155Gd-Mössbauer studies of the T′-, T ∗- and T-phases of La 2- x-yGd xSr yCuO 4

Abstract

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- y Gd x Sr y CuO 4. In T′ -phase Gd 2CuO 4, 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 0.15CuO 4, 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 0.15CuO 4, 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.