First-principles Study Of Magnetism And Ferroelectricity In Multiferroic Ag（Au）CrS₂

Multiferroics, in which two or three ferroic orders (such as ferroelectric, ferromagnetic and ferroelastic) coexist in the same phase, have attracted great interests recently, due to the coupling of various ferroic orders such as magnetoelectric effect, magnetoelastic effect, and piezoelectric effect. In this thesis, based on the First-principles simulation softwares WIEN2K and Wannier90, we perform a systematic study of the magnetic and ferroelectric properties in Ag(Au)CrS2.Firstly, we analyze the magnetic and ferroelectric properties of AgCrS2based on the First-principle calculations. By comparing the energies of six different magnetic structures, we found that the structure with a collinear double-striped antiferromagnetic ordering in layer and antiferromagnetic arrangement between layers has the lowest energy. This result is consistent with the experimental finding. In order to understand the magnetic exchange mechanism, we used Heisenberg model to extract the exchange constants, which indicate that the ferromagnetic interaction along the parallel spin stripes and antiferromagnetic interaction across the stripes play an important role in stabilizing the unusual magnetic structure. Moreover, because the inter-layer exchange is comparable to the intra-layer ones, AgCrS2belongs to a three-dimensional magnetic structure. In order to understand the ferroelectric property, we calculated the Born effective charge based on the modern polarization theory, and it was found that the Bonn effective charges of Ag and S1are relatively anomalous. Across the structure phase transition, the location of Cr ion has little change, while there exists obvious change for the tetrahedral symmetry of AgS4, suggesting that the ferroelectricity may be produced by Ag and S. To confirm our conjecture, we have examined the dependence of total energy on the short Ag-S distance, and the results show that the origin of ferroelectricity in AgCrS2is due to the breaking of inverse symmetry of AgS4.Secondly, we explore the reason for absence of ferroelectricity in AuCrS2, which has a similar magnetic structure to AgCrS2. A comparison of the energies of five different magnetic structures indicates that the structure with a collinear double-striped antiferromagnetic ordering in layer and antiferromagnetic arrangement between layers has the lowest energy, which is consistent with the experimental result. To understand the magnetic exchange mechanism, we used Heisenberg model to extract the exchange constant, and found that similar to AgCrS2, the ferromagnetic interaction along the parallel spin stripes and antiferromagnetic interaction across the stripes play an important role in stabilizing the unusual magnetic structure. To explore the reason for absence of ferroelectricity, we produced Cm-AuCrS2by replacing Ag with Au in AgCrS2. Compared with the energy in the ground state of AuCrS2with a C2/m crystal structure, the energy of Cm-AuCrS2is much higher. In the Cm structure, Au ion and S ion form an asymmetric AUS4tetrahedral structure, which induces ferroelectricity. In contrast, in the C2/m structure, Au ion and S ion form AuS2symmetrical linear structure, which does not exhibit ferroelectric property.