The Heterogenous Interface And Magnetoelectric Effect Between Ferroelectric HfO₂ And Ni:a First-principles Study

The ferroelectric/ferromagnetic composite multiferroic thin films possess great potential in the application of low power consumption non-volatile memory devices with voltage control of magnetism due to their strong magnetoelectric coupling coefficients.However,traditional ferroelectric thin films（e.g.typical perovskite structure ferroelectrics）have hindered the development of ferroelectric and multiferroic devices due to their incompatibility with CMOS process,obvious size effect,and other problems.In 2011,ferroelectricity has been found in the HfO₂ thin film,which is compatibile with the CMOS process.Subsequently,a strong magnetoelectric coupling effect was found at the interface between the ferroelectric HfO₂ and 3d ferromagnetic metals.There are also studies showing the magnetoelectric effect and magnetoresistance of hafnium-based multiferroic tunnel junctions vary signisicantly after a number of electric field cyclings.However,underlying the physical mechanism under the found magnetoelectric effect at the ferroelectric/ferromagnetic interface still needs further study.Therefore,the HfO₂/Ni interface and its magnetoelectric coupling properties has been studies in this paper by systematic first-principles calculations.The electronic structure and ferroelectric polarization induced magnetoelectric coupling effect at the HfO₂/Ni interface are studied with taking into account of the influences of interfacial oxygen vacancies and metal oxidation.The main conclutions of this essay are as follows:（1）Atomic and electronic structures of the HfO₂/Ni interface.Since the interfacial structure and band offset between ferroelectric films and electrodes have a decisive effect on the performance of electronic devices,we investigated the effects of interfacial oxygen vacancies and metal oxidation on the atomic and electronic structures of HfO₂/Ni interface.By the calculation,it is found that the value band offset of the HfO₂/Ni interface increases with the increases of interfacial oxygen vacancies and metal oxidation due to the electronegativity of oxygen.The calculated value band offset is about 3.0-3.2 e V at the non-polar HfO₂/Ni interface when the stoichiometric ratio between Hf and O is 1:2,agreeing well with the experimental value.This result reveals the atomic structure and energy band offset characteristics of HfO₂/Ni interface.（2）Magnetoelectric coupling at defect-free HfO₂/Ni interface.We find that a strong ferroelectric polarization-controlled magnetoelectric effect appearing at the HfO₂/Ni interface,by analyzing the interfacial magnetic moments,density of states,and spin density.The interfacial magnetic moments decrease（increase）when the ferroelectric polarization points into（away from）the Ni layer due to the accumulation（depletion）of screening charge dominated by the minority-spin electrons and weaker（stronger）Ni-3d and O-2p orbital hybridization,leading a strong magnetoelectric coupling effect.（3）Influence of interfacial oxygen vacancies and metal oxidation on the HfO₂/Ni interfacial magnetoelectric coupling effect.The magnetoelectric coupling effect is weaken with the increase of interfacial oxygen vacancies due to weaker polarization-induced electron screen and interfacial Ni-O orbital hybridization.However,the Ni-O antibonding state at E_F appeares when a Ni O layer is formed at the interface by the diffusion ofO atoms.The majority-spin electrons dominated charge screen of ferroelectric polarization for the antibonding state leads to the reversal of magnetoelectric coupling effect as compared to that at the unoxidized interface,illustrating well the experimental results.These results are of great significance for the understanding of the magnetoelectric coupling mechanism at the ferroelectric/ferromagnetic interface and the development and application of multiferroic heterostructures.