No abstract

The temperature (T) dependence of the anisotropic magnetic penetration depth (T) of magnetically aligned powders of crystalline HgBa 2 Ca 2 Cu 3 O 8ϩ␦ is reported. Measurements were performed in the Meissner state using the ac-susceptibility technique. The temperature dependences of the in-plane, ab (T), and out-of-plane, c (T), penetration depths are markedly different. This is believed to arise from the large anisotropy ratio ␥ϭ͓ c ͑0͒/ ab ͑0͔͒Ӎ30. The behavior of ab (T) is indicative of d-wave superconductivity while c (T) is similar to the behavior expected for a superconductor with intrinsic Josephson coupling between the CuO 2 planes. Similar measurements were performed on Ba 0.6 K 0.4 BiO 3 powders for comparison.

The temperature variation of the magnetic susceptibility of the compounds LiNiO2 and NaNiO2 has been measured. Contrary to a previous report by Hirakawa and co-workers (1985), which claimed that these materials were antiferromagnetically coupled, and so could potentially possess quantum liquid ground states, the authors' data suggest that these materials behave as weakly coupled 2D Ising ferromagnets. They show that this conclusion is in fact support by much of the data in the previous report.

The anisotropy of the static homogeneous magnetic spin susceptibility of the antiferromagnetic CuO 2 bilayers and the crystal-field parameters are measured in Gd-doped YBa 2 Cu 3 O 6ϩx ͑small x͒ single crystals using Gd 3ϩ ESR at 9, 75, 150, and 225 GHz. We show that the easy magnetization direction is along ͓100͔ and that there is a magnetostriction leading to an orthorhombic lattice distortion. We observe an antiferromagnetic domain structure corresponding to the two equivalent orthorhombic distortions of the tetragonal lattice which depends on magnetic fields of the order of 1 T. The domain structure is unchanged between 10 and 150 K and is independent of thermal and magnetic history. We discuss two models: ͑i͒ charged domain walls, ͑ii͒ magnetization pinned to a small number of defective oxygen-rich Cu͑1͒ layers.