Investigation of SmFe$_{1-x}$Co$_x$AsO using $^{57}$Fe M\"{o}ssbauer spectroscopy as a function of temperature and applied magnetic field

The M\"{o}ssbauer spectra(MS) of SmFe$_{1-x}$Co$_x$AsO, where $x$=0.0, 0.05, and 0.1, were measured in applied fields up to 9 T and at temperatures between 4.2 K and 298 K. SmFeAsO is magnetically ordered with T$_N$=138 K and has a hyperfine magnetic field of 5.05 T at 4.2 K. In a 9 T magnetic field the MS is consistent with a distribution of hyperfine magnetic fields and has a width of $H_{apllied}+H_{hyperfine} \simeq 13 T$, suggesting that the sample is not polarized and evidence of strong magnetic anisotropy. The MS of SmFe$_{0.95}$Co$_{.05}$AsO is well-described by a Lorentzian peak from room temperature to 4.2 K. From room temperature to $\sim$25 K the linewidth is 0.30 mm/sec but then increases to 0.72 mm/sec when cooled from 25 K to 10 K. Below 10 K it remains constant. The MS of SmFe$_{0.9}$Co$_{.1}$AsO shows no evidence of internal field at any temperature.