Effect of thermal and stacking order on pinning induced coercivity mechanism in the FeCo/FePt exchange spring bilayers

Abstract

Recent rapid progress in magnetic storage and processing devices require controlled characteristics of good thermal stability of magnetization (high coercive field) as well as low switching field which is possible by multilayer and nano-structuring. Here we study the implications of FeCo underlayer and overlayer on the static and dynamic magnetic properties of FeCo/FePt exchange spring system along with post annealing kinetics. Probing of FePt/FeCo/Si and FeCo/FePt/Si bilayers using high energy synchrotron X-Ray Diffraction by varying glancing angles unveiled structural variations at the interface, where FeCo is observed to inhibit the crystalline growth of FePt. The hysteresis loop establishes soft ferromagnetic in-plane behaviour and defect-driven hard ferromagnetic properties in the outplane configuration. In addition to the out-plane hysteresis, the magnetization dynamics studied by Ferromagnetic Resonance depicts an out-plane magnetization component of FePt only in FeCo/FePt/Si. X-Ray Absorption Spectroscopy in the Fe, Co and Pt L-edge regions reveal dissimilar interface in the two series due to thermal agitation effects. Fe and Co Kedges ensure similar local coordination of Co immaterial of stack order, contrary to altered local geometry of Fe observed in the two series ascribed to varied crystallinity of FePt. Such structures are useful for spin-wave assisted low field magnetization switching.