Magnetic ordering in ~(57)Fe-doped EuNiO_3 and NdNiO_3 studied by Moessbauer spectroscopy

摘要

~(57)Fe Moessbauer spectra of EuNiO_3 and NdNiO_3 doped with 1 at. % ~(57)Fe taken in the temperature range 1.5 ≤ T ≤ 300 K reveal two magnetically inequivalent Fe sites. About half of the Fe atoms show a magnetic hyperfine interaction that follows the macroscopic magnetization: a second-order magnetic phase transition is observed for EuNiO_3 at T_N =190(10) K, while a first-order magnetic transition is seen for NdNiO_3 at T_N = 150(10) K. The other half of the Fe atoms reveals a slowing down of fluctuating hyperfine fields on decreasing temperature, indicating that these sites are magnetically frustrated. This can be well explained by a charge disproportionation occurring at the Ni sites.