Physics and Astronomy, Department of


Date of this Version

August 2006


A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy
Major: Physics and Astronomy
Under the Supervision of Professor Sy-Hwang Liou
Lincoln, Nebraska
August, 2006
Copyright © 2006 Lu Yuan.


Interest in highly sensitive magnetic sensors has been great due to their wide applications ranging from data storage to geomagnetic exploration. To achieve better performance, magnetic sensors are usually fabricated with micrometer-sized or submicrometer-sized multilayer structures. The thickness of each layer can be as thin as a few angstroms. The magnetic properties of these small and thin layers are quite different from those of the bulk. As the size of the magnetic devices shrinks and the thickness of the ferromagnetic films decreases, the chance of having defects becomes higher. Those defects may be formed during thin film deposition, annealing and the lithography process etc. To have a better understanding the origin of those nanometer sized defects is important for improving sensitivity and signal-to-noise ratio of those magnetic sensors.

In this thesis, a magnetic sensitivity mapping (MSM) system is developed to locate the inhomogeneous regions in the ferromagnetic layer of magnetic sensors. An ultrasensitive microcantilever torque magnetometer (MTM) system is developed to characterize the submicrometer-sized magnetic films and arrays. The detailed magnetic microstructures of both the free layer and the pinned layer in magnetic tunneling junctions are studied by the analysis of the temperature and voltage dependence of the tunneling magnetoresistance data.

We have correlated the microstructures to the sources of magnetic noise using the developed MSM system. In this study, a scanning nanometer-sized magnetic tip was used to generate a localized magnetic field and excite the free-layer magnetic moment at the air-bearing surface (ABS). By mapping out the magnetic noise as a function of position, the inhomogeneous regions in the ferromagnetic layer of the magnetic sensors that relate to magnetic instabilities inside the recording heads are identified.

We studied the voltage and temperature dependence of resistance and magnetoresistance of two types of magnetic tunneling junctions (MTJs). These two types of MTJ samples have different free layer structures but the same pinned structures and the same material for free and reference layers. The tunneling magnetoresistance ratio (TMR), defined as (RAP-RP)/RP, is 26% and 70% for type 1 and type 2, respectively. From the analysis of our results, we conclude that: (1) There are more magnetic inhomogeneous regions in the free magnetic layer of type 1 MTJ samples than in those of type 2 MTJ samples; (2) There are possible additional spin-glass-like states that occur at the interface between the magnetic layer and the insulating layer in the type 1 MTJ sample at low temperature. These results clearly indicate that the micro-magnetization orientation in the free layer and its interfaces plays an important role in determining the TMR ratio in these two types of MTJ samples.

An ultra-sensitive MTM system is developed to characterize the magnetic nanostructures. The MTM system can be operated in temperature from 10 K to 300 K and under vacuum of 5 x 10-8 torr. We have also developed a new method to deposit magnetic patterns on cantilevers that allows us to have more flexibility in magnetic studies using MTM in the future.

Advisor: Sy-Hwang Liou

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