A study on the oxidation kinetics of ultrathin cobalt films

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The tunneling magnetoresistance (TMR) as high as 40% has been reported with proper ferromagnetic materials, insulating materials, which is mainly Al_2O_3, and/or postanneal, indicating that these spin-dependent tunnel junctions are potentially applicable in magnetoresistive random access memory and high areal density read heads. For TMR heads to be commercially used, however, large resistance should be decreased down to considerable level. To this purpose, tunneling barrier should be thinner or new other tunneling barrier than Al_2O_3 should be developed. Co oxide has a low resistance and can be oxidized at relatively low temperature due to its high free energy of formation. We have prepared polycrystalline Co films on 4 degree tilt-cut Si (111) wafers and studied their oxidation kinetics under atmospheric conditions at room temperature and elevated temperatures. Mtallic cobalt is a ferromagnet but CoO is an antiferromagnet and therefore practically nonmagnetic. As a consequence, oxidation is associated with a loss in magnetism. Experimental results show that as soon as exposed to air, Co takes an oxidation process up to some thickness. As a environmental temperature goes high, it is revealed that oxidation rate for Co increases and a critical thickness up to which metallic Co can be oxidized for each temperature goes high, too.
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2000-11-08