First Principles Design Of Some Novel Two-dimensional Spintronic Materials

In traditional semiconductor devices,the storage and transmission of information is based on the charge properties of electrons.Thanks to the advancement of semi-conductor technology,traditional electronics has developed rapidly since the advent of transistors,and device functions have become more and more diverse.As Moore's Law describes,the number of components that can be accommodated in integrated cir-cuits doubles every 18-24 months.However,current leakage and thermal effect restrict the further miniaturization of traditional electronic devices.Spin-based devices avoid thermal effects and may have faster information processing speed.Therefore,people hope to keep Moore's law with the help of spintronic materials.On the other hand,the miniaturization of devices will inevitably lead to a significant reduction in material size,which will be a potential challenge to the keep of its properties.Since the discovery of graphene in 2004,two-dimensional materials have attracted wide attention because of their unique physical properties.Compared with three-dimensional thin film materi-als,two-dimensional materials have incomparable advantages.Firstly,its thickness is down to the atomic level,which perfectly meets the needs of miniaturization of devices.Secondly,there is no hanging bond on its surface,and we can easily stack different two-dimensional materials together to form multifunctional materials.Last but not least,the new physics brought about by dimensionality reduction is expected to help people de-sign devices with new concepts.In the past decade,spintronics has developed rapidly,and a series of new physics have been discovered,which has resulted in many new concepts of device design.The key issue of spintronics is how to control the direction of spin.Of course,the best way to control spin is by means of electric field.However,few spintronics materials can do this.In this paper,we design some new two-dimensional spintronics materials from the basic principles of quantum mechanics with the aid of first-principles calculation,trying to solve the problem of spin manipulation by electric field.Here,we will give a brief introduction to our main work,especially the innovation.(1)when stacking Bi bilayer on two kinds of 2D substrates(In2Se2 and In2Se3),we find a novel spin spiltting.Different with traditional Rashba type spin splitting,its spin texture reverse in one spin subband.Based on the proposed tight binding model,we find this reversal is derived from the singificant change of orbit component and the interac-tion from substrates hybride different orbits,giving rise to this change.Furthermore,the electrical polarization in ferroelectrical In2Se3 provides an accessible and non-volatile way to tune the spin splitting.(2)We propose a scheme to realize two-dimensional ferroelectricity by intercalat-ing atoms between transition metal dichalcogenides(TMDC)layers,and design a sys-tematic,strict and rigorous framework to screen out the system that meets the require-ments.The two-dimensional ferroelectricity originates from the "tetrahedral" bonding formed between the intercalated atoms and the upper and lower chalcogenide atoms.This special dipole formation mechanism determines the lock relationship between in-plane polarization and out-of-plane polarization,therefore we can apply a small in-plane electric field to reverse the more important out-of-plane polarization,which is the unique excellent property of these two-dimensional ferroelectric systems.(3)Based on the spontaneous polarization of the intercalated TMDC system,we further look for a system with excellent spin-dependent properties.In heavy metal in-tercalation systems such as Pt1/3(MoS2)2 and Ir1/3(MoS2)2,the spin both split in out-of-plane and in-plane directions,and there spin texture are both strongly coupled with the polarization.The polarization reversal will lead to the spin texture reversal.(4)Excellent half-metallic properties were found in the double-layer CrS2 inter-calation system with 1.0 density of Cu.The Curie temperature of the system is higher than room temperature,the half-metallic gap is very large and the magnetic anisotropy is also very prominent.(5)In a four-layer CrS2 system intercalated with Cu at 1.0 density,we found a new magnetoelectric coupling effect caused by a general mechanism.When the layered system holds an interlayer antiferromagnetic interaction and has a vertical electrical polarization,there is a shift between the spin-up part and the spin-down part of density of states due to the existence of the out-of-plane polarization.This will result in a net residual magnetic moment in the system.When the direction of polarization is reversed,the shift will be reversed,and the direction of residual magnetic moment will also be reversed.Therefore,we can change the direction of the magnetic moment of the system through the external electric field.