Nature of coupling and origin of coercivity in giant magnetoresistance NiO-Co-Cu-based spin valves

The effect of various couplings on the switching field and coercivity in Ni O-Co-Cu-based giant magnetoresistance (GMR) bottom spin valves is investiga ted. Bottom spin valves as well as different layer permutations that make u p a bottom spin valve, viz., Co single films, Co/Cu/Co trilayers, and Co/Ni O bilayers (deposited under similar growth conditions), were investigated f or their magnetic, crystal, and interfacial structure. As-deposited bottom spin valves exhibit a large GMR of approximate to 16.5%, and a small net fe rromagnetic coupling (+0.36 mT) between the "free" Co layer and the NiO-pin ned Co layer. The high resolution transmission electron microscopy (HRTEM) and in situ scanning tunneling microscopy (STM) studies on spin valves and trilayers show that the average grain size in these films is approximate to 20 nm and average roughness approximate to 0.3 nm. Using these values, the Observed ferromagnetic coupling in spin valves could largely be accounted for by Neel's so-called "orange-peel" coupling. Results also show that the "free" Co layer exhibits an enhanced, coercivity (H-c(Free-Co) = 6.7 mT) wi th respect to Co single films of comparable thickness (H-c(Co) = 2.7 mT). T he TEM studies did not reveal the presence of any pin-holes, and "orange-pe el" or oscillatory exchange coupling mechanisms cannot adequately account f or this observed coercivity enhancement in the "free" Co layer of spin valv es. The present study shows that the often observed and undesirable coerciv ity enhancement in the "free" Co layer results from magnetostatic coupling between domain walls in the "free" Co layer and high coercivity NiO-pinned Co layer (H-c(Pinned-Co) approximate to 45 mT); without NiO, the coercivity of Co layers in the corresponding Co/Cu/Co trilayer remains largely unchan ged (H-c(Co/Cu/Co) = 3.0 mT) with respect to Co single films, Evidence of: magnetostatically coupled domain walls was confirmed by direct observation of magnetization reversal, which revealed that domain walls in the "free" C o layer are magnetostatically locked-in with stray fields due to domain wal ls or magnetization ripples in the high coercivity NiO-pinned Co layer of t he spin valves. The observed escape fields (defined as fields in excess of intrinsic coercivity of Co single him that are required to overcome magneto static coupling between domain walls) are in agreement with theoretically c alculated values of escape fields.