Magnetoelastic and magnetostatic interactions in exchange-spring multilayers

Abstract

The macroscopic or average aggregate behavior of Kneller's exchange-spring mechanism is well established and widely examined; less is known about its microscopic details. In the present study, the microscopic nature of exchange spring was investigated using $\mathrm{TbFe}∕\mathrm{FeCo}$ multilayers and correlated with their aggregate behavior (magnetostriction, torque, and magnetization curves). Results show that, as predicted, the exchange-coupled geometry reduces the switching field by increasing the average magnetization and decreasing the average anisotropy of the multilayers. However, it is the magnetostatic interactions between domain walls in adjacent layers that lead to a reduction in coercivity. Stray fields emanating from domain walls in a given layer magnetostatically lock in with the stray fields from walls in adjacent layers, giving rise to low energy, low coercivity ``twin'' walls. Since in a multilayer a wall can magnetostatically lock in with walls in layers both above and below it for optimum flux closure, a sharp drop in coercivity is observed when the number of bilayers exceeds two. Preliminary results on Fe-Pd based shape memory alloy (SMA) thin films and multilayers are also given to further emphasize the efficacy of the exchange-spring mechanism as well as to highlight a key micromagnetic difference between magnetostrictive and magnetic SMA films. In particular, the number of martensite variants is greatly reduced because variants with easy axis normal to the plane of the film are prohibited due to magnetostatic considerations.