Fe Mossbauer-effect studies on grain-aligned YBaz(Cul "Fe")408samples have permitted us to establish spatial anisotropy of the Fe Debye-Wailer factor and electric-field-gradient parameters in the allimportant CuO, planes. The ratio of the f factor parallel and perpendicular to the c axis, f, I f "=2. 0(1). Our analysis shows V" to be positive and directed along the c axis but motionally averaged because of rapid fluctuations of oxygen atoms in the planes. Vibrating-sample-magnetometry measurements with the applied field parallel and perpendicular to the c axis show that the critical-current anisotropy J,/Jj, =3.25 and is insensitive to Fe doping concentrations in the range 0~x 0.03.

(Cu, .xZnx)30,.4 superconductors have been synthesized, and characterized according to resistivity, AC susceptibility and oc magnetization measurements. oc magnetization measurements on randomly oriented powdered systems exhibit pronounced M(H) hysteresis loops, indicative of extensive flux pinning. We feel that this flux pinning arises due to the Zn acting as a pinning centre. The critical current density J, has been deduced from hysteresis data

A 57Fe Mossbauer spectroscopy study of Mn,-,Fe,As with x = 0.01, 0.03 and 0.15 has been undertaken. For x = 0.01 a peculiar temperature dependence of the hyperfine field has been observed in the ferromagnetic phase. The change in the electric field gradient at the first order transition at 275(5) K has been explained on the basis of a change in the electron configuration of the 3d-electrons of Fe using the crystal field splitting model developed for MnAs by Haneda and coworkers. The magnetic hyperfine field at the Fe nucleus is very small as compared to the corresponding hyperfine field at the Mn nucleus. Stable extra phases, not observed by X-ray diffraction, involving Fe atoms are formed for x = 0.01 and x = 0.03 when heating the sample above 400 K and 540 K respectively.

Stoichiometric Ca2CuO3, having square-planar Cu-chains [C. L. Teske and H. Müller-Buschbaum, Z. Anorg. Allg. Chem. 379, 234 (1970); M. Hjorth and J. Hyldtoft, Acta Chem. Scand. 44, 516 (1990)], is expectedly antiferromagnetic due to 1D intrachain superexchange [K. Okuda, S. Noguchi, K. Konishi, H. Deguchi, and K. Takeda, J. Magn. Magn. Mater. 104–107, 817 (1992)]. Nonetheless, we report remarkable spontaneous ferromagnetism at 293 K after sintering, prominently in oxygen. This apparently introduced excess oxygen at vacant quasioctahedral sites, promoting spin-flip and ferromagnetic interchain coupling. Thermogravimetry (TGA) revealed excess oxygen, ≈0.17 O/Cu. X-ray diffraction (XRD) yielded a comparatively smaller unit cell. Ferromagnetism disappeared by oxygen depletion, mimicking reported nonmagnetism of Ca2CuO3−δ [Okuda et al. (1992)]. Elemental analysis showed insignificant magnetic impurity traces. Tc ranged between 680 K and 723 K, depending on freshness and purity. Saturation magnetization varied with processing, optimally 0.30 A · m2/kg at 1.0 T applied field. Coercivity and remanence varied with purity.