Magnetization reversal and coercivity mechanism in truncated conical nanowires of permalloy

Magnetization reversal mechanism and stable remanent states of truncated conical nanowire of high aspect ratio are examined using micromagnetic simulation tool. The length of nanowire is fixed at 1 mu m, while its base radius R is varied from 50 to 10 nm and top radius r is varied R-10 nm to R-5 nm. The axial magnetization reversal proceeds predominantly through domain wall motion which is hindered as the tapering of the conical nanowire is increased. At remanence, a homogeneous magnetization persists throughout the nanowire except at the ends. Vortex states appear at both the ends in case of nanowires close to cylindrical shape, whereas a flower state of magnetization is exhibited at the tapered end if r is as small as <= 20 nm. Control of magnetization reversal through propagation of domain wall can be observed as tapered end with flower configuration successfully restricts the movement of domain wall. Expressions for total energy density of both vortex-vortex state and vortex flower state are constructed, and the obtained values are compared with homogeneous configuration to understand the extent of stability. The transition of remanent state is successfully obtained by energy calculations. Angular dependency of coercivity shows the existence of a critical angle above which the coercivity follows Stoner-Wohlfarth model. Below the critical angle, the coercivity mechanism is explained by Kondorsky model if R is large, whereas a modified Kondorsky model is applicable for small base radius.