Current and resistance fluctuations provide insight into the electronic and magnetic properties of spin electronic devices based on giant- and tunneling-magnetoresistance. The on going research on tunneling magnetoresistance (TMR) devices, namely magnetic tunnel junctions (MTJs), has revealed novel physics of spin dependent transport. To date, the majority of studies have involved conventional transport measurements and structural studies. Many studies are motivated by the desire to achieve higher TMR values as experimental observations of TMR are still well below theoretical predictions. The voltage bias dependence and temperature dependence of the TMR indicate that electronic transport in MTJs depends on structural quality. In particular, the role of defect states inside the barrier and interface effects are poorly understood. This thesis focuses on using noise as a complementary probe of magneto-electronic transport in MTJs. The emphasis is on relating barrier resistance noise and current shot noise to spin dependent transport processes in MTJs. Understanding and controlling noise is also important from a technological perspective. TMR devices are now being used as magnetic sensors in computer hard-drives and magnetoresistive random access memories. The trend towards miniaturization is leading to devices in which resistance and current noise become a limiting factor in their performance.
1/f resistance noise was measured over a wide range of magnetic fields and temperatures in a number of MTJs having both Mg O and AlOx tunnel barriers. The electronic 1/ f noise in MTJs is associated with defects in the tunnel barrier that act like charge traps. The temperature dependence of 1/f noise indicates that the population kinetics of charge traps are, in part, thermally activated and involve a broad distribution of charge trap depths. A Hooge-like noise parameter, α, is defined and it was found to exhibit an anomalous dependence on bias voltage, in contrast to the usual quadratic dependence found in metals and semiconductors. An exponential scaling between α and the differential resistance is revealed and indicates that the mechanisms behind the bias dependence of the differential resistance may also be responsible for the bias dependence of the barrier noise. Although the origin of such a scaling is not understood the data is consistent with a model for spin transport that involves the opening of low-noise, tunneling channels for electrons. Through this scaling, the measured bias dependence of the tunneling resistance quantitatively accounts for the bias dependence of the magnetoresistive 1/f noise that occurs when magnetization of the magnetic layers undergoes reversal.
Current fluctuations in MTJs have also been investigated at low temperatures. For some MTJs, the current noise is in quantitative agreement with the shot noise predictions for random, uncorrelated tunneling events. However, other MTJs show shot noise suppression. Suppression is observed to be higher in thin barrier MTJs that also have large areas. Possibility defect assisted tunneling though localized states in the barrier is argued to be the reason for the suppression. Shot noise data in linear arrays of MTJs was found to be consistent with theoretical predictions of current noise for incoherent tunneling of charge carriers through N barriers; that is, a suppression of shot noise that scales with 1/N is observed.
|Advisor:||Nowak, Edmund R.|
|Commitee:||Ji, Yi, Rakesh, Rakesh, Safronova, Marianna|
|School:||University of Delaware|
|Department:||Department of Physics and Astronomy|
|School Location:||United States -- Delaware|
|Source:||DAI-B 70/09, Dissertation Abstracts International|
|Subjects:||Electrical engineering, Electromagnetics, Condensed matter physics|
|Keywords:||Magnetic tunnel junctions, Noise, Resistance fluctuations, Shot noise, Spintronics, Tunneling magnetoresistance|
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