2-DIMENSIONAL SPIN FREEZING IN Y1-ZCAZBA2CU3-XFEXO7

Hyperfine electric and magnetic interactions in Y1-zCaBa2Cu3-xFexO6+y compounds have been studied for different rates of Fe and Ca substitut ion (x = 0.09 and 0.24; z = 0.24; y = 0-1) by Fe-57 Mossbauer spectros copy of powder samples obtained by different thermal treatments. Calcu lations of the Fe3+ EFG tensor have been performed for different coord ination polyhedra in Cu1 and Cu2 sites. Variations of the direction, a mplitude and of the sip of the principal component of the EFG are repo rted versus the iron displacement from the ideal Cu sites inside the p yramidal and tetrahedral coordination polyhedra. Calculated DELTAE(Q) and eta values are compared with the experimental ones. For the Cu2 si te, faithfully probed in both oxygen-depleted and oxygen-saturated sam ples by fairly large fraction of residing iron atoms, the mutual orien tations of the axes of the principal EFG component and that of easiest magnetization are deduced from interrelations between quadrupole para meters for paramagnetic quadrupolar and magnetic Zeeman Mossbauer spec tra. In the desoxygenated samples, the coexistence of a long range 2D antiferromagnetic order and of a spin-glass order has been evidenced. For the oxygen-saturated samples, the two-dimensional spin-glass order is observed below T(f) = 10 K. The Mossbauer spectra under applied fi elds at 4.2 K show an easy polarization of the spins in the Cu2 sites.