Study On The Multiferroic Of Manganites With The Mechanism Of Double Ferroelectric Polarization

The ferroelectric polarization in the type-II multiferroic manganites RMnO₃ is derived from the special magnetic structure of the system.The ferroelectric order and magnetic order correspond to each other.The type-II multiferroic manganites RMnO₃ has a very strong intrinsic magnetoelectric coupling effect.It also has a wide application prospect in the fields of high-density information storage,electromagnetic energy conversion and detection processing technology.The origin mechanism of ferroelectric polarization in the type-II multiferroic manganites RMnO₃ can be divided into two types.The first type is that in the spiral spin order manganites represented by TdMnO₃ and DyMnO₃,ferroelectric polarization originates from spin orbit coupling interaction,ferroelectric polarization P?e_ij×(S_i×S_j),where e_ij is the unit vector connecting two nearest spin vectors;The second type is in the E-type antiferromagnetic order manganites represented by HoMnO₃,ferroelectric polarization comes from spin phonon coupling interaction,ferroelectric polarization P?(S_i·S_j).Recent research results show that these two kinds of ferroelectric polarization mechanisms can co-exist in some orthorhombic manganites RMnO₃ system with strong R-Mn coupling interaction,which makes the system not only show rich physical phenomenon,but also has more excellent ferroelectric properties.In this paper,the orthorhombic HoMnO₃ with double ferroelectric polarization mechanisms is taken as the research object.By using Gd³⁺ions to replace Ho³⁺,the multiferroic phase separation is realized in the orthorhombic HoMnO₃,so as to improve the ferroelectric polarization intensity and magnetoelectric coupling effect of the system.In addition,we also pay attention to the orthorhombic GdMnO₃ located at the boundary of A-type antiferromagnetic order and spiral spin order.Using Lu³⁺ions with smaller ion radius to partially replace Gd³⁺,we can realize the coexistence and competition of the spiral spin order and E-type antiferromagnetic order in the system,so as to enhance the multiferroic properties of GdMnO₃.The main results are as follows:(1)The effects of partial substitution of Gd³⁺ions on the crystal structure,magnetic structure and ferroelectric transport properties of HoMnO₃ were studied.Due to the difference of ion radius,the partial substitution of Gd³⁺ions makes the cell volume of Ho_1-xGd_xMnO₃ sample expand,and the distortion degree of MnO₆ octahedron weakens,so that the magnetic structure of the system gradually changes to spiral spin order.At a certain substitution concentration of Gd³⁺ions(0.1<x<0.4),the coexistence and competition of spiral spin order and E-type antiferromagnetic order appeared in Ho_1-xGd_xMnO₃ samples,and the multiferroic phase separation was realized at the two-phase boundary.This phase separation effectively improves the ferroelectric polarization and magnetoelectric coupling effect of the system.(2)The effects of partial substitution of Lu³⁺ions on the crystal structure,magnetic structure and ferroelectric transport properties of GdMnO₃ were studied.Through the analysis of XRD and Raman spectra of Gd_1-xLu_xMnO₃(0?x?0.4)samples,it is pointed out that the partial substitution of Lu³⁺ions effectively enhances the lattice distortion of the system.The enhanced lattice distortion of the Gd Fe O₃-type distortion and staggered orbital ordering make the interaction intensity of the nearest-neighbor ferromagnetic superexchange(NN-FM-SE)close to that of the next-nearest-neighbor antiferromagnetic superexchange(NNN-AFM-SE).The competition between the NN FM interaction(J_FM(NN))and the NNN AFM(J_AFM(NNN))brings about the spin frustration and makes the magnetic state transform from the spiral spin order to the E-type antiferromagnetic order.The change of magnetic structure can effectively enhance the ferroelectric polarization of the system and optimize the performance of the system.The above experimental results indicate that in the orthorhombic RMnO₃(R=Gd-Lu),the coexistence of multiferroic magnetic structures and multiferroic ferroelectric polarization mechanisms can be realized in the system through appropriate substitution of A-site rare earth ions,so as to effectively enhance the multiferroic properties of the system.