The Perpendicular Magnetic Anisotropy In Low-Dimensional Magnetic Materials

The unique crystal structure and emerging properties of low dimensional magnetic materials have attracted much attention from condensed matter physicists.More importantly,the low-dimensional magnetic materials have great application value in spintronics and other fields.The single-atom bit has drawn a lot of interest,which represents the ultimate limit of the classical approach to high-density magnetic storage.Exploring perpendicular magnetic anisotropy(PMA)in single-atom bit is greatly needed for the development of nanomagnetic devices.Thus,it is a great challenge to search suitable materials for achieving large perpendicular magnetic anisotropy in a single-atom magnetic system.Recently,the two-dimensional covalent organic frameworks(COFs)have attracted much attention due to their great physical and chemical properties in optoelectronic devices,such as organic light emitting diodes and organic field transistors.Importantly,inspired by the several prominent advantages of COFs,such as highly ordered porous structures,low cost and strong covalent interactions,are greatly suitable for single-atom magnetic system.In addition to the low-dimensional magnetic materials based on the COFs,the two-dimensional intrinsic ferromagnetic thin films had been realized,such as the CrI₃ monolayer and Cr₂Ge₂Te₆bilayers.Moreover,the magnetic anisotropy of two-dimensional magnetic materials is reason for the existence of the long-range magnetism.Importantly,the two-dimensional magnetic materials with large PMA have the advantage over applying to spintronics devices with high-storage density and low-energy consumption.Thus,the main contents about the origin and manipulation of PMA for low dimensional magnetic systems are as follows:(1)The Tetraoxa[8]circulene(TOC)structure is one of the COFs,which can be polymerized into zero-dimensional nanoclusters,one-dimensional nanoribbons and two dimensional nanosheets with high stability.Here,we systemically study the MA of the TOC doped by single 3d tradition metal(TM)atoms(V,Cr,Mn,Fe,Co and Ni)via first-principles calculations.Specifically,we find that the Co@TOC system has large perpendicular magnetic anisotropy(PMA),up to 1.76 me V/atom.To perform the deep analysis of PMA of Co@TOC,we take band structure,orbital hybridization-resolved MAE and spin splitting at?point into account.The PMA of Co@TOC originates from the band level splitting of Co-(d_xz,d_yz)by Spin-Orbit coupling and the hybridization of Co-(d_xz,d_yz)and O-P_z orbits near the Fermi level.Furthermore,the PMA of Co@TOC can be enhanced up to 7.28 me V/atom by applying biaxial tensile strain of 9%.The tensile strain can promote the band level splitting of Co-(d_xz,d_yz)and the hybridization of Co-(d_xz,d_yz)and O-P_z orbits near the Fermi level,which relust in the enhancement of PMA in Co@TOC.(2)Based on the two-dimensional monolayer MnSe₂,we can effectively manipulate the PMA of Mn₄Se_8-xO_x(x=1,2,3,4)by changing the concentration of oxygen.Within the increase of oxygen concentration,the PMA of Mn₄Se_8-xO_x(x=1,2,3,4)presents an oscillating behaviour.To perform the deep analysis of PMA in Mn₄Se_8-xO_x(x=1,2,3,4),we take element-resolved MAE and orbital hybridization-resolved MAE into account.The main contribution of SOC in Mn₄Se_8-xO_x(x=1,2,3,4)originates from the Se atoms.However,the different Se atom has different contributions under the impact of oxygen.Furtherly,the competition between the hybridization of Se-(P_z,P_y)and Se-(P_x,P_y)result in the oscillation of PMA in Mn₄Se_8-xO_x(x=1,2,3,4).