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A caged compound CeOs2Al10, crystallizing in the orthorhombic YbFe2Al10-type structure, undergoes a mysterious phase transition at T0 = 29 K. We report the results of electron diffraction, magnetization, and magnetoresistance for single crystals. Superlattice reflections characterized by a wave vector q = (0, −2/3, 2/3) observed at 15 K indicate a structural modification in the ordered state. Activation-type behavior of the electrical resistivity along the three principal axes below 50 K suggests gap opening in the conduction band. The magnetic susceptibility χ = M/B is highly anisotropic, χa > χc > χ b , all of which sharply decrease on cooling below T0. Furthermore, a metamagnetic anomaly in the magnetization and a step in the magnetoresistance occur at B = 6-8 T only when the magnetic field is applied parallel to the orthorhombic c axis. However, T0 hardly changes under magnetic fields up to 14 T, irrespective of the field direction. By using these data, we present a B-T phase diagram and discuss several scenarios for the mysterious transition.

We report the observation of a long-range magnetic order in a quasicrystalline approximant, i.e., Cd 6 Tb, demonstrating that a spin glass is not the ground state for the binary approximant but localized spins on the vertices of an icosahedron become antiferromagnetically ordered below 24 K. The result is in contrast to the spin-glasslike behaviors reported for Cd-Mg-R quasicrystals composed of same icosahedral clusters.

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Neutron scattering experiments have been performed to elucidate magnetic properties of the quasicrystal approximant Au 70 Si 17 Tb 13 , consisting of icosahedral spin clusters in a body-centered-cubic lattice. Bulk magnetic measurements performed on the single crystalline sample unambiguously confirm long-range ordering at T C = 11.6 ± 1 K. In contrast to the simple ferromagnetic response in the bulk measurements, single crystal neutron diffraction confirms a formation of intriguing non-collinear and non-coplanar magnetic order. The magnetic moment direction was found to be nearly tangential to the icosahedral cluster surface in the local mirror plane, which is quite similar to that recently found in the antiferromagnetic quasicrystal approximant Au 72 Al 14 Tb 14 . Inelastic neutron scattering on the powdered sample exhibits a very broad peak centered at ω 4 meV. The observed inelastic spectrum was explained by the crystalline-electric-field model taking account of the chemical disorder at the fractional Au/Si sites. The resulting averaged anisotropy axis for the crystalline-electric-field ground state is consistent with the ordered moment direction determined in the magnetic structure analysis, confirming that the non-coplanar magnetic order is stabilized by the local uniaxial anisotropy.

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The structure of an Al-Ni-Co decagonal ͑d-͒ quasicrystal has been investigated by scanning tunneling microscopy ͑STM͒. STM images with atomic-scale resolution have been obtained successfully for the surfaces of both tenfold and twofold planes. On the tenfold surface, large terraces and monoatomic-layer steps are formed. The symmetry of each layer is not decagonal but pentagonal and the two adjacent layers are related by the inversion symmetry. The step lines are rough, which can be attributed to the existence of many symmetrically equivalent low-energy steps. The atom adsorptions are often observed at locally symmetric sites. An analysis based on the high-dimensional description of the quasicrystalline structure has shown that the structure has nearly perfect quasiperiodic order for the decagonal quasicrystal. On the twofold surface, interlayer phason defects are observed, but the density of them is quite low. This fact indicates that the d-quasicrystal of the present sample is not in the random tiling state in which the configurational entropy related to the phason disorder stabilizes the quasicrystal.

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