Under the assumption that in a double-layer film both layers are equally deformed, the following formula for the magnetostriction energy

has been derived where

is the coupling constant and α is the angle between the magnetization vectors. This formula is valid if we can neglect interaction with the substrate. The sign of the coupling constant is discussed. It turns out that where the composition of the two layers are different, the value of K
ccan be, in some special cases, negative.