GD-155-MOSSBAUER STUDIES OF THE T'-PHASE, T-ASTERISK-PHASE AND T-PHASE OF LA2-X-YGDXSRYCUO4

Gd-157-Mossbauer studies determining the electric and magnetic hyperfi ne interactions reveal the location and magnetic properties of the Gd ions in the three possible phases of La2-x-yGdxSryCuO4. In T'-phase Gd 2CuO4, the Gd ions are located in lower local point symmetry than expe cted from the tetragonal structure, order at T(N)=6.65 K and interact with the weak ferromagnetic component of the Cu sublattice. The Gd mag netic moments are oriented at an angle of 45-degrees relative to the c rystallographic c-axis. The analysis of the temperature dependence of the observed hyperfine fields reveals a zero-field splitting of the Gd S7/2 ground state. In the superconducting T-phase of La1.775Gd0.075Sr 0.15CuO4, Gd is paramagnetic and the Mossbauer spectra exhibit a large quadrupole splitting of 4.2 mm/s, reflecting the different oxygen sur rounding in comparison to the T'-phase (1.39 mm/s). The T-phase, LaGd 0.85Sr0.15CuO4, exhibits below 4 K a dominant subspectrum of magnetica lly ordered Gd ions at M' sites (eight-fold oxygen coordination) with hyperfine parameters similar to the T'-phase and minority sites with l arger quadrupole splittings and paramagnetic behaviour.