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Exchange bias training effect in magnetically coupled bilayers

Srinivas Polisetty, University of Nebraska - Lincoln

Abstract

Interfaces in magnetically coupled bilayer heterostructures play a vital role in novel spintronics devices. Particularly, control of the interface spin structure enables the development of progressively down-scalable magnetic read-heads which are of major importance for non volatile magnetic recording media. Exchange bias and its accompanying training effect are fundamental magnetic coupling phenomena taking place at the interfaces of antiferromagnetic/ferromagnetic and hard/soft ferromagnetic bilayers.^ Here, in my thesis I present the experimental results of exchange bias training in the prototypical antiferromagnetic/ferromagnetic exchange bias system CoO/Co and the corresponding coupling and aging phenomena in the all ferromagnetic hard/soft bilayer CoPtCrB/CoCr. The latter system provides experimental access to its pinning layer magnetization thereby allowing to measure fundamental properties of exchange bias and its corresponding training phenomenon. A phenomenological theory is best fitted to all experimental training data of antiferromagnetic/ferromagnetic and hard/soft ferromagnetic bilayers evidencing the universality of the theory. My studies are further extended to the temperature dependence of the exchange bias training effect. Again, excellent agreement between experiment and theory confirms the remarkable universality of the underlying phenomenological approach. Furthermore, the dependence of the exchange bias training on the ferromagnetic film thickness is investigated in a CoO/Co-wedge sample. Scaling behavior with collapse of the temperature and thickness dependent parameters onto a single master curve is presented. Magnetotransport measurements are used for complementary studies of exchange bias in CoO/Co-heterostructures. Here, exchange bias produces a shift of the magnetoresistance curve along the magnetic field axis and an additional asymmetry along the resistance axis. The dynamic non-equilibrium properties of the exchange bias training effect are investigated via the sweep rate dependence of the exchange bias field. A dynamical enhancement of the exchange bias training effect has been observed in both CoO/Co and CoPtCrB/CoCr bilayers with increasing sweep rate of the applied magnetic field. A generalized theory has been developed for the dynamical enhancement studies confirming once more the consistency and universality of the phenomenological approach. ^

Subject Area

Nanoscience|Physics, Condensed Matter

Recommended Citation

Polisetty, Srinivas, "Exchange bias training effect in magnetically coupled bilayers" (2009). ETD collection for University of Nebraska - Lincoln. AAI3379026.
http://digitalcommons.unl.edu/dissertations/AAI3379026

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