperatures, with its transition to the Néel state due to the interlayer interactions. Effects of field-induced anisotropy have been observed. © 2009 The American Physical Society.

Abstract:

The recent observation of superconductivity with critical temperatures (Tc) up to 55 K in the pnictide RFeAsO(1-x)F(x), where R is a lanthanide, marks the first discovery of a non-copper-oxide-based layered high-Tc superconductor. It has raised the suspicion that these new materials share a similar pairing mechanism to the cuprate superconductors, as both families exhibit superconductivity following charge doping of a magnetic parent material. In this context, it is important to follow the evolution of the microscopic magnetic properties of the pnictides with doping and hence to determine whether magnetic correlations coexist with superconductivity. Here, we present a muon spin rotation study on SmFeAsO(1-x)F(x), with x=0-0.30 that shows that, as in the cuprates, static magnetism persists well into the superconducting regime. This analogy is quite surprising as the parent compounds of the two families have rather different magnetic ground states: itinerant spin density wave for the pnictides contrasted with the Mott-Hubbard insulator in the cuprates. Our findings therefore suggest that the proximity to magnetic order and associated soft magnetic fluctuations, rather than strong electronic correlations in the vicinity of a Mott-Hubbard transition, may be the key ingredients of high-Tc superconductors.