Magnetic reversal processes and critical thickness in FePt/{\alpha}-Fe/FePt trilayers

TL;DR

This study investigates the magnetic reversal processes in FePt/{}-Fe/FePt trilayers, revealing how the reversal mechanism and coercivity change with soft layer thickness, and deriving the critical thickness analytically.

Contribution

It provides a detailed micromagnetic analysis of reversal mechanisms and derives an analytical expression for the critical thickness where the coercivity mechanism changes.

Findings

Reversal process involves three steps: nucleation, domain wall motion, and hard phase reversal.

Critical thickness for mechanism change is inversely proportional to the square root of the hard phase anisotropy.

Mechanism shifts from single-step to three-step with increasing soft layer thickness.

Abstract

Magnetic reversal processes of a FePt/{\alpha}-Fe/FePt trilayer system with in-plane easy axes have been investigated within a micromagnetic approach. It is found that the magnetic reversal process consists of three steps: nucleation of a prototype of domain wall in the soft phase, the evolution as well as the motion of the domain wall from the soft to the hard phase and finally, the magnetic reversal of the hard phase. For small soft layer thickness Ls, the three steps are reduced to one single step, where the magnetizations in the two phases reverse simultaneously and the hysteresis loops are square with nucleation as the coercivity mechanism. As Ls increases, both nucleation and pinning fields decrease. In the meantime, the single-step reversal expands to a standard three-step one and the coercivity mechanism changes from nucleation to pinning. The critical thickness where the coercivity mechanism alters, could be derived analytically, which is found to be inversely proportional to the square root of the crystalline anisotropy of the hard phase. Further increase of Ls leads to the change of the coercivity mechanism from pinning to nucleation.