Magnetization reversal and switching field distribution in Co-Tb based bit patterned media

We have fabricated ordered array of ferromagnetic nanodots, so-called nanobumps by depositing Ta(5nm)/Pt(5nm)/Co88Tb12(5nm)/Cu(2nm)/Pt(5nm) mutilayers onto the barrier layer of auto-assembled anodic alumina template with 100 nm period. The same multilayers was deposited on a flat Si/SiOx substrate (the so-called reference sample) for comparison. We used extraordinary Hall Effect (EHE) measurements to probe magnetization reversal mechanism and switching field distribution (SFD) of these two kinds of materials. The extraordinary Hall resistivity measurements were performed by a standard four-probe method. The measurement of the coercivity as a function of magnetic field angle with respect to the sample surface reveal that ferromagnetic nanodots follow Stoner-Wohlfarth model with a shallower variation, which is typical of a dot-by-dot reversal but with a nucleation/propagation process for each dot. On the other hand, multilayers deposited on flat substrate follow Kondorskey model, which indicate nucleation/ propagation type reversal. Finally, we have calculated switching field distribution for nanobump material