Mössbauer studies of Ba(Fe_1-xNi_x) ₂As₂

We present detailed 57Fe Mössbauer effect spectroscopy (MS) measurements at various temperatures for Ba(Fe_1-xNi_x) ₂As₂. The isomer shift values for all samples are in the range of 0.42 ± 0.02 mm s^-1, indicating a typical metallic state of divalent Fe ions. The MS spectra of the magnetic samples (up to x = 0.024) are well reproduced by spin density waves subspectra. Both the magnetic ordering temperatures TM and the average magnetic hyperfine fields H _eff, decrease with x. The H_eff values scale linearly with TM- For higher x values the samples become superconducting and the MS spectra below and above Tc are almost identical, indicating that the MS technique are not sensitive enough to the superconducting transition.