¹⁵⁵Gd-Mössbauer studies of the T′-, T^*- and T-phases of La_2-x-yGd_xSr_yCuO₄

¹⁵⁵Gd-Mössbauer studies determining the electric and magnetic hyperfine interactions reveal the location and magnetic properties of the Gd ions in the three possible phases of La_2-x-yGd_xSr_yCuO₄. In T′ -phase Gd₂CuO₄, the Gd ions are located in lower local point symmetry than expected from the tetragonal structure, order at T_N = 6.65 K and interact with the weak ferromagnetic component of the Cu sublattice. The Gd magnetic moments are oriented at an angle of 45° relative to the crystallographic c-axis. The analysis of the temperature dependence of the observed hyperfine fields reveals a zero-field splitting of the Gd S _{7 2} ground state. In the superconducting T-phase of La_1.775Gd_0.075Sr_0.15CuO₄, Gd is paramagnetic and the Mössbauer spectra exhibit a large quadrupole splitting of 4.2 mm/s, reflecting the different oxygen surrounding in comparison to the T′-phase (1.39 mm/s). The T^*-phase, LaGd_0.85Sr_0.15CuO₄, exhibits below 4 K a dominant subspectrum of magnetically ordered Gd ions at M′ sites (eight-fold oxygen coordination) with hyperfine parameters similar to the T′-phase and minority sites with larger quadrupole splittings and paramagnetic behaviour.