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Exchange bias effects of CrSe/Fe/GaAs thin film structure

Authors Wang, Chao
Issue Date 2008
Summary A lot of effort has recently been devoted to the research of antiferromagnetic (AFM)/ferromagnetic (FM) bilayer because its exchange bias effect plays a key role in enabling high-level performance of spin-valve heads, perpendicular recording media, magnetic memories, novel permanent magnets and domain stabilizer in recording heads based on anisotropy magnetoresistance. A variety of AFM materials have been investigated in such AFM/FM bilayer systems and many models have been proposed to explain the underlying mechanisms of the exchange biasing phenomena. However, a quantitative understanding of these phenomena, such as the magnitude of the exchange bias and the coercivity enhancement, has not been achieved . Moreover, the roles of the many different parameters involved in exchange bias, such as magnetic anisotropy, roughness of the bilayer structure, spin configuration and magnetic domain structure, are far from being understood. In this study, a novel AFM/FM bilayer structure of CrSe/Fe was fabricated by the molecular beam epitaxy (MBE) technique. CrSe/Fe bilayer structures were grown on three GaAs substrates with different orientations in the same growth run. In the as-grown CrSe/Fe/GaAs (100) structure, it was found that its Fe layer is a single-crystalline bcc structure while its CrSe layer consists of four hexagonal domains with their c-axis each along one of the four <111> directions of the Fe layer. This bilayer structure exhibits a strong negative exchange bias (NEB) effect while a Au/Fe/GaAs (100) structure shows no sign of exchange bias, it can be concluded that the NEB observed in the CrSe/Fe/GaAs (100) structure is attributed to the exchange coupling between CrSe and Fe layers. Based on the exchange spring mode, we have provided complementary interpretations on the observed magnetization reversal characteristics, the temperature dependence of the exchange bias field, as well as the coercivity enhancement in this bilayer structure. For the CrSe/Fe/GaAs (110) structure, its structural quality is poor since its Fe layer shows obvious lattice imperfections and its top CrSe layer is almost amorphous in nature. However, the MH loops of this structure show a similar NEB effect as that exists in the CrSe/Fe/GaAs (100) structur Together with the fact that the corresponding Au/Fe/GaAs (110) structure shows no sign of NEB, we can conclude that the observed NEB effect is an intrinsic nature of the CrSe/Fe bilayer structure that does not depend on the crystalline quality of the bilayer structure. For the CrSe/Fe/GaAs (111)B structure, micro-structural characterization indicates that its Fe layer is a bcc structure and the top CrSe is a hexagonal NiAs-type structure with its c-axis along the [111] direction of the substrate. Regarding the magnetic properties of this structure, an interesting and unexpected observation is that the polarity of the centre shift of the loops changes from negative to positive at around 40 K. This transition exists under magnetic cooling fields from 0.3 to 2T. This observed transition of the exchange bias polarity in the CrSe/Fe/GaAs(111)B bilayer structure indicates the coexistence of both FM and AFM exchange coupling in this structure. A corresponding Au/Fe/GaAs (111)B structure was also found to show a positive exchange bais (PEB) effect and a monotonical decrease in both the coecivity and exchange bias as the temperature increases from 5 to 300K. These observations indicate that the observed PEB effect in the CrSe/Fe/GaAs(111)B structure originates from the Fe/GaAs(111)B interface while the NEB effect comes from the intrinsic nature of the CrSe/Fe bilayer. We believe that the PEB effect of the Fe/GaAs (111)B may be attributed to the direct bonding interaction between the first Fe layer and the terminated As layer of the GaAs (111)B substrate, which results in an AFM coupling between the first and the second Fe layers. In addition, we have observed different asymmetries in the M-H loops of the CrSe/Fe/GaAs (111)B structure as a function of temperature. The asymmetry observed at T < 20K is possibly due to the fact that the decreasing-field reversal begins in the thicker regions of the Fe layer and the increasing-field reversal begins in the thinner regions of the Fe layer in this structure. The “opposite” asymmetry observed in the range of 20 K ≤ T ≤ 60 K is attributed to some possible mechanisms that are related to the relationship between the chiralities in the winding and unwinding of the exchange spring in the AFM layer as well as the domain structure of the Fe layer at the interface of the Fe/GaAs(111)B hetero-junction. In summary, this thesis research has helped obtain a better understanding of the underlying physics behind the exchange bias phenomenon. An unusual PEB effect arising from the interface of Fe/GaAs (111) B was discovered, which will likely stimulate further fundamental research and find new applications in spintronics .
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2008
Language English
Format Thesis
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