  Nucleon momentum distribution (NMD), particularly its high-momentum
components, is essential for understanding the nucleon--nucleon ($ NN $)
correlations in nuclei. Herein, we develop the studies of NMD of
$^{56}\text{Fe}$ from the axially deformed relativistic mean-field (RMF) model.
Moreover, we introduce the effects of $ NN $ correlation into the RMF model
from phenomenological models based on deuteron and nuclear matter. For the
region $ k<k_{\text{F}} $, the effects of deformation on the NMD of the RMF
model are investigated using the total and single-particle NMDs. For the region
$ k>k_{\text{F}} $, the high-momentum components of the RMF model are modified
by the effects of $ NN $ correlation, which agree with the experimental data.
Comparing the NMD of relativistic and non-relativistic mean-field models, the
relativistic effects on nuclear structures in momentum space are analyzed.
Finally, by analogizing the tensor correlations in deuteron and Jastrow-type
correlations in nuclear matter, the behaviors and contributions of $ NN $
correlations in $^{56}\text{Fe}$ are further analyzed, which helps clarify the
effects of the tensor force on the NMD of heavy nuclei.
