Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Electrical and Computer Engineering

First Advisor

Gary Tuttle


Ultra-thin alumina barrier magnetic tunnel junctions (MTJs) were examined. Such thin barriers can be imperfect and have pinholes. Examination of bias and temperature dependence of the MTJs supports a model of two resistors in parallel: one a tunnel magnetoresistor, the other an Ohmic resistor. Moreover, the results indicate that in addition to the tunneling conductance, hopping conductance through the barrier is present. It was found that even in the presence of pinholes, the dynamic conductance exhibits a parabolic dependence on the applied voltage as well as an insulating-like resistance dependence on temperature. These results suggest commonly used criteria for judging the presence of pinholes in tunnel junction barriers is insufficient and unreliable.;The presence of pinholes in the ultra-thin barrier is of major concern, as is the reliability of the barrier layer under stress. Two types of breakdown mechanisms are observed in ultra-thin barrier MTJs. It was found that the breakdown mechanism is an indicator of the barrier quality. Junctions showing an abrupt change in resistance at the breaking point have relatively large breakdown voltage (VB > 0.6 V) and fail due to intrinsic breakdown of a well-formed oxide that follows the E model of dielectric breakdown. On the other hand, the breakdown of junctions showing a gradual decrease in resistance at the breaking point occur at a much lower voltage (VB ~ 0.3--0.4 V), which is related to the presence of pinholes in the barrier. Current concentrated at the pinhole after breakdown generates a strong, circular magnetic field, which curls the local magnetization in the free-layer around the pinhole. This makes the free-layer reversal very sensitive to the location of the breakdown point in the junction area. Thus, the presence of pinholes can have strong implications on the parametric margins of the ultra-thin barrier MTJ for read sensor applications. It is proposed to include a statistical analysis of breakdown mechanisms to determine the presence of pinholes in ultra-thin barrier MTJs.



Digital Repository @ Iowa State University,

Copyright Owner

Bryan Kent Oliver



Proquest ID


File Format


File Size

91 pages