Investigation Of Exchange Bias Effect In Ir₂₀Mn₈₀/Co₄₀Fe₄₀B₂₀ Bilayer

This paper is devoted to investigating the mechanism of exchange bias in IrMn/CoFeB magnetic films,mainly analyzing whether there is a domain-wall-like spin structure parallel to the interface in the process of magnetization reversal in ferromagnetic film and the evolution and motion of the magnetic domain walls of ferromagnetic layer.Based on the experimental observation,we give the direct evidence of the theoretical judgment and make an experimental supplement to the exchange bias theory model.In this paper,the DC magnetron sputtering has been used to develop a series of IrMn/CoFeB layer films with different thickness.The macroscopic magnetic properties are characterized by the magneto-optic Kerr effect microscope.As the thickness of the ferromagnetic layer increasing from 15 nm to 120 nm,the bias field of the sample is decreasing gradually from 155 Oe to 12 Oe and the coercivity of the sample is decreasing from 60 Oe to 35 Oe also.As the thickness of antiferromagnetic layer increasing from 0 nm to 30 nm,the bias field of sample increases rapidly to 170 Oe and the coercivity is gradually increased from 8 Oe to 15 Oe.The spins rotation process in the magnetic reversal is explored by Lorentz transmission electron microscope.The cross-section samples processed by Focus Ion Beam present the microstructure of amorphous CoFeB layer,in which there is not a uniform structure and an obvious elements segregation.We get the spins mapping of sample in the process of magnetic reversal by the electron holography and a detailed analysis of the reversal process.The in-plan samples developed on the SiN window films present the change of magnetic reversal mechanism of the ferromagnetic layer with the thickness change of antiferromagnetic layer by the in-situ magnetic holder and Fresnel imaging mode.In the IrMn/CoFeB bilayer system,we can conclude that the there is no large-scale spin spring structure inside the ferromagnetic layer during the magnetization reversal process.Through analysis of the residual magnetic state by the high spatial resolution electron holography,we can make sure that there is not a magnetic domain wall structure with the thickness exceeding 2.3 nm.We find that the elements segregation leads to the existence of ripple domains.When the thickness of IrMn layer is less than 5 nm,there is a process of domain wall rapid movement in the magnetic reversal.When the thickness of IrMn layer is larger,there is a process of domain nucleation and growth in the magnetic reversal.