Determination of equilibrium coupling angles in magnetic multilayers by polarized neutron reflectometry

We have performed polarized neutron reflectometry (PNR) on Co/Cu multilayer s grown by sputter deposition at the first antiferromagnetic (AF) maximum o f the coupling oscillation. The growth of the Cu spacer layers was paused h alfway through each layer for a variable amount of time to allow residual g ases to be adsorbed onto the surface. A sample with clean Cu spacers shows good AF coupling, with low remanence and high saturation field. The PNR spe ctra show a strong 1/2-order Bragg peak and little splitting between the re flectivities for incident up arrow and down arrow spin neutrons at zero fie ld, characteristic of AF ordering. Meanwhile, a more heavily gas-damaged sa mple with a remanent fraction of similar to root2/2 has strongly spin-split PNR spectra at the critical edge and nuclear Bragg peak, showing a signifi cant ferromagnetic component. A strong 1/2-order Bragg peak is still presen t. We are able to fit accurately the magnetization and PNR data by assuming that such a sample shows considerable biquadratic coupling, with moments c oupled close to 90 degrees at zero field.