Exploring Effective Methods To Increase The Curie Temperature Of Two-dimensional Ferromagnetic Material

As a hot topic in condensed matter physics,two-dimensional materials have been widely studied in the fields of mechanics,optics and electricity.However,the research progress on magnetism is relatively slow,and it has not been paid attention until recent years.So far,there have been many reports about two-dimensional magnetic materials in experimental and theoretical research,which exhibit many unique physical properties,broaden the path for the development of low-dimensional spintronics,and have broad prospects for development.Curie temperature（Tc）,as one of the important parameters that affect the practical application of ferromagnetic materials,is the critical temperature at which materials change from ferromagnetism to paramagnetism,and is the intrinsic property of materials.Although many new two-dimensional magnetic materials have emerged at present,two-dimensional materials with Tc close to room temperature are not common,which limits the practical application of two-dimensional magnetic materials.Finding high-temperature two-dimensional ferromagnetic materials is the top priority.In this paper,a high temperature ferromagnetic two-dimensional material Mn₂N₆C₆is predicted by first principle calculation,and based on this,a magnetic ion dimerization method is proposed,which can effectively improve the Curie temperature;In addition,the experimental phenomenon that the Curie temperature of Mn Sn film increases sharply with the increase of the number of layers is explained theoretically,and it is found that the interlayer coupling also has some influence on the increase of Tc.The main research contents of this paper are as follows:1.We predicted a two-dimensional ferromagnetic material Mn₂N₆C₆monolayer with high Curie temperature,verified its stability and analyzed its stability mechanism by first principles,and calculated the magnetic properties of Mn₂N₆C₆monolayer using GGA+U,GGA+SOC+U and Monte Carlo simulation.The results show that the Mn₂N₆C₆monolayer has good dynamic and thermodynamic stability,which is mainly due to theπconjugation effect in the C-N skeleton and theπ-d coupling between the Mn atom and the N atom.The magnetic ground state of Mn₂N₆C₆monolayer is ferromagnetic,and the magnetic moment of each magnetic unit can reach 7.2μB.Based on Ising model and Heisenberg model,we use Monte Carlo method to calculate its Curie temperature,and the results are 974K and 272K,respectively.It is proved that Mn₂N₆C₆monolayer is a high-temperature two-dimensional ferromagnetic material.In addition,we propose a completely different strategy,namely magnetic ion dimerization,which integrates two magnetic ions into a ferromagnetic dimer and increases the Curie temperature by increasing the magnetic moment of the magnetic unit.2.It is found experimentally that the Curie temperature of Mn Sn film is thickness dependent,that is,when the number of layers increases,the Curie temperature rises sharply.We use first-principles calculation and Monte Carlo simulation to verify this phenomenon and make a theoretical explanation.The magnetic ground states of single layer,double layer and three layers of Mn Sn are verified by GGA+U method.The results show that the ferromagnetic state is present in and between layers of Mn Sn film,and The local magnetic moment of each unit cell in single-layer Mn Sn can reach up to 13.5μ_B.This is mainly provided by Mn atoms.Based on Heisenberg model,the Tc of Mn Sn single layer is 43K,and the Tc of Mn Sn double layer is 142K,which is consistent with the experimental phenomenon.The reason for this phenomenon is the strong ferromagnetic coupling between Mn Sn layers.We regard it as the magnetic field of vertical Mn Sn layer.It is because of its existence that the ferromagnetic interaction in the system is more stable.In addition,when we calculate the Curie temperature,we take the atoms between the upper and lower layers as a ferromagnetic dimer,and the calculated result is 141K,which is almost the same as the 142K calculated by the traditional method,which also verifies the correctness of the dimer method.We not only explain the sharp rise of Curie temperature of Mn Sn thin film in the experiment,but also reveal that interlayer coupling is a new way to realize high temperature ferromagnetism in two-dimensional materials.