Abstract
Exchange bias(EB) is an interfacial magnetic phenomenon. After a bilayer film made of a ferromagnet and antiferromagnet goes through a field-cooling process from above the antiferromagnet's Néel temperature, these two materials will couple magnetically at their interface. Thus, a nickel-iron/nickel-oxide bilayer can magnetically couple and enable EB. This phenomenon is related to the interfacial arrangement of atomic magnetic moments. However, it is quite difficult to experimentally observe the direction of magnetic moments for individual atoms, and even harder to resolve the necessary atomic arrangement in polycrystalline films such as NiFe/NiO. Thus, we try to simulate the magnetic moments of each atom coupling to permit EB at the interface of nickel-iron/nickel-oxide system by theoretical simulations and mapping these results onto experiments on the films grown on a crystalline MgO substrate. According to random-field model, we can obtain the exchange bias from the difference of interfacial energies before and after the change of magnetic field direction, then incorporating the thickness effects of the ferromagnetic layer and its magnetization. Afterwards, we compared the simulated results with the experimental data and determined reasonable configurations of atomic magnetic moments by comparison with results from HRTEM (high resolution transmission electron microscopy). The result of VASP simulations for exchange bias without roughness and with two kinds of roughness (2.20Å and 4.43Å) at the interface is 3.08±4.59(Oe), -24.27±3.13(Oe) and -195.53±10.88(Oe), respectively, and 0.00±3.88(Oe), -4.17±1.96(Oe) and -11.02±3.12(Oe) for different directions of applied field. All simulations revealed that interfacial roughness can strengthen the EB, which is consistent with experimental results. Thus, we can conclude that the configuration of atomic magnetic moments is similar to real case with the interfacial roughness as also shown from HRTEM. Afterwards, we calculated for the effect of interfacial dislocations on EB. When dislocations appear around the interface with 2.20Å roughness, EB increased from -24.27±3.13(Oe) to -143.46±4.37(Oe), and from -4.17±1.96(Oe) to -82.30±7.29(Oe) for different directions of applied field. As EB is essentially a spin-spin interaction between ferromagnet and antiferromagnet at the interface, any kind of modification of the interfacial spin configuration (e.g. quantity of the uncompensated interfacial antiferromagnetic spins and the relative orientation between antiferromagnetic and ferromagnetic spins) will affect the final exchange coupling. Thus, it is reasonable to conclude that an interface roughness can strengthen the interfacial exchange coupling by enhancing the defect-generated uncompensated condition. This is in agreement with recent Monte Carlo simulations [J. Spray and U. Nowak, J. Phys. D: Appl. Phys. 39, 4536 (2006)].