  We have studied a spin-1/2, ABA, Ising trilayer system with two different
types of interactions, in-plane ferromagnetic and out-of-plane
anti-ferromagnetic, among the lattice sites, on different layers. In the pure
case, devoid of any impurity, we employed Monte-Carlo method with single
spin-flip Metropolis algorithm to find out the anti-ferromagnetic critical
temperature (N\'eel temperature) and another lower temperature, called the
compensation temperature, both with total magnetisation zero, in accordance
with already established results for square lattice. Then non-magnetic
impurities, (spin value= 0) were implanted on each layer at randomly picked
sites and their concentration was increased in steps from 5% to 20%. The ratio
of inter-planar anti-ferromagnetic to mid-layer ferromagnetic coupling strength
as well as the ratio of top and bottom layer ferromagnetic to mid layer
ferromagnetic coupling strength were also varied and N\'eel and compensation
temperatures, both were observed shifting towards lower temperature values with
increase in concentration of impurities in the lattice for any fixed ratio of
different coupling strengths, in absence of any external magnetic field. In
addition, the magnitude of the residual magnetisation i.e. the ratio of the
peak value of the magnetisation in between N\'eel and compensation points and
the saturated value of magnetisation, was also observed to vary with different
values of controlling parameters.
