Theoretical Studies On The Physical Properties Of Multifierroic BiMn₂O₅ And Organic Magnets (EDT-TSF)₂FeCl₄ Et Al

At present, the development of magnetic and ferroelectric materials has penetrated in all areas of modern science and technology. For example, electronic information products usually use the ferromagnets to store information. Accordingly, the sensor industry is strongly dependent on a class known as ferroelectric materials, which have a spontaneous polarization, and the direction of its polarization with the electric field can be changed. Multiferroics are an interesting group of compounds that have electric, magnetic and structural order parameters that result in simultaneous ferroelectrics, ferromagnetism and ferroelasticity in the same phase. With the development of large scale integrated circuits, device miniaturization has become an inevitable trend, and this trend promotes people to look for multi-functional magnetoelectric materials.In this paper, we have studied the electronic structure, charge transfer and magnetic coupling of the two chains compounds of [M(N3)2 (HCOO)][(CH3)2NH2] (M= Fe and Co) by employing the first-principles method based on density functional theory (DFT). The results reveal that AFM state is the ground state and FM state is the metastable one for the two compounds, and the ferromagnetic interaction is mainly transmitted by end-on azido. The two compounds exhibit semiconductor character with small gap in the FM state, while metallic in the AFM state. Using the same method, we have studied the electronic structure of Co and Cu doped zinc-blende ZnO, band structure calculations show that the Co-doped ZnO is semiconductor, while the Cu-doped ZnO shows an interesting half-metallic character.For LN-type ZnSnO3, we have investigated the ferroelectric and nonlinear optical properties by using DFPT method. The results show that the LN-type ZnSnO3 is a direct-band-gap semiconductor and has a large spontaneous polarization. The large dielectric constants and NLO susceptibilities indicate that the LN-type ZnSnO3 would be a candidate for high-performance dielectric and nonlinear optical material. In addition, by comparing the NLO susceptibilities and EO coefficients of LN-type ZnSnO3 with that of LiNbO3, although the two compounds belong to the same point group (3m), it is found that the symmetry of them is different. For mutiferroic BiMn2O5, we have calculated its electronic structure and spontaneous polarization. The results reveal that antiferromagnetic state is the ground state. The ferroelectricity of BiMn2O5 arises from two factors:the role of Bi lone pair electrons and the noncollinear magnetic ordering.