Mossbauer spectroscopy investigation of the Immm phase nickelates Gd2BaNiO5 and Tm2BaNiO5

The indirect coupling between Ni-O-Ni chains of the Immm phase nickelates R 2BaNiO5 (R = rare earth) results in a range of interesting magnetic behavio r. In this work Gd-155 Mossbauer spectroscopy data for Gd2BaNiO5 (T-N = 55 K) are interpreted in terms of a constant electric-field gradient tensor an d a temperature-dependent magnetic hyperfine field with a saturation value of 24.5 T. The sensitivity of the spectra to the projection of the hyperfin e field enables a magnetic feature at T = 24 K to be identified with magnet ic reorientation from the a to the b axis. These results assist with the ch aracterization of the crystal-field interaction for the Tm site in isostruc tural Tm2BaNiO5 (the first such characterization for this particular struct ural series).