Nature of Coupling and Origin of Coercivity in Giant Magnetoresistance NiO-Co-Cu-Based Spin Valves
H D. Chopra, D X. Yang, P J. Chen, D Parks, William F. Egelhoff Jr.
The effect of various couplings on the switching field and coercivity in NiO-Co-Cu-based giant magnetoresistance (GMR) bottom spin valves is systematically investigated. A difficulty in controlling contributions to switching field and coercivity from various coupling mechanisms result, in large part, from an inability to make a clear distinction between them. Therefore, in order to identify the origin and magnitude of operative coupling mechanisms, bottom spin valves as well as different layer permutations that make up a bottom spin valve, viz., Co single films, Co/Cu/Co trilayers and Co/NiO bilayers were deposited under similar growth conditions. These films were investigated for their magnetic, crystal, and interfacial structure using domain studies, magnetization measurements, high resolution transmission electron microscopy (HRTEM), and in-situ scanning tunneling microscopy (STM). As-deposited bottom spin valves exhibit a large GMR of nearly equal to} 16.5%. And a small net ferromagnetic coupling (+0.36 mT) between the 'free' Co layer and the NiO-pinned Co layer. The HRTEM and in-situ STM studies on spin valves and trilayers show that the average grain size in these films is nearly equal to} 20 nm and average roughness nearly equal 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 arising due to correlated interfaces. Results also show that the 'free' Co layer exhibits an enhanced coercivity (Hc'Free'-Co = 6.7 mT) with respect to Co single films of comparable thickness (HcCo = 2.7 mT). The TEM studies did not reveal the presence of any pin-holes, and 'orange-peel' or oscillatory exchange coupling mechanisms cannot adequately account for this observed coercivity enhancement in the 'free' Co layer of spin valves. The present study shows, for the first time, that the observed and undesirable coercivity enhanvement in the 'free' Co layer results from magnetostatic coupling between domain walls in 'free' Co layer and high coercivity NiO pinned Co layer (HcPinned-Co nearly equal to} 45 mT). Whose magnetization state is modified by an adjacent NiO layer due to exhange anisotropy, without NiO, the coercivity of Co layers in the corresponding Co/Cu/Co trilayer remains largely unchanged (HcCo/Cu/Co = 3.0 mT) with respect to Co single films. That magnetostatically coupled domain walls are the origin of coercivity enhancement in the 'free' Co layer of spin valves was confirmed by direct observation of magnetization reversal, which revealed that domain walls in the 'free' Co layer are magnetostatically locked in with stray field due to domain walls or magnetization ripples in the high coercivity NiO-pinned Co layer of the spin valves. The observed escape fields, in excess of intrinsic coercivity of Co single film, required to overcome magnetostatic coupling between domain walls are in agreement with theoretically calculated values of excape fields.
Physical Review B (Condensed Matter and Materials Physics)
, Yang, D.
, Chen, P.
, Parks, D.
and Egelhoff Jr., W.
Nature of Coupling and Origin of Coercivity in Giant Magnetoresistance NiO-Co-Cu-Based Spin Valves, Physical Review B (Condensed Matter and Materials Physics)
(Accessed February 29, 2024)