  We investigate the exchange coupling constant $J_{ij}$ in Nd$_x$Fe$_{1-x}$
amorphous alloys with different compositions within the framework of
first-principles calculation. We observed a strong atomic-dependence of
$J_{ij}$ and its fluctuations. We show that the composition strongly affects
the distance dependence of $J_{ij}$. Composition dependence of calculated Curie
temperatures is modest for $x<0.5$. To examine the effect of the local
environment on the exchange couplings, we demonstrate combined analyses of the
coordination structure and exchange coupling constants using the Gabriel graph.
Our study reveals that the Curie temperatures are mostly dominated by the
averaged $J_{ij}$ and coordination numbers determined by the pairs of
neighboring atoms. We also observed that the exchange couplings between Fe--Fe
and Fe--Nd become stronger with increasing the number of surrounding Nd atoms.
