A model of magnetostriction interaction in thin double-layer magnetic films of Ni-Fe alloy

Under the assumption that in a double-layer film both layers are equally deformed, the following formula for the magnetostriction energy
E = K_{c} \sin^{2} \alpha
has been derived where
K_{c} = (9/40)(2U'U"/U'_{0} + U_{0}")
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
c
can be, in some special cases, negative.