In recent years, the exchange bias between ferromagnetic (FM) and antiferromagnetic (AFM) thin films has received increasing attention in physics and material science because it plays an important role of pinning the ferromagnetic layer in giant magnetoresistance spin valve and tunneling magnetoresistance junction. This fact has promoted the study on the improvement of the exchange bias consequently. Extensive studies have been done in order to increase the exchange bias. As far as studied until now, although the fundamental mechanism is not yet fully understood, the factors which could influence the exchange bias are texture of AFM, interfacial roughness, interfacial atomic interdiffusion, topology of AFM, growth mode of AFM, stoichiometry of AFM, stress of FM and crystalline quality. As antiferromagnetic material, NiO has some advantages like superior corrosion resistance and higher resistivity over many other antiferromagnetic metals. Because of these characteristics, NiO could be used not only in magnetoresistance read heads but also in magnetic random access memory (MRAM) devices. Besides, NiO could improve magnetoresistance in spin valve by making electrons of the adjacent ferromagnetic layer specular scattering. But NiO has some disadvantages like low blocking temperature and weak exchange bias.
In this study, the exchange bias and magnetoresistance of the multilayer using NiO were investigated. In order to find the optimum deposition condition of NiO, deposition conditions like Ar gas pressure, rf power, the distance between substrate and target and thickness were varied. It is found that Ar gas pressure is one of the most important factors for the exchange bias and less than 1mTorr is good. The effect of rf power is less than other variations. The distance between substrate and target is also important and short distance is relatively good. The exchange bias is found differently with thickness and it is generally better when NiO film is thicker. The optimum deposition condition is 60W or 80W-0.2mTorr-55mm-1000Å and the maximum exchange bias is 118Oe. In order to find the cause of the optimum deposition condition, (111) texture, surface roughness and topology were analyzed by XRD and AFM. It seems that (111) texture, surface roughness and grain size effect the exchange bias. Especially the possibility of the effect of grain size is found out from the distance between substrate and target by SEM. The effect of (111) texture for the exchange bias was investigated by comparing NiO grown on MgO (111) substrate with NiO grown on MgO (100) substrate. NiO was grown epitaxially on MgO substrate by controlling deposition rate and temperature and growth mode of NiO on MgO substrate was analyzed by HRXRD and XRD φ-scan. It is found that (111) texture improves the exchange bias but not preferentially. By controlling thickness of pinned layer and spacer layer, MR ratio was increased. The addition of NiO on free layer increased MR ratio because of specular scattering effect. Finally, the heat treatment improves the exchange bias. But when NiO is thicker than 1000Å, hysteresis loop of pinned layer were separated and it seems that antiferromagnetic characteristics of thick NiO disappears locally by annealing.
