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Abstract
The production, structure and magnetic properties of $\rm Co\sb{x}Ni\sb{1-x}O$ thin film alloys, CoO-NiO superlattices and CoO-NiO bilayers are studied. Interesting exchange anisotropy behavior and novel ordering behavior is observed. Both polycrystalline and epitaxial single crystal films are produced by reactive sputtering. Single phase $\rm Co\sb{x}Ni\sb{1-x}O$ metastable alloys were produced. The epitaxial single crystals are of high crystalline quality and the interfaces of CoO-NiO superlattices are highly coherent. Shifted hysteresis loops are used to explore the exchange anisotropy properties of $\rm Co\sb{x}Ni\sb{1-x}O$/Ni$\sb{81}$Fe$\sb{19 }$ and CoO-NiO superlattice/Ni$\sb{81}$Fe$\sb{19}$ coupled bilayers. A shifted hysteresis loop is observed for $\rm Co\sb{x}Ni\sb{1-x}O$/Ni$\sb{81}$Fe$\sb{19}$ couples, in contrast to earlier reports. $\rm Co\sb{x}Ni\sb{1-x}O$/Ni$\sb{81}$Fe$\sb{19}$ couples exhibit an optimum exchange anisotropy near compositions of x = 0.5, with properties comparable to FeMn/Ni$\sb{81}$Fe$\sb{19}$ suggesting the possibility of using the $\rm Co\sb{x}Ni\sb{1-x}O$ alloys in exchange bias applications. The ordering temperature of $\rm Co\sb{x}Ni\sb{1-x}O$ varies linearly with x. CoO-NiO superlattices are ideal for studying the effects of interfacial magnetic coupling since the superexchange interaction restricts the coupling to near neighbors. Even with this purely interfacial coupling, the effects of the interaction are strong well away from the interfaces. CoO-NiO superlattices have a single magnetic ordering temperature between the ordering temperatures of CoO and NiO when the constituent layers are thinner than 20A. In addition, the exchange anisotropy observed in CoO-NiO superlattices suggests that the interlayer coupling has a strong effect on the magnetocrystalline anisotropy of the layers within the superlattice. Thick-NiO/thin-CoO/Ni$\sb{81}$Fe$\sb{19}$ trilayers were produced to investigate the thickness dependence of the oxide-oxide interaction in the superlattices. When the CoO layers are sufficiently thin ($\le$20A), the CoO ordering temperature approximates the NiO value, indicating quite strong coupling throughout the CoO layer. The effect of the coupling is much weaker for thicker CoO layers ($\ge$30A). The superlattice ordering behavior is modeled using a mean field formalism. In superlattices with thick constituent layers, a surprising magnetic coupling is observed whereby the NiO layers couple through paramagnetic CoO layers.
