Cd 2 Os 2 O 7 crystallizes in the pyrochlore structure and undergoes a metal-insulator transition ͑MIT͒ near 226 K. We have characterized the MIT in Cd 2 Os 2 O 7 using x-ray diffraction, resistivity at ambient and high pressure, specific heat, magnetization, thermopower, Hall coefficient, and thermal conductivity. Both single crystals and polycrystalline material were examined. The MIT is accompanied by no change in crystal symmetry and a change in unit-cell volume of less than 0.05%. The resistivity shows little temperature dependence above 226 K, but increases by 3 orders of magnitude as the sample is cooled to 4 K. The specific heat anomaly resembles a mean-field transition and shows no hysteresis or latent heat. Cd 2 Os 2 O 7 orders magnetically at the MIT. The magnetization data are consistent with antiferromagnetic order, with a small parasitic ferromagnetic component. The Hall and Seebeck coefficients are consistent with a semiconducting gap opening at the Fermi energy at the MIT. We have also performed electronic structure calculations on Cd 2 Os 2 O 7. These calculations indicate that Cd 2 Os 2 O 7 is metallic, with a sharp peak in the density of states at the Fermi energy. We interpret the data in terms of a Slater transition. In this scenario, the MIT is produced by a doubling of the unit cell due to the establishment of antiferromagnetic order. A Slater transition-unlike a Mott transition-is predicted to be continuous, with a semiconducting energy gap opening much like a BCS gap as the material is cooled below T MIT .

A spin-density wave (SDW) is shown to mediate a strongly temperature-dependent magnetic coupling between magnetic proximity layers. For parallel or antiparallel moments in the proximity layers, the order parameters of the SDW oscillate as a function of the spacer thickness with a two monolayer period. The SDW phase transition between incommensurate and commensurate phases can be controlled by flipping the magnetization of one of the proximity layers. [S0031-9007(97)02435-6] PACS numbers: 75.70.Cn, 75.30.Ds, 75.30.Fv Spin-density waves (SDW) have been the subject of intensive study for many years [1]. While the SDW in bulk Cr is incommensurate (I), a commensurate (C) SDW can be stabilized by doping. Recently, the SDW phase in thin films or in the spacers of magnetic multilayers has received a great deal of attention. In scanning electron microscopy with polarization analysis (SEMPA) studies [2], Cr films on Fe substrates exhibit an I SDW even well above the Néel temperature T N ϳ 311 K of bulk Cr. Although the SEMPA study cannot distinguish whether the observed SDW is induced by the magnetic coupling with the Fe proximity layers or by the strong Fermi-surface nesting in Cr, perturbed angular correlation spectroscopy on Fe͞Cr multilayers [3] attributes the magnetic behavior of the Cr spacer to the intrinsic antiferromagnetism of bulk Cr. First-principles calculations have shown that the oscillatory magnetic coupling between two Fe layers can be explained by either paramagnetic or antiferromagnetic Cr spacers [4]. Despite these remarkable studies, the relationship between the interface coupling and bulk SDW magnetism is still poorly understood. In this Letter, we study the competition between the bulk SDW antiferromagnetism of a Cr spacer and the magnetic coupling at the interfaces. As described below, this competition affords a novel way to select the I and C SDW phases by switching the magnetic configurations of the proximity magnetic layers.The SDW in bulk Cr is produced by the coherent motion of electrons and holes coupled by the Coulomb interaction [5]. Although usually believed to coincide with the Fermi surface nesting wave vectors Q 6 ͑2p͞a͒ ͑1 6 d͒, the SDW ordering wave vectors are actually given by [6] Q 0 6 Q 6 7 L d 2p͞a, where a is the lattice constant of the conventional bcc unit cell. For pure Cr, d ഠ 0.05 [7] so that the hole Fermi surface is somewhat larger than the electron Fermi surface. When L 0, the SDW wave vector equals the nesting wave vector; when L 1 the SDW is commensurate with the underlying lattice. In order to minimize the nesting free energy DF, 0 , L # 1 so the SDW wave vectors Q 0 6 lie closer to 2p͞a than the nesting wave vectors Q 6 . If the SDW wave vector lies along theẑ direction normal to the multilayer interface, the spin at each atomic layer can be writtenwhere a s is a constant, u is an arbitrary phase, g is an order parameter, and a s g 0.6m B for bulk Cr at zero temperature. For a spacer consisting of N ML's sandwiched by two ferromagnetic layers with moments S M , the...

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The theoretical study of spin diffusion in double-exchange magnets by means of dynamical mean-field theory is presented. We demonstrate that the spin-diffusion coefficient becomes independent of the Hund's coupling J(H) in the range of parameters J(H)S>>W>>T, W being the bandwidth, relevant to colossal magnetoresistive manganites in the metallic part of their phase diagram. Our study reveals a close correspondence as well as some counterintuitive differences between the results on Bethe and hypercubic lattices. Our results are in accord with neutron-scattering data and with previous theoretical work for high temperatures.

Although absent in bulk transition metals, a noncollinear, helical (H) spin-density wave (SDW) is stabilized by steps at the interfaces in Fe͞Cr multilayers. Using the random-phase approximation, we evaluate the phase boundary between the H SDW and the collinear, incommensurate (I) SDW found in bulk Cr. In agreement with neutron-scattering results, the I-to-H transition temperature T IH is always lower than the bulk Néel temperature T N and the nodes of the I SDW lie near the Fe-Cr interfaces. While a H SDW with a single 6p͞2 twist has lower free energy than a I SDW above T N , H SDW's with larger twists are stable between T IH and T N . [S0031-9007(98)