Study of magnetization of a bilayer nanoststructure CoCu/Co (GF/F) by polarized neutron reflectometry

Magnetization reversal of the exchange-coupled granulated ferromagnetic (GF) CoCu and ferromagnetic (F) Co nanolayers is studied by polarized neutron reflectometry. The main parameters that specify the structure and saturation magnetization of the GF and F layers are determined for Si(substrate)/Co0.5Cu0.5(5nm)/Co(x)/Si(3nm) samples with a thickness x varying from 6 to 20 nm. Neutron data allow some suggestions to be made about the features of the GF/F bilayer magnetization reversal. For small x the main mechanism of the magnetization reversal is the domain wall motion hindered by the exchange interaction between GF and F. As a consequence, the magnetization of the Co layer contacting with numerous granules is reversed in fields exceeding its own coercivity by an order of magnitude. For large x the magnetization reversal undergoes several stages conditioned by interaction of GF and F nanolayers. These stages may be characterized by three fields. For an oppositely magnetized sample, when the field approaches H1, there appear regions with flipped moments coupled to non-flipped moments in neighboring regions ("lateral magnetic springs") in the F layer. The field H1 sets off the overturn of magnetic moments of the granules, which is accompanied by the pinning of the regions with reversed magnetization in the F layer. The fields H2 and H3 tag the completion of the overturn of the F layer magnetization and the granule moments, respectively. When the applied field reaches H3, any further change of the magnetic state is due to a reversible rotation of the granule moments and the F layer magnetization. The reversibility of magnetic states persists with fields both increasing above H3 and decreasing down to 0 and further to -H1.