Magnetic structure determination for annealed Ni₈₀Fe₂₀/Ag multilayers using polarized-neutron reflectivity

Sputtered Ni₈₀Fe₂₀/Ag multilayers, annealed post-growth, show giant magnetoresistive (GMR) effects at unusually low magnetic fields (≈ 5 Oe)[1]. Structural characterization by cross-sectional TEM[2] and x-ray diffraction indicates that the Ag preferentially diffuses into the Ni₈₀Fe₂₀ layers at the interfaces. Using polarized-neutron specular reflectivity, we have obtained magnetization depth profiles for a series of annealed [Ni₈₀Fe₂₀(20 angstrom)/Ag(40 angstrom)]₄ multilayers. Though GMR in related materials is associated with coherent antiferromagnetic alignment of the ferromagnetic layers, specular neutron data for the Ni₈₀Fe₂₀/Ag multilayers show no trace of half-order spin-flip intensity characteristic of this simple structure. In small applied fields, transverse scans at the half-order position show a broad feature which disappears upon saturation. These data suggest that while the Ni₈₀Fe₂₀ moments are antiferromagnetically correlated along the growth axis, the in-plane magnetic domains are only of micron-order size and are thus not apparent in a specular measurement.