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We present the results of muon-spin-relaxation experiments for two materials which show geometric frustration. ZnCr 2 O 4 has a spinel structure with S = 3 2 spins on a lattice of corner-sharing tetrahedra. Our experiments show that a local magnetic field which is quasi-static on the muon timescale develops below T c = 12.5 K, a transition which has been associated with a three-dimensional analogue of the spin-Peierls transition. In contrast, Gd 3 Ga 5 O 12 has a garnet structure with S = 7 2 spins arranged on interpenetrating triangular sublattices. In this material the muon data exhibit a temperature-dependent spin-relaxation rate indicative of slow spin fluctuations. We discuss these differing behaviours and relate them to the underlying physics in the two materials.

Epitaxial films of the well-known alpha ͑orthorhombic͒ structure and an unusual hcp form of uranium have been grown on Nb and Gd buffers, respectively, by sputtering techniques. In a 5000 Å film of ␣-U a chargedensity wave has been observed, and its properties are different from those found in the bulk. The 500 Å hcp-U film has a c / a ratio of 1.90͑1͒, which is unusually large for the hcp structure. Theoretical calculations show that this hcp form is metastable and predict that it orders magnetically.

X-ray magnetic circular dichroism ͑XMCD͒ measurements were performed at the U M 4,5 edges and Fe K edge on well-defined uranium/iron multilayers with different compositions. The multilayers have layer thicknesses in the range 9 -40 Å for uranium and 9 -34 Å for iron. At both 10 K and room temperature, the U layers are magnetically polarized in all of the multilayers studied. To deduce the magnetic moment on the uranium from the XMCD results requires assumptions about the magnetic dipole operator ͗T Z ͘ in the analysis, and this is discussed in detail. Given the most likely scenario of strong hybridization between the U 5f and Fe 3d states, the largest value of the induced U moment is ϳ0.12 B , which is located primarily at the interface and oscillates within the uranium layer.

Epitaxial layers of uranium have been grown on a variety of buffer/seed layers on sapphire substrates by UHV magnetron sputtering and their structure determined using x-ray diffraction. The buffer layers were epitaxial layers of niobium, tungsten and niobium covered by a seed layer of hcp gadolinium, on which uranium layers were grown to a thickness of 600 Å. The x-ray diffraction results establish that the α-orthorhombic phase of uranium grows epitaxially in the (110) orientation on the niobium (110) buffer, while on the tungsten (110) buffer the growth planes of the α-uranium were (002) and for the growth on the gadolinium buffer the α-uranium was predominantly (021) oriented. These results show that epitaxial uranium films in selected orientations can be grown by using an appropriate buffer. To our knowledge this is the first report of epitaxial α-uranium films, and it is significant because of the difficulty of growing single crystals of α-uranium due to the occurrence of high temperature structural transformations.