Coexistence of antiferromagnetic ordering and superconductivity in the Ba(Fe0.961Rh0.039)2As2 compound studied by Mössbauer spectroscopy

The results of a $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy study between 2.0 and 294 K of superconducting Ba(Fe${}_{0.961}$Rh${}_{0.039}$)${}_{2}$As${}_{2}$ are reported. The main component of the electric field gradient tensor at 294 K is shown to be positive and its increase with decreasing temperature is well described by a ${T}^{3/2}$ power-law relation. The shape of the M\"ossbauer spectra below the N\'eel temperature ${T}_{N}=55.5(1)$ K is shown to result from the presence of doping-induced disorder rather than of incommensurate spin-density-wave order. The measured hyperfine magnetic field reaches its maximum value at the critical temperature ${T}_{c}=14$ K and then decreases by 4.2% upon further cooling to 2.0 K. This constitutes direct evidence of the coexistence of and competition between superconductivity and magnetic order. The extrapolated value of the Fe magnetic moment at 0 K is determined to be 0.35(1) ${\ensuremath{\mu}}_{B}$. The Debye temperature of Ba(Fe${}_{0.961}$Rh${}_{0.039}$)${}_{2}$As${}_{2}$ is found to be 357(3) K.