Switching field distribution of bit-patterned media: A combined experimental and micromagnetic modeling study.

摘要

The increase in the areal density of hard disk drives will be limited by the thermal instability of granular thin film media. Patterned medium has been targeted as one of the most promising candidates to overcome this limit and to enable the higher recording density. One of the fundamental issues associated with patterned media is the element-to-element variation in intrinsic magnetic properties, particularly the switching field distribution (SFD).;The main focus of this thesis work is trying to understand the SFD of patterned magnetic media and its correlation with different microstructure. To study the SFD of bit-patterned media, Co/Pt based multilayers, both quasi-single and polycrystlline films, have been first fabricated using magnetron sputtering technique. The continuous films are patterned into different sizes down to 50 nm using E-beam lithography. The experimental work has been focused on studying the size effect of the SFD as well as the effect of micro structures on the SFD. Methodology for isolating the domain wall motion from magnetization reversal process has been established. In comparison to the conventional perpendicular ac demagnetization remanent curve, the remanent loop with an in-place ac demagnetization initial state proved that the domain wall pinning field has size-independent characteristics. The magnetization reversal process has been identified as nucleation-triggered and followed by the fast domain wall propagation. For all the patterned structures, nucleation fields of most of the elements are higher than the domain wall pinning fields. This implies that once there is a reversed domain formed, domain wall can not be pinned but instead moving across the whole element, leading a uniform magnetization state. Therefore, for remanent states after perpendicular ac fields demagnetization, the patterned structures all exhibit uniform magnetization configuration. However, the uniform magnetization configuration does not mean the patterned element is or should behave like a single domain particle.;Micromagnetic modeling has been utilized to study the physics behind the size dependence of the SFD. The switching field of different patterned sizes exhibit an ascending trend as the sizes get smaller. Increase in the switching field can be explained by the association with various nucleation modes. The dynamic nucleation process has also been studied and it is found that the nucleation mode shows a damping constant dependence characteristic. Finally, the effect of fluctuation in magnetic properties on the SFD has been simulated, including the saturation magnetization, magnetic crystalline anisotropy and the exchange coupling. Considering the two different microstructures under study, the simulation results show that the exchange coupling can play an important role for the two distinguished SFD trends.