A Modified Two-Phase Model to Address the Magnetization Reversal Mechanism and Angular Dependence of Coercivity in Conical Wire–Tube Heterostructures

Abstract

Integrating different geometries into a single nanostructure paves the way to obtain magnetic properties available in both isolated geometries. Wire and tube nanostructures are among the most explored morphologies in the ferromagnetic cylindrical structure area. This work examines wire–tube heterostructures with the inclusion of bulged and tapered diameter modulations, which enhance the control over static magnetic properties. An extra step is observed in the hysteresis loops corresponding to the switching originated initially in the tube region, followed by the wire region. The pinning of the domain wall at the wire–tube interface can also be observed. Remanent states show that vortex domain walls can be nucleated at the ends along with the interface, depending on the radius and the type of modulation. The core region of the vortex configuration is shifted for low values of radius, resulting in a unique arrangement of spins just before reversal. Angular variation of coercivity dictates that the reversal mechanism follows propagation of domain walls along with rotation to initially switch the spins in the tube region and later in the wire region till a certain critical angle. Later, the entire wire–tube structure shows pseudocoherent rotation. A modified two-phase (MTP) model is formulated to fit the simulated data of angular coercivity below the critical angle. Above the critical angle, the Stoner–Wohlfarth (SW) model fits well with simulated data. It has been demonstrated that in the case of extremely tapered wire–tube structures, the critical angle is almost nonexistent, and the MTP model explains the reversal mechanism at all field inclinations.