  We report on the synthesis and structural characterization of the
magnetocaloric FeMnP$_{0.75}$Si$_{0.25}$ compound. Two types of samples (as
quenched and annealed) were synthesized and characterized structurally and
magnetically. We have found that minute changes in the degree of
crystallographic order causes a large change in the magnetic properties. The
annealed sample, with higher degree of order is antiferromagnetic with a zero
net moment. The as-quenched sample has a net moment of 1.26 $\mu_B$/f.u. and
ferrimagnetic-like behavior. Theoretical calculations give rather large values
for the Fe and Mn magnetic moments, both when occupied on the tetrahedral and
pyramidal lattice site. The largest being the Mn moment for the pyramidal site
reaches values as high as 2.8 $\mu_B$/atom.
