  We study the two-step spin crossover in a double-layered elastic model based
on transition metal complexes each taking high spin (HS) and low spin (LS)
states. Here, only the simplest elastic interactions between adjacent molecules
are considered and the system is exposed to the external pressure within the
framework of $NPT$-Monte Carlo method. As a certain amount of pressure is
applied, the first order thermal transition between uniform HS and LS phases
transforms to a two-step transition with an emergent intermediate spin (IS)
phase, where the HS and LS molecules are paired face to face between layers and
form diagonally striped clusters within the layer. The difference in the size
of HS and LS molecules is reflected both in the elastic interactions and in the
enthalpy, and the IS phase could gain the latter over the loss of the former by
significantly reducing its volume. The present pressure effect is interpreted
to the chemical one in double-layered transition metal materials, which
actually reveals a variety of multistep spin crossover transitions relevant to
our numerical result.
