Application of Gettering Layers for Low Temperature Conversion of Magnetic Oxides into Ferromagnetic Metals in Thin Films, Multilayers, and Nanostructured Arrays
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Nanoscale patterning of magnetic metals and their alloys remains a significant challenge due to the lack of reactive-ion etching (RIE) chemistries producing volatile compounds of magnetic elements. Patterning is typically achieved either via a pre-etched substrate deposition, lift-off process, ion milling, or wet etching, all lacking the needed fidelity to achieve the high resolution and high areal density because of fabrication issues such as fencing, shadowing, edge damage, redeposition, or magnetic material residual. Alternatively, electrochemical deposition may circumvent these fabrication issues and produce high aspect ratio structures, but is difficult to make complex structures, i.e., Co/Pd or Co/Pt multilayers, and CoCrPtX alloys, which are widely studied to be the potential candidates for magnetic recording media with areal density beyond 1 Tb/in2. This work demonstrates conversion of nonmagnetic cobalt oxide (CoO) into ferromagnetic cobalt (Co) in thin films, multilayers, and nanostructured arrays by low temperature annealing in the presence of tantalum (Ta) gettering layers. Thin film of CoO sandwiched between Ta seed and capping layers can be effectively reduced to a magnetic Co thin film by annealing at 200◦C, whereas CoO does not exhibit ferromagnetic properties at room temperature and is stable at up to ∼ 400◦C. The CoO reduction is attributed to the thermodynamically driven gettering of oxygen by Ta, similar to the exothermic reduction-oxidation reaction observed in thermite systems. Likewise, annealing at 200◦C of a nonmagnetic CoO/Pd multilayer results in the conversion into a magnetic Co/Pd multilayer with perpendicular anisotropy. A nanopatterning approach is introduced where CoO/Pd multilayer is locally reduced into Co/Pd multilayer to achieve magnetic nanostructured array in the presence of Ta islands. Magnetic properties of thin films, multilayers, and patterned arrays in this work are measured on MicroMag alternating gradient force magnetometer and self-built polar magneto-optical Kerr effect magnetometer. A Physical Electronics 5700 X-ray photoelectron spectroscopy is used to characterize the chemical states and compositions. This technique can potentially be adapted to nanoscale patterning of other systems for which thermodynamically favorable combination of oxide and gettering layers can be identified.