ISBN 9783868538380

978-3-86853-838-0, Reihe Physik

Syed Rizwan Ali
Studies on exchange bias, current induced magnetization switching and spin injection in magnetic nanostructures

190 Seiten, Dissertation Rheinisch-Westfälische Technische Hochschule Aachen (2011), Softcover, A5

Zusammenfassung / Abstract

This thesis investigates the exchange bias EB phenomena in a variety of magnetic systems such as diluted metallic and oxide antiferromagnets (AFMs) and metallic cluster spin glass (SG). The major part of the work is carried out on samples grown by a molecular beam epitaxy (MBE) system. However, some of the samples discussed were grown by sputtering. The capability of the MBE technique for the growth of films with better structural quality and with abrupt interfaces has resulted in a significant expansion of the work beyond EB into other projects like "spin torque in MBE grown nanopillars". Additionally, the MBE technique is also incorporated for the growth of smooth MgO films as tunnel barriers for spin injection into graphene. The main results of the thesis are summarized in the following.

The so far unexplained features of the EB in the Fe/Cr bilayers are investigated. These features include the sign reversal of the EB field HEB and its enhancement at low temperature. These phenomena are explained by considering interface alloying at Fe/Cr interface and the formation of a Fe-Cr spin glass (SG) which is stabilized after field cooling. Inherent competing interactions in Fe-Cr SG are shown to be responsible for the above mentioned HEB features. The temperature tuning through the SG-to-AFM phases yields at the interface to the Fe overlayer a sign change of the exchange couplings, and hence of HEB. The HEB of samples containing the intentionally deposited Fe-Cr SG alloy underneath the Fe overlayer shows all the features of HEB observed in the Fe/Cr bilayers and is therefore in support of the above explanation. Additionally, the study on intentionally alloyed Cr1-xFex/Fe bilayers demonstrates that the sign of HEB can be tuned by using by using competing exchange interactions of a SG. This opens a new direction for future studies of EB in systems involving competing interactions.

The effects of nonmagnetic impurity (Cu) in an intermediate-anisotropy metallic AFM IrMn are studied for exchange biased CoFe/(IrMn)1−xCux bilayers. The results demonstrate that the HEB in CoFe/(IrMn)1−xCux bilayers can be enhanced significantly (for 5 K ≤ T ≤ 350 K) due to Cu dilution in the AFM IrMn. The underlying origin is identified by the close qualitative connection between the temperature and dilution dependence of the thermoremanent magnetization mTRM of the (IrMn)1−xCux and that of the HEB of CoFe/(IrMn)1−xCux bilayers. A reduction in the blocking temperature TB is also observed with increasing Cu dilution in IrMn which is attributed to a reduced AFM grain size and thermal stability. Monte Carlo simulations based on a Heisenberg model and temperature dependent uncompensated AFM moments support the experimental results. This model predicts a further increase of HEB and less reduction of thermal stability and blocking temperature if the grain size upon dilution could be kept constant. In future, the latter could be achieved by controlling the suitable sample growth conditions, e.g. substrate temperature, post deposition annealing etc. Additionally, for an AFM with high Néel temperature (TN) and better thermal stability (e.g. NiMn, PtMn, PdMn etc.) the present study predicts HEB enhancements larger than the one achieved for IrMn.