We obtained optical and near-infrared spectra of Type$,$Ia supernovae (SNe$,$Ia) at epochs ranging from 224 to 496 days after the explosion. The spectra show emission lines from forbidden transitions of singly ionised iron and cobalt atoms. We used non-local thermodynamic equilibrium (NLTE) modelling of the first and second ionisation stages of iron, nickel, and cobalt to fit the spectra using a sampling algorithm allowing us to probe a broad parameter space. We derive velocity shifts, line widths, and abundance ratios for iron and cobalt. The measured line widths and velocity shifts of the singly ionised ions suggest a shared emitting region. Our data are fully compatible with radioactive $^{56}$Ni decay as the origin for cobalt and iron. We compare the measured abundance ratios of iron and cobalt to theoretical predictions of various SN$,$Ia explosion models. These models include, in addition to $^{56}$Ni, different amounts of $^{57}$Ni and stable $^{54,56}$Fe. We can exclude models that produced only $^{54,56}$Fe or only $^{57}$Ni in addition to $^{56}$Ni. If we consider a model that has $^{56}$Ni, $^{57}$Ni, and $^{54,56}$Fe then our data imply that these ratios are $^{54,56}$Fe / $^{56}$Ni $=0.272pm0.086$ and $^{57}$Ni / $^{56}$Ni $=0.032pm0.011$.
Download