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.

Magneto-optical Kerr effect and nuclear resonant scattering study of uni-directional anisotropy in hard-soft magnetic bilayers

“…This indicates that the H SAT aligns the HM layer moment in one direction that induce unidirectional anisotropy and, therefore, exhibits the EB phenomena similar to that of conventional FM/AFM bilayers. 1,3,[7][8][9] These results are similar to our previous report on the similar kind of sample,…”

“…Recent studies indicate that exchange bias (EB) phenomena are not only restricted to ferromagnetic-antiferromagnetic (FM-AFM) systems but can also be observed in exchange-coupled magnetic layers with different magnitudes of coercive fields such as exchange spring magnets. [1][2][3][4][5][6][7][8] The unidirectional anisotropy can be induced in such exchange spring magnets by approaching the remanent state of the hard magnetic (HM) layer after subjecting it to a saturating field (H SAT ). The EB effect is observed in the minor loop of the soft magnetic (SM) layer, when the HM layer is under the remanent state.…”

“…The advantages of exchange spring magnets as compared to conventional EB systems (FM-AFM) include (i) one does not need to field-cool the samples across T N of AFM for obtaining the EB phenomena, (ii) the EB phenomena are functional even at ambient temperatures, and (iii) the magnitude of the EB shift (H B ) can be tuned by reaching the remanent state from different H SAT and also by varying the angle between the external magnetic field (H EXT ) and H SAT . 3,[7][8][9] However, the role of the SM layer thickness is not specifically addressed in the context of tuning the H B in the literature.…”

Domain state modulation by interfacial diffusion in FePt/FeCo thin films: experimental approach with micromagnetic modelling

“…To overcome this problem, FePt is exchange coupled with other soft magnetic materials which enhances the energy product of the system with a reduced switching field [13][14][15]. According to previous investigations, various FePt-based exchange coupled systems such as FePt/Fe [16,17], FePt/Co [18], FePt/Fe 19 Ni 81 [19], FePt/FeCo [20] etc have been used to attain favourable characteristics. Most of these studies focus on demonstrating the magnetic properties of epitaxial films.…”