Studies On Exchange Bias Induced By Fast Laser Annealing And On Perpendicular Magnetic Anisotropy In Polycrystalline BiFeO₃/FM Films

BiFeO3 is a multiferroic material possessing ferroelectricity and anti-ferromagnetism simultaneously at room temperature, which has stimulated extensive studies recently. Furthermore, the BFO/FM bilayer system provides a possible way in electric-manipulated exchange bias (EB) in spintronic devices. However, for the time being, it is still a big challenge to obtain significant EB field in the BFO/FM bilayers. The field cooling with conventional heat treatment results in mutual inter-diffusion and other interaction between the BFO and FM layers near the interface, which is not able to induce or enhance the EB field effectively. On the other hand, the EB field will be weakened and even removed in those samples which have fairly large EB fields in the deposited states. In this thesis, in order to solve the problems mentioned above, the fast laser annealing method was used in our fabricated BFO/FM samples and the in-plane EB field could be modified successfully. In addition, this provides new clue for inducing perpendicular EB in the BFO/ perpendicularly-magnetized FM films.1. Studies on in-plane EB effect induced by fast laser annealing in polycrystalline BFO/FM bilayersPrevious studies show that the conventional field cooling for BFO/FM bilayer system could strongly damage the interface between the BFO and FM layers, leading to significant deterioration of EB. In this thesis, this method has been replaced by fast laser annealing, which can depress the extent of interface damage and modify the EB successfully in the polycrystalline BFO/Co bilayers. In those samples with obvious EB in the deposited states, it is found that the EB field could be increased significantly if the laser fluence rises to a certain value. On the other hand, in those samples with negligible EB effect, EB field could be easily induced after field cooling with proper laser fluence. In addition, the sign of EB field could also be changed with respect to the direction of the cooling field. In contrast, after field cooling by conventional heat treatment, EB could be neither induced nor enhanced. The feasibility of fast laser annealing accompanied with field cooling to enhance or induce EB in the BFO/Co bilayer can be understood by much less interfacial diffusion in comparison with conventional field cooling.2. Studies on perpendicular magnetic anisotropy and perpendicular exchange bias in polycrystalline BFO/FM bilayersFirst, the dependences of perpendicular magnetic anisotropy (PMA) on the buffer layer thickness, the middle layer thickness and the period number have been investigated carefully in the polycrystalline BFO/[Co/Pt]n multilayer samples at room temperature. It shows that the PMA is enhanced with increasing the thickness of the Pt buffer layer. However, the PMA is weakened to some extent with increasing the thickness of middle Co or Pt layer. Moreover, the PMA is enhanced with increasing the period number n. Then, the preliminary study of perpendicular EB effect against the temperature in this series of samples was performed. It is found that weak perpendicular EB could be realized at low temperature after field cooling, which is attributed to spin-glass EB effect. With further increasing the temperature, the EB field shows a decreasing trend and disappears finally at room temperature. Further studies will be performed on these samples with fast laser annealing as mentioned previously, which is hopeful to achieve perpendicular EB at room temperature.