Exchange bias effect in L10-ordered FePt and FeCo-based bilayer structure: effect of increasing applied field

Abstract

The applied magnetic field (HAPP) dependence of the exchange bias (EB) is studied in an exchange-coupled thin-film bilayer composed of a hard ferromagnetic FePt layer in the proximity of a soft ferromagnetic FeCo layer. FePt/FeCo structure is deposited in an ultra-high vacuum chamber, where the FePt layer was first annealed at 823 K for 30 min and subsequently cooled to room temperature in the presence of an in-plane magnetic field, HMAX ~ 1.5 kOe to promote L10-ordered hard magnetic phase with magnetic moments aligned in one of the in-plane directions in the FePt layer. In-situ magneto-optical Kerr effect measurements during different stages of bilayer growth and detailed ex-situ superconducting quantum interference device-vibrating sample magnetometer measurements jointly revealed that due to the interplay between exchange coupling at the interface and dipolar energies of the saturated hard FePt layer, a hysteresis loop of FeCo layer shifts along the magnetic field axis. A clear dependence of EB field (HEB) on increasing maximum value of the HAPP during the hysteresis loop measurement is understood in terms of the magnetic state of soft and hard magnetic layers, where EB increases with increasing HAPP until the hard layer moment remains undisturbed in its remanence state. As soon as the field was sufficient to rotate the spins of the FePt layer, the loop became symmetric with respect to the field axis.