| For the purpose of making magnetic read heads in computer hard disk drives, magnetic tunneling junctions (MTJs) with ultrathin barriers have to be fabricated because the heads are required to have a low resistance area product. One of the challenges of making ultrathin barriers was the magnetoresistance (MR) reduction in the low resistance area product regime. The focus of this dissertation is to understand the underlying mechanisms for this MR reduction.; We have found that some MR reductions can be explained by the geometrical effect, due to inhomogeneous current distribution when the junction and electrode resistances are comparable. For MTJs with ultrathin barriers, geometrical effects become more pronounced due to the small resistance area product associated with ultrathin barriers. We have studied the intermixing effects of the ferromagnetic layer, NiFe, and Al in our MTJs with ultrathin barriers that were fabricated by epitaxial growth. Magnetotransport measurements of several junctions with ultrathin barriers show that the non-ideal oxidation of the intermixing region, which is ∼0.6 nm in our system, degrades the device performance, posing a fundamental limit to the optimization of ultrathin barrier tunnel junctions. We have treated the magnetic oxide, due to over-oxidation, as part of a composite barrier system in the framework of a free-electron model. According to this model, as the normal barrier gets thinner and the magnetic oxide gets thicker, the MR decreases. This model may explain why the MR drop, due to over-oxidation, is more pronounced in ultrathin barrier tunneling junctions than in ones with thicker barriers. |
