Barrier oxidation related paradigmatic transport behavior of magnetic tunnel junctions.

摘要

Rapid implementation of a magnetic tunnel junction (MTJ) in magnetic sensors and memories is expected in the foreseeable future. Fabrication of ultra thin barriers, typically accomplished by deposition of an Al layer followed by in-situ natural or plasma oxidation, is the most critical step in achieving high quality MTJs. In this dissertation, we address some issues related to the barrier oxidation process, which may profoundly affect the quality of MTJs.; We first describe the systematic increase of the exchange bias H_ex in the NiFe seed layer underneath the FeMn antiferromagnet during plasma oxidation of tunnel barrier. The increase in H _ex was found to closely correlate with barrier oxidation rate. Based on the model of oxidation by cation current proposed by Cabrera and Mott, we propose that the increase in H_ex stems from the ferromagnet/antiferromagnet interface modulation incurred by the diffusion of metals in multilayers underneath the tunnel barrier.; We then discuss the paradigmatic behavior of MTJs with under- and over-oxidized tunnel barriers. For the strongly over-oxidized case, the junctions exhibited zero bias anomalies (giant conductance dip or equivalently, giant resistance peak) in dynamic conductance curves, and striking bias and temperature dependencies of TMR, i.e. suppression of TMR at zero bias and the decrease of TMR with decreasing temperature. Logarithmic dependencies of the conductance on bias and temperature agree well with Applebaum's theory describing Kodno-type spin flip scattering between tunneling electrons and impurity spins. These results confirm that an over-oxidation of the tunnel barrier leads to diffusion of Co (Fe) ions from the bottom electrode into the barrier.; For the under-oxidized case, TMR of the junctions showed unusually strong bias and temperature dependencies. TMR was found to decrease more rapidly with increasing temperature and bias. We attribute this to the interface mixing at the CoFe/Al interface, leading to magnon assisted tunneling to occur at much lower energy scales as manifested by inelastic electron tunneling spectroscopy (IETS). Proper or over-oxidation of the barrier seems to cure the interface mixing, by driving Al in the mixed region to diffuse back to the barrier during oxidation, thereby eliminating the unusual magnon features.