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Magnetic structures are investigated by means of neutron diffraction to shine light on the intricate details which are believed key to understanding the magnetoelectric effect in LiCoPO4. At zero field, a spontaneous spin canting of ϕ = 7(1) • is found. The spins tilt away from the easy b-axis towards c. Symmetry considerations lead to the magnetic point group m ′ z which is consistent with the previously observed magnetoelectric tensor form and weak ferromagnetic moment along b. For magnetic fields applied along a, the induced ferromagnetic moment couples via the Dzyaloshinskii-Moriya interaction to yield an additional field-induced spin canting. An upper limit to the size of the interaction is estimated from the canting angle.

Abstract:We investigated the antiferromagnetic phase transition in the frustrated and multiferroic hexagonal manganites h-YMnO 3 (YMO) and h-(Y 0.98 Eu 0.02 )MnO 3 (YEMO). Elastic neutron scattering was used to study, in detail, the phase transition in YMO and YEMO under zero pressure and in YMO under a hydrostatic pressure of 1.5 GPa. Under conditions of zero pressure, we found critical temperatures of T N = 71.3(1) K and 72.11(5) K and the critical exponent 0.22(2) and β = 0.206(3), for YMO and YEMO, respectively. This is in agreement with earlier work by Roessli et al. Under an applied hydrostatic pressure of 1.5 GPa, the ordering temperature increased to T N = 75.2(5) K, in agreement with earlier reports, while β was unchanged. Inelastic neutron scattering was used to determine the size of the anisotropy spin wave gap close to the phase transition. From spin wave theory, the gap is expected to close with a critical exponent, β , identical to the order parameter β. Our results indicate that the gap in YEMO indeed closes at T N = 72.4(3) K with β = 0.24(2), while the in-pressure gap in YMO closes at 75.2(5) K with an exponent of β = 0.19(3). In addition, the low temperature anisotropy gap was found to have a slightly higher absolute value under pressure. The consistent values obtained for β in the two systems support the likelihood of a new universality class for triangular, frustrated antiferromagnets.

We describe a systematic approach for the design of long, ballistic cold, and thermal neutron guides for the European Spallation Source (ESS). The guides investigated in this work are 170 m long and are required to have a narrowing point with room for a pulse shaping chopper placed 6 m from the moderator. In addition, most guides avoid line-of-sight from the moderator to the sample. The guides are optimized in order to find a reasonable trade-off between neutronics performance and construction price. The geometries simulated are closely related to the thermal-neutron multi-length-scale diffractometer HEIMDAL and the cold-neutron multi-analyser spectrometer BIFROST. For the cold-neutron guide an inexpensive solution was found that maintains good transport properties, while avoiding line-of-sight. However, for the thermal-neutron guide the losses when avoiding line-of-sight are large and it seems a good choice to stay in line-of-sight, even though this will increase both the shielding costs and fast-neutron background. The results are of general relevance for the understanding of the relation between transport, background, and price of long neutron guides.