  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
$=0.272\pm0.086$ and $^{57}$Ni / $^{56}$Ni $=0.032\pm0.011$.
