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Sources of magnetic fields--magnetic monopoles--have so far proven elusive as elementary particles. Condensed-matter physicists have recently proposed several scenarios of emergent quasiparticles resembling monopoles. A particularly simple proposition pertains to spin ice on the highly frustrated pyrochlore lattice. The spin-ice state is argued to be well described by networks of aligned dipoles resembling solenoidal tubes--classical, and observable, versions of a Dirac string. Where these tubes end, the resulting defects look like magnetic monopoles. We demonstrated, by diffuse neutron scattering, the presence of such strings in the spin ice dysprosium titanate ([Dy.sub.2][Ti.sub.2][O.sub.7]). This is achieved by applying a symmetry-breaking magnetic field with which we can manipulate the density and orientation of the strings. In turn, heat capacity is described by a gas of magnetic monopoles interacting via a magnetic Coulomb interaction. 10.1126/science.1178868

In the course of their day, sample environment professionals can be confronted by numerous technical challenges applicable to a range of scientific questions. This paper presents three successful outcomes from user-posed sample environment challenges for in situ neutron scattering experiments undertaken at the Australian Centre for Neutron Scattering (formerly the Bragg Institute). The sample environments presented here have nothing in common other than their novelty. They may not be the best solution but have been constrained by time, resources and ability. The questions the users posed were: Can we mount a cylinder in cylinder (CIC) rheometer, more regularly used on a small angle scattering instrument, on a diffraction instrument and obtain usable data? Can we supply high-voltage (up to 10 kV) across a sample within the Paris-Edinburgh press while mounted on a powder diffraction instrument? And finally can a Lakeshore 340 and an in-house built liquid conductivity cell do the job of a commercial liquid conductivity meter? This paper presents the engineering and equipment solutions that were used to answer these questions, and in each case the scientific users left with useful, intriguing and, hopefully, publishable data. [ABSTRACT FROM AUTHOR]

Recent powder patterns of the antiferromagnet FePS3 below its Ne´el temperature show unusual magnetic diffuse intensity in the Q range 0.5–0.8 A˚−1. This was initially thought to be a group of unresolved magnetic Bragg peaks. However subsequent powder patterns taken with longer wavelengths and better resolution have failed to resolve the peaks except to confirm the existence of a Bragg peak at Q=0.61 A˚−1. In particular the powder pattern taken with λ=5 A˚ neutrons shows a marked diminution of the rest of the diffuse intensity with respect to this Bragg peak. To test whether the diffuse intensity is due to magnon scattering, the low Q region of the scattering from the powder sample of FePS3 was measured on LONGPOL at 100 K with both polarization and energy analysis. The diffuse intensity was confirmed as magnetic but energy analysis showed that it is at least overwhelmingly elastic with no suggestion of scattering with an energy gain of 13 meV, the energy expected for the magnon band gap in this highly anisotropic antiferromagnet. [Copyright &y& Elsevier]

Transition-metal thiophosphates, MPS[sub 3] (M = Fe, Mn, etc.), make up a class of antiferromagnetic materials with quasi-two-dimensional magnetic behaviour. The metal atoms occupy a honeycomb lattice. In MnPS[sub 3], the first-neighbour interaction is 400 times stronger than the exchange between the planes and the Mn moments show little anisotropy. The antiferromagnetic order is not well established perpendicular to the planes. Here we present the first powder neutron diffraction patterns of FePS[sub 3]. The absence of trailing edges on the magnetic Bragg peaks, such as have been observed in MnPS[sub 3], indicates that the magnetic order is threedimensional in FePS[sub 3]. [ABSTRACT FROM AUTHOR]

Abstract: The upgrade of the cold neutron triple-axis spectrometer FLEXX is described. We discuss the characterisation of the gains from the new primary spectrometer, including a larger guide and double focussing monochromator, and present measurements of the energy and momentum resolution and of the neutron flux of the instrument. We found an order of magnitude gain in intensity (at the cost of coarser momentum resolution), and that the incoherent elastic energy widths are measurably narrower than before the upgrade. The much improved count rate should allow the use of smaller single crystals samples and thus enable the upgraded FLEXX spectrometer to continue making leading edge measurements. [Copyright &y& Elsevier]

We present a detailed study in the paramagnetic regime of the frustrated s = 1/2 spin-compound linarite PbCuSO4(OH)2 with competing ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neiglibor exchange interactions. Our data reveal highly anisotropic values for the saturation field along the crystallographic main directions, with ∼7.6, ∼10.5, and ∼8.5 T for the a, b, and c axes, respectively. In the paramagnetic regime, this behavior is explained mainly by the anisotropy of the g factor, but leaving room for an easy-axis exchange anisotropy. Within the isotropic J1-J2 spin model, our experimental data are described by various theoretical approaches, yielding values for the exchange interactions J1 ∼ --100 K and J2 ∼ 36 K. These main intrachain exchange integrals are significantly larger as compared to the values derived in two previous studies in the literature and shift the frustration ratio &agr; = J2/|J1| ≈ 0.36 of linarite closer to the one-dimensional critical point at 0.25. Electron spin resonance (ESR) and nuclear magnetic resonance (NMR) measurements further prove that the static susceptibility is dominated by the intrinsic spin susceptibility. The Knight shift as well as the broadening of the linewidth in ESR and NMR at elevated temperatures indicate a highly frustrated system with the onset of magnetic correlations far above the magnetic ordering temperature TN = 2.75(5) K, in agreement with the calculated exchange constants. [ABSTRACT FROM AUTHOR]