Magnetostriction of a ferromagnetic material is caused by lattice strain when spin
moments rotate with an applied magnetic field. When the spin moment rotates
the spin orbit coupling, through the rotation of any unquenched orbital moment,
will cause a small reorientation of the electron clouds which will result in lattice
strain. The measured magnetostriction is proportional to the lattice strain times the
thickness of the film. Our calculated magnetostriction values assumed a constant
films thickness of 150 nm, so the resulting decrease with number of layers would
imply a decrease in film stress. Thus the Ru is effectively causing a reduction in
stress of the total film by isolating the layers from each other. If we assume that
the films stress is constant in each of the layers, then our results are proportional
to the thickness of each individual layer, showing that the Ru effectively lets each
layer act independently. The stress in each CoFe layer is not changed, but the total
film has a magnetostriction that is determined by the thickness of each individual
CoFe layer. The FMR results, which should be confirmed by VSM measurements,
show that the total moment is not significantly changed by the Ru so the moment
is independent of the Ru, but the magnetostriction of the total film is reduced.