Theoretical Study Of Doping-Induced Low-Dimensional Magnetism Based On Flat Bands

The increasing demand for miniaturized electronic devices has prompted scientists to search for materials in small size.The low-dimensional magnetic materials can be used to construct spintronic devices of small size and high density with low power consumption.This has become a research hotspot.They are easily integrated into electronic devices due to the smaller size and ultra-thin thickness.And the reduction of dimension also makes low-dimensional magnetic materials vulnerable to external conditions.These highlight the advantages of low-dimensional magnetic materials.However,there are still few low-dimensional magnetic materials available at present.Most two-dimensional ferromagnetic materials have low Curie temperature.And a lot has been predicted in theory but only a few have been synthesized in experiments.Therefore,it is very important to seek low-dimensional magnetic materials with high Curie temperature and strong stability,which is urgent for people to explore.Magnetism can be induced by doping,defect engineering,magnetic nearest neighbor effect,applying strain or electric field to expand the family of low-dimensional magnetic materials.Doping carriers is a good method to induce and regulate magnetism without damaging the intrinsic periodic structure of materials.As the bridge between theory and experiment,theoretical calculation can be used to design and screen more low-dimensional magnetic materials.And it can provide theoretical support and direction guidance for experiments.In this paper,based on first-principles calculations of density functional theory,the following two works are carried out in order to obtain more low-dimensional magnetic materials.1.Ferromagnetism induced by doping and nanoribbons in two-dimensional HfP₂Based on first-principles calculations,we found that magnetism can be induced by doping electrons in the single-layer HfP₂,and it can be maintained above room temperature.The strain can regulate the band gap of single-layer HfP₂ and change the band structure from indirect band gap to direct band gap.In addition,we also found that the zigzag HfP₂ nanoribbon is a nonmagnetic semiconductor,while the armchair HfP₂ nanoribbon is a metal with magnetic property.The magnetism of armchair nanoribbon mainly originates from the Hf atoms at the unsaturated edge,the width has a certain impact on the magnetism at edges.We designed a type of two-dimensional magnetic material and a type of magnetic nanoribbon in this work.They can be used to manufacture microelectronics and spintronics devices.This adds new members to low-dimensional magnetic materials.2.Doping-induced magnetism and magnetoelectric coupling in 1D NbOCl₃ and NbOBr₃Based on first-principles calculations,we demonstrated 1D ferroelectric NbOCl₃ and NbOBr₃nanochains recently proposed have good stabilities,and speculated that they can be easily separated from the bulk.And the itinerant ferromagnetism can be induced over a wide range of electron-doping concentrations in 1D NbOCl₃ and NbOBr₃ nanochains.Electrons can also be introduced into 1D NbOCl₃and NbOBr₃ nanochains through oxygen vacancy defects.And one oxygen vacancy defect can result in a magnetic moment of 2μ_B per defect in the system.More importantly,this can lead to the coexistence of ferroelectricity and ferromagnetism by doping.Interestingly,the internal electric field generated by spontaneous ferroelectric polarization in the system has a great influence on spin,leading to a large magnetic moment difference in short nanochains.At a certain concentration,the ferroelectric polarization intensity of the system can be regulated by changing the length of the nanochain and applying different strains.It can influence the distribution of charge carriers in short nanochains,and achieve the regulation on magnetism by adjusting ferroelectric polarization.This work not only discovered two kinds of 1D multiferroics through doping electrons,but also realized magnetoelectric coupling in 1D system,which can be used to manufacture high-density and multifunctional storage devices.