Fourfold Magnetic Anisotropy Induced By Interface Exchange Coupling In Epitaxial IrMn/Ferromagnetic Bilayers

The phenomenon of exchange bias（EB）induced by the interface exchange coupling in ferromagnetic（FM）/antiferromagnetic（AFM）bilayers conventionally can lead to a unidirectional magnetic anisotropy K_eb as well as an accompanied uniaxial magnetic anisotropy K_u in the system.Especially for epitaxial EB system,there is also an intrinsic magnetocrystalline anisotropy K₁ in its FM layer.Under the joint action of these three magnetic anisotropies,the magnetization process of FM layer may undergo a complex multi-step reversal.On this basis,a large number of experiments and theoretical calculations have focused on the magnetization reversal behavior and the corresponding magnetization mechanism of FM layer.For AFM layer under the EB system,the magnetization reversal is not easy to be detected due to its overall compensated spin structure,hence it lacks necessary research.In addition,the application and promotion of spin valves,magnetic tunnel junctions and other spintronic devices based on EB effect make it urgent for people to carry out in-depth research on the magnetic properties of AFM materials.In this process,AFM IrMn alloy has shown great practical value due to its high Neel temperature and related characteristics that can induce large EB field.In recent years,the heated discussion on noncollinear AFM has also made IrMn₃ with a stoichiometric ratio of 1:3 and noncollinear spin structure become a research hotspot again.A deeper understanding of IrMn₃ spin structure and related magnetic properties has led people to reconsider its specific role in magnetic anisotropy in EB system.Therefore,this project attempts to construct the epitaxial IrMn/FM bilayer and study its magnetic anisotropy induced by interface exchange coupling in EB system in detail.First,the amorphous CoFeB/epitaxial γ-IrMn₃ bilayers were constructed on MgO（001）single-crystal substrates by using magnetron sputtering deposition system,in which the EB direction is along IrMn[100]or[110]axes.An additional fourfold magnetic anisotropy K₄ induced by interfacial exchange coupling was observed in the amorphous CoFeB layer.Because of the combined effect of K₄,K_eb and K_u,one-and two-step magnetic switching processes were observed at different magnetic field orientations,which usually appear in single-crystal FM layer with an intrinsic magnetocrystalline anisotropy K₁ but not in amorphous FM layer.In other words,in this system,AFM IrMn successfully induced an extrinsic K₄ in FM layer through interface exchange coupling.The angular dependent magnetic switching fields can be nicely fitted by a phenomenological model based on domain wall nucleation and propagation with the in-plane K₄ along<100>.The ferromagnetic resonance measurements indicate that the specific strength of K₄ for EB along[100]is larger than that for EB along[110].The induced K₄ can be well understood by considering two types of AFM domains caused by both monatomic steps and defects and their induced net uncompensated spins along the in-plane<100>axes.The different dependence of K₄ on the EB direction are because of the different effects of growth magnetic field H_growth on the presence of AFM domains.So far,it is known that the epitaxial γ-IrMn₃ layer can induce an interface fourfold magnetic anisotropy along IrMn<100>directions in FM layer through interface exchange coupling.After that,the amorphous CoFeB layer was changed into an epitaxial single-crystal Fe layer with intrinsic magnetocrystalline anisotropy,and an attempt was made to regulate the in-plane fourfold magnetic anisotropy of the Fe layer.First,the growth of epitaxial-IrMn₃ layers was realized on MgO（001）substrates by magnetron sputtering system,followed by the growth of epitaxial single-crystal Fe layers,during which H_growth was applied along the direction of Fe[100]to induce EB.By changing the thickness of FM layer,the competitive relationship between the intrinsic magnetocrystalline anisotropy of Fe layer and the interface fourfold magnetic anisotropy is explored.The MOKE test results show that as the thickness of Fe layer gradually decreases,the hard and easy axes of the system have changed,which is an inevitable result of the competition between the intrinsic magnetocrystalline anisotropy of Fe layer and the interface fourfold magnetic anisotropy.The ferromagnetic resonance test results quantitatively show the dependence of the total fourfold magnetic anisotropy of the system on the thickness of Fe layer,from which the specific strength of the interface fourfold magnetic anisotropy can be analyzed.Through the study of samples with different AFM thickness,it is found that the prerequisite for the generation of this interface fourfold magnetic anisotropy is the AFM order established by IrMn.In other words,the establishment of IrMn AFM order is a necessary condition for the system to generate EB,and then the fourfold magnetic anisotropy at the interface is induced in FM layer through the interface exchange coupling.