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Fabrication and magnetic characterization of artificially nano-structured materials for ultra-high density magnetic storage media

Final Activity Report Summary - NANOMAGNET2 (Fabrication and magnetic characterisation of artificially nanostructured materials for ultra-high density magnetic storage media)

The main objective of the project was to investigate the magnetic properties of exchange-biased materials in both continuous and nanostructured thin films. The Exchange bias (EB) phenomenon is of great importance in electronic devices such as read heads, spin valves, magnetic sensors, etc. Another possible application is the use of exchange-biased materials for ultra-high density magnetic recording.

In this project, we investigated the magnetisation reversal mechanisms of Antiferromagnetic and ferromagnetic (AF/FM) thin films induced by the exchange coupling at the AF/FM interface. FeF2/FM, with FM = Fe, Ni, Co and permalloy, bilayers were prepared by electron beam evaporation. Structural and magnetic characterisation of continuous films and patterned nanostructures was done using X-ray diffractometry, neutron scattering, atomic force microscopy, magnetometry and the magneto-optical Kerr effect.

Several remarkable results were obtained:
1. In terms of the continuous AF/FM thin films, incomplete domain walls were induced in FM materials due to the exchange coupling at the AF/FM interface. This kind of domain walls was predicted by theoretical models, however their existence was not experimentally demonstrated.
2. Cooling field experiments of Ni/FeF2/Py thin films demonstrated that the magnetic bulk structure of the AF played a crucial role to determine the EB magnitude. This was a significant result since the role of bulk AF spins on EB was still controversial. While some models neglected them others considered them relevant for triggering the interfacial configuration that yielded EB.
3. FeF2/FM thin films exhibited a peculiar feature. The system showed positive EB at high cooling fields. The transition between negative and positive EB was investigated as a function of the crystalline quality. Samples with low interfacial roughness and high crystalline quality transited through double hysteresis loops. These results demonstrated the creation of a bi-domain state at intermediate cooling fields.
4. Regarding the nanostructured AF/FM thin films, arrays of FeF2/FM dots were successfully prepared using electron beam lithography and Ar ion milling. The magnetisation reversal mechanism was studied via magneto-optical Kerr effect in a dot diameter range from 100 nm to 1200 nm.
5. The onset of positive EB, i.e. the minimum cooling field necessary for getting positive EB, was found to be dependent on the dot size. It was observed that the smaller the dot diameter, the lower the minimum cooling field. This result enhanced the importance of the AF/FM domain size ratio in the appearance of double hysteresis loops.