Nature of magnetization reversal in exchange-coupled polycrystalline NiO-Co bilayers

The nature of magnetization reversal in exchange-coupled NiO-Co polycrystalline bilayers was investigated. As-deposited bilayers exhibit a moderate value of exchange bias Hg (= -0.9 mT) and a significantly enhanced coercivity (H-c(NiO.CO) = 12,4mT), which is roughly 5 times the coercivity of a reference Co single film (H-c(Co) = 2.7 mT). Real time investigation of magnetization reversal in exchange-coupled NiO-Co bilayers shows that reversal is highly local and nonuniform in nature. It is preceded by the formation of precursors or embryos of reversed domains as the applied field reaches a critical value congruent to 8.8-9.0 mT. Once this critical applied field value is reached, numerous reversed domains are formed. Growths of such reversed domains occur primarily by the abrupt nucleation and the subsequent coalescence together of reversed domains; wall motion is not the dominant growth mode. Clear evidence is presented which shows that the strength of exchange bias varies at the microscopic scale across the sample. This manifests itself as different microscopic regions switching abruptly at different fields, and a given microscopic area switching at different fields in the positive and negative field directions. When the applied field is along the unidirectional anisotropy, reversal of a given strongly coupled microscopic region is aided by exchange bias, and such a region switches first; the same region undergoes reversal last when the polarity of the applied field is changed to oppose unidirectional anisotropy. Significantly, it was found that, locally, the measured value of exchange bias may vary by a factor of 3 or more from the macroscopically measured value of H-E (= -0.9 mT) obtained from the shift of the M-H loop. High-resolution transmission electron microscopy (HRTEM) shows that that the local variation in HE may be explained by considering the underlying microstructure and interfacial topography of the NiO-Co interface. HRTEM results show that the NiO surface parallel to the interface may be a fully compensated (200) plane (low H-E), an uncompensated plane (111) (high H-E), or a partially compensated plane (moderate H-E) of antiferromagnetic NiO. Clear evidence of partial compensation of the moments of antiferromagnetic NiO at the interface is found. Partial compensation (and hence a reduced value of H-E) results from a highly faceted NiO surface at the atomic level, with different facets having different orientation (different sublattices). Finally, a magnetic effect is observed, which suggests that it may be energetically more favorable to have steps on the NiO surface that are el en multiples of the unit cell dimensions of NiO ao (= 2ma(0), m epsilon integer > 0) and energetically unfavorable to have steps that are odd multiples of the unit cell dimension a(0) [ = (2m -1)a(0)m epsilon integer > 0].