Observation of antiparallel magnetic order in weakly coupled Co/Cu multilayers

Polarized neutron reflectivity and scanning electron microscopy with polarization analysis are combined to determine the magnetic structure of Co(6 nm)/Cu(6 nm) multilayers. These data resolve a controversy regarding the low-field state of giant-magnetoresistive (GMR) multilayers with weak coupling. As-prepared samples show a strong antiparallel correlation of in-plane ferromagnetic Co domains across the Cu. At the coercive field, the Co domains are uncorrelated. This irreversible transition explains the decrease in magnetoresistance from the as-prepared to the coercive state. For both states, the Co moments reside in domains with in-plane sizes of approximate to 0.5-1.5 mu m. [S0031-9007(99)08797-9].