Tuning of exchange coupling in FePt based multilayer systems
1Department of Physics and Astrophysics, University of Delhi, Delhi, India 110007
Abstract:FePt system with co-existence of A1 and L10 phase with inter-granular exchange coupling can be looked upon as a natural exchange spring system. The dual exchange spring behavior (inter-granular and interfacial exchange coupling) is investigated systematically by varying the thickness of soft ferromagnetic layer Fe3Pt in FePt/Fe3Pt and non-magnetic spacer layer Al2O3 in FePt/Al2O3/Fe3Pt. The extent of coupling in these systems can be analyzed using recoil curves. Monte Carlo simulation predicts strength of inter-granular exchange coupling increases with thickness of FePt film.
The fabrication of magnetic devices such as ultra-high density magnetic recording media, micro electromagnetic devices etc. requires large energy product which is markedly dependent on coercivity and remanence magnetization.Kneller and Hawig proposed that an optimum energy product could be achieved by combining the soft and hard magnetic material to provide high coercivity . Theoretically, it has been predicted that the FePt nanocomposite can exhibit energy product ~ 90 MGOe . Several attempts have been made to improve exchange spring properties by coupling the different soft and hard magnetic layers, annealing at different temperatures, introducing a thin non-magnetic spacer between the soft and hard FM layers.FePt thin film with high ordering exhibits large coercivity due to large magnetocrystalline anisotropy (~ 107 J/m3) and therefore, can be used to design exchange spring system.
The FePt/Fe3Pt direct and FePt/Al2O3/Fe3Pt indirect exchange spring systemswere synthesized by depositing different thickness (estimated using RBS) of Fe3Pt and Al2O3layers. Fig.1 (b)-(d) shows the typical hysteresis recorded at room temperature for as-deposited as well as 450 oC annealed FePt thin films of different thickness (24 nm, 137 nm and 370 nm). It can be easily observed that the shoulder decreases with increase in thickness of the film and completely vanishes for 370 nm thick film. This indicates the improvement in inter-granular exchange coupling.
Fig. 2 (a)-(b) shows the recoil curves recorded at room temperature for 24 nm and 370 nm thick films. For 24 nm thick films almost closed recoil loops are observed upto a reverse field of 5kOe while 370 nm thick film exhibits closed recoil loops upto a reverse field ~ 7 kOe. This behavior further confirms the strong inter-granular exchange coupling between A1-L10 FePt phases.
Fig.1 Hysteresis curve for (a) as-deposited and 450 oC annealed thin films of thickness (b) 24 nm (c) 137 nm (d) 370 nm
Fig.2. Recoil curves for (a) 24 nm (b) 370 nm thick FePt films.
Magnetic studies reveal that the inter-granular exchange coupling enhances with increase in thickness of the film. The strength of exchange coupling as estimated from Monte Carlo simulations is directly related with the amount of coupled and uncoupled soft phase present in the system.
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