No abstract

We attempt to solve the magnetic structure of the gadolinium analogue of 'spin-ice', using a mixture of experimental and theoretical assumptions. The eventual predictions are essentially consistent with both the Mössbauer and neutron measurements but are unrelated to previous proposals. We find two possible distinct states, one of which is coplanar and the other is fully three-dimensional. We predict that close to the initial transition the preferred state is coplanar but that at the lowest temperature the ground-state becomes fully three-dimensional. Unfortunately the energetics are consequently complicated. There is a dominant nearest-neighbour Heisenberg interaction but then a compromise solution for lifting the final degeneracy resulting from a competition between longer-range Heisenberg interactions and direct dipolar interactions on similar energy scales.

The intermetallic compound YbCu 2 Si 2 is a well-known nonmagnetic ͑NM͒ Yb intermediate-valent compound with a Yb valence of 2.9 at ambient pressure and 300 K. In the present work we have investigated the effect of high pressure on the ground state properties of YbCu 2 Si 2 on both microscopic and macroscopic levels by using the 170 Yb Mössbauer effect, electrical resistance, and x-ray diffraction techniques, respectively. High-pressure x-ray diffraction data indicate that the lattice structure of YbCu 2 Si 2 is stable up to 22.2 GPa. The value of the bulk modulus ͓B 0 ϭ168(10) GPa Ϫ1 ͔ is found to be close to the value expected for trivalent RCu 2 Si 2 compounds. The pressure dependence of the electrical resistance reveals evidence for a pressureinduced magnetic order for pу8 GPa. From our Mössbauer data, we conclude a crossover from the NM to a magnetically ordered state of localized Yb moments for pу8 GPa and below 2 K. The pressure-induced change of the electric quadrupole splitting indicates that this transition is accompanied by a valence change towards the Yb 3ϩ state. ͓S0163-1829͑99͒01729-4͔