Research Of Multiferroic Material TbMnO₃, Doping With Fe, Co

Single-phase multiferroics are materials in which two or more primary ferroic orderparameters coexist simultaneously. These materials with the coexistence of ferromagnetism andferroelectricity also show magneto-electric effect which has great potential application in theareas of information storage, spin electronics, non-linear magnet-light effect and transducer.Magneto-electric effect is that we can control the electric polarization through the magneticfield, conversely, control the magnetic polarization through the electric field. At persent, thereare so many explanations for the mechanism of multiferroic materials. For example, in theBiFeO₃and BiMnO₃of the Bi based compounds, the ferroelectric properties are related to thestructural aberration caused by the Bi lone pair electrons. In the YMnO₃system, the ferroelectriceffect is thought to be the results of MnO₆coordination polyhedron distortion and Y iontranslocation induced electric polarization. Aken et al. through theoretical study, think these aredue to the size effect and electrostatic. In the rare earth manganese oxygen compounds, such asTbMnO₃, DyMnO₃; magneto elasticity on lattice modulation causes ferroelectric, in addition,Efremov et al. think that magnetic and charge ordered coupling can also produce a ferroelectriceffect. Although there are many causes of multiferroic performance mechanism, but in thesemechanisms, some mechanisms are calculated theoretically, and also have not beenexperimentally demonstrated; part mechanisms are opposite or controversial, therefore, in-depthstudy of the multiferroic materials properties, exploration of magnetic and electric polarizationbetween coexisting mechanisms have very important significance of science.As an example of multiferroic materials, TbMnO₃, its performance and mechanism attractso much interest. TbMnO₃is a perovskite manganese oxide, and has a helical magnetic structure.At low temperature, the multiferroic properties mechanism also has many views. Kimura et al.considered Mn-O-Mn structure deformation to be the mainly reason; some research group basedon the" theory of electrons " mechanism, thougt its attributed to spin ordering of induced chargepolarization; some groups put forward the "lattice theory" based on Dzyaloshinskii-Moriya （DM）model of the mechanism, the magnetic field ion translocation phenomenon caused the ferroelectric behavior. All the views are attributed to a problem, that is to say, in low temperature,the multiferroic properties of TbMnO₃should attributed to which ion, Mn atoms play thedominant role, or Tb atoms play a leading role. Doping method will affect the nature of thematerial, which may be helpful to know of the multiferroic properties.In this thesis, we add suitable content of Fe2O3, Co3O4into TbMnO₃, using solid statesintering reaction to fabricate the polycrystalline TbMn_1-xFe_xO₃and TbMn1-xCoxO3samples, andthen the polycrystalline structures of the samples are analyzed by using X ray diffraction （XRD）,Fourier-transform infrared （FT-IR） and other instruments. The low temperature electrical andmagnetic properties are characterizaed by the low temperature resistance-temperaturemeasurement systems, ferroelectric tester; superconducting quantum interference device; andother methods of analysis and testing equipments, the results are explored. The major results andconclusions can be summarized as following:1. We fabricated the TbMn_1-xFe_xO₃and TbMn1-xCoxO3polycrystalline samples, and foundthat the structures changed with varying the doping contents. For the Fe doped TbMnO₃samples,the lattice constant c increased gradually with adding the Fe content; while lattice constant bdecreased, the lattice contant a and the lattice volume v did not show any rules. As for the Codoped TbMnO₃study, we found that with the increase of the Co content; the lattice constant a, band the lattice volume v decreased gradually, while the lattice constant c did not have theperformance of a certain rule. These were due to the difference of radius between the doping ionsand the normal position ions. And as the absence of oxygen vacancy, there were so manydifferent ions, which aggravated the change of lattice constants.2. As for the electrical properties, we measured the resistance of the samples with thechange of temperature using four point methods; and then mapped the R-T curves as well as theI-V curves. Generally speaking, we found that the resistances of doping materials were sodifferent but clearly. The conductive performances of Fe doping TbMnO₃samples became poorwith the temperature reducing, while these of Co doping TbMnO₃samples became better. Allthese were relevanted to the hybrid effect of different ions. Fe occupied the Mn site, causing thedisorder structure and influencing the exchange between Mn-O-Mn; then leading to theelectronic transmission blocked. For the Co doping TbMnO₃samples, although Mn-O-Mn keyswere destroyed, the number of Mn-O-Co keys increased. On the one hand, Co³⁺changed spin state from high to low spin state at the low temperature; and became more stable, more benefitfor the electronic transmission between Mn³⁺and Co³⁺. On the other hand, doping damaged thelong-range ferromagnetic ordered structure, provided more electronic pairs between Mn³⁺andCo³⁺and gave rise to more cruise electronics, as well as improved the performance of electricaltransport.3. We tested the magnetic properties of the samples with Fe, Co doping TbMnO₃, observedthe change of magnetization with temperature. After analyzing, we found that the magneticproperties changed with different content of Fe or Co; the influence was defferent for thedifferent doping content. As to the samples of TbMn1-xCoxO3, when the doping content reachedto0.5, the change of magnetism performance became most obvious. The Co doping damaged theexchange action between Mn³⁺and Co³⁺, and resulted in the disorder structure; destroyed thelong-range ferromagnetic order. Also, there were so many short-range charges ordered state inthe styem, corresponding to the antiferromagnetic structure. Such AFM and FM coexistence andcompitetition leaded to bifurcation phenomenon in the FC and ZFC curves. There was amagnetic transition in TbMn_0.5Co0.5_O3sample, which happened at92K, due to the existences ofdifferent magnetic ions reversed exchange polarization and leaded to the net magnetismreversed.