The eigenproblems of spin waves in a symmetrical ferromagnetic bilayered system with periodic boundary conditions are solved using the interface-rescaling approach (IRA).The results show that interface coupling between two sublayers would not change the excitation energy of odd bulk modes,but change excitation energy of even bulk modes.We call this peculiar phenomenon the phenomenon of even bulk mode variance (PEBMV).There are two kinds of mechanisms which cause PEBMV:phase reversal and phase translation of the magnon at the interface,corresponding,respectively,to the antiferromagnetic and ferromagnetic interface coupling cases.PEBMV embodies the selective effect of the interface on different bulk magnons.
