The phase transition mechanism and ferroelectric polarization of CaMn 7 O 12 are investigated by using the density functional theory. Our results show that the P3 space group should be the ground-state structure with R3 as the intermediate phase. It is the helicoidal magnetic order that induces the first transition to the R3 phase while the second transition to P3 structure results from soft modes under the constraint of the magnetic structure. The two phase transitions from R3 through R3 to P3 are second order and first order, respectively, which is consistent with experimental observations. Group theoretical analysis shows that the particular domain states and multi-domain structure of the P3 phase match with the observed coexistence of two magnetic modulations below T N2 =48 K. Our calculated electric polarization is also in good agreement with experiment. By analyzing the polarization contributions from both mode and atomic-decomposition viewpoints, we find that the Raman-type distortions give rise to a significant contribution to the total polarization. This unexpected result can be understood through the asymmetric change of the Born effective charges caused by the particular helicoidal magnetic order, which leads to an abnormal infrared character of the purely Raman-active modes.
We report on first-principles calculations of a Ni monolayer inserted at one interface in the epitaxial Fe/PbTiO3/Fe multiferroic heterostructure, focusing on the magnetoelectric coupling and the spin-dependent transport properties. The results of magnetoelectric coupling calculations reveal an attractive approach to realize cumulative magnetoelectric effects in the ferromagnetic/ferroelectric/ferromagnetic superlattices. The underlying physics is attributed to the combinations of several different magnetoelectric coupling mechanisms such as interface bonding, spin-dependent screening, and different types of magnetic interactions. We also demonstrate that inserting a Ni monolayer at one interface in the Fe/PbTiO3/Fe multiferroic tunnel junction is an efficient method to produce considerable tunneling electroresistance effect by modifying the tunnel potential barrier and the interfacial electronic structure. Furthermore, coexistence of tunneling magnetoresistance and tunneling electroresistance leads to the emergence of four distinct resistance states, which can be served as a multistate-storage device. The complicated influencing factors including bulk properties of the ferromagnetic electrodes, decay rates of the evanescent states in the tunnel barrier, and the specific interfacial electronic structure provide us promising opportunities to design novel multiferroic tunnel junctions with excellent performances.
First-principles calculations were used to investigate the interfacial electronic structure and magnetoelectric effect in the Fe/PbTiO3 heterointerface. We demonstrate that the large magnetoelectric effect in this system is determined by the combination of different magnetoelectric coupling mechanisms, i.e., the conjunction of interface bonding mechanism and the electrostatic screening of the spin-polarized carriers. The change of induced magnetic moments on interfacial Ti atoms is due to the variation of interface bonding when the ferroelectric polarization reverses, while the contribution to magnetoelectric coupling from interfacial Fe atoms is determined by the electrostatic screening of spin-polarized carriers. The combination of different interfacial magnetoelectric coupling mechanisms enhances the magnetoelectric coefficient at the Fe/PbTiO3 heterointerface to be several times larger in magnitude than that of individual magnetoelectric coupling mechanism. Our results indicate a new strategy to design multiferroic materials with large magnetoelectric effects.
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