Mössbauer spectroscopy investigation of the $\mathrm{Immm}$ phase nickelates ${\mathrm{Gd}}_2{\mathrm{BaNiO}}_5$ and ${\mathrm{Tm}}_2{\mathrm{BaNiO}}_5$

The indirect coupling between Ni-O-Ni chains of the $\mathrm{Immm}$ phase nickelates $R_2{\mathrm{BaNiO}}_5\hspace{0.5em}(R=$rare earth) results in a range of interesting magnetic behavior. In this work ${}^{155}\mathrm{Gd}$ Mössbauer spectroscopy data for ${\mathrm{Gd}}_2{\mathrm{BaNiO}}_5\hspace{0.5em}{(T}_{\mathrm{N}}=55\mathrm{}$ K) are interpreted in terms of a constant electric-field gradient tensor and a temperature-dependent magnetic hyperfine field with a saturation value of $24.5$ T. The sensitivity of the spectra to the projection of the hyperfine field enables a magnetic feature at $T=24\mathrm{}$ K to be identified with magnetic reorientation from the a to the b axis. These results assist with the characterization of the crystal-field interaction for the Tm site in isostructural ${\mathrm{Tm}}_2{\mathrm{BaNiO}}_5$ (the first such characterization for this particular structural series).