Hydrothermal Synthesis And Characterization Of The Single Phase Multiferroic Materials

The21^stcentury enters the information age, with the digital and multimediabecoming the mainstream. With economical development and social progress, theinformation capacity increases exponentially.The standard of a country’s strength isbased on the information technology and the development of information industry,thedegree of the usage of information by people is a key point. Therefore,when theresearch of the giant magnetoresistance and high temperature superconductingmaterials become smoothly, ferroelectric and ferromagnetic can be realizedsimultaneously, with the function of storage, they can promote multifunctionall andmulti-iron materials of microelectronic devices to set off new research boom.Recent experiemental studies have shown that its ferroelectric and magneticproperties can not only coexist in the same material, but also have a strong couplingeffect. This means when the electric field is applied,the magnetic spin direction isfree to flip; when the electrod in the direction of the applied magnetic field can also befree to flip. All these properties indicate that they can be used widely in the field oftechonology. The strong magnetic coupling, the mutual regulation between magneticand electricity can realize ferroelectric data entry and ferromagnetic read data,which will lead to a new field of multiferroic following the giant magneticresistance materials.But in practical applications, in order to achieve the strong magnetic couplingeffect, we need two conditions,the first one is ferroelectric and ferromagnetic mustcoexist, the other one is the temperature must be above the room temperature. Untilnow we haven’t found multiferroic materials with three properties above, but onlywith one properties of them. Such as BiMnO₃, it has strong ferroelectric and ferromagnetic, but the magnetic coupling is very weak, the temperature is also belowroom temperature, only100K. another materials can reach the above roomtemperature and also have strong ferroelectric,but their ferromagnetic is very weak,such as BiFeO₃, it is antiferromagnetic.This main functions of the thesis are to develop new multiferroic materials andimprove the ferroelectric and ferromagnetic of multiferroic materials at roomtemperature. We mainly research GdFe1-xCrxO₃series of compound, the usage ofMn replace Cr and B Mn mixture.1.Synthesis with a mild hydrothermal method with a perovskite ABO3structure seriescompounds GdFe_1-xCr_xO₃,by analyzing the magnetic characteristic,we can know it iswell sheet-shaped crystallized. As for the synthesis, hydrothermal and solid-phasemethod can achieve more perfect crystals and the higher synthesis of orderedmaterials. what’s more,the quantity of alkalinity in the compounds plays an importantrole in the compostion of the final product, only through the precise control ofalkalinity can we get the required stoichiometric product.A site selection ofGd³⁺,Gd³⁺is a kind of strong paramagnetic iron, the effective magnetic moment isrelatively large, it can produce strong ferromagnetic.B site Cr³⁺doped to make thesystem Fe-O-Cr ferromagnetic super-exchange weaken, the competition of the threesuper-exchange interaction of Fe-O-Cr, Fe-O-Fe and Cr-O-Cr.we can see theferromagnetic from the magnetic data of all samples, as a result of the disorderedarrangements of the Fe ion and Cr ion in the B-site, the super-exchange interactionbetween ions generated relatively weak ferromagnetism. Under the influence of thecompetition between the ferromagnetic and anti-ferromagnetic,the magnetic orderingtransition has a relationship with the doping concentration of Cr. despite this,thesamples synthesized by hydrothermal method is better than other samples ofsynthesis with high crystallinity, a high degree of ferromagnetic order.2.By mild hydrothermal synthesis GdFe_0.5Cr（Mn）_0.5O₃multiferroic materials,electron micrograph of two compounds with the rules of sheet-shaped crystals. Magnetic studyfound GdFe_0.5Cr_0.5O₃phase transition point Tn=225K, from250K-300K linear,whicn is consistent with the Curie-Weiss law occured in225K from paramagnetic toantiferromagnetic, the change of ferromagnetic and weak ferromagnetism at lowtemperatures. Which may be due to the role of super-exchange between Fe³⁺and Cr³⁺,also may be subject to the influence of Gd³⁺, as Gd³⁺is a strong paramagnetic ion andweak ferromagnetism at low temperatures,GdFe_0.5Mn_0.5O₃the magnetic properties ofparamagnetic and anti-ferromagnetic transition in the25K or so, which maybeaffected by Gd³⁺, whose transition point is50K below.hysteriesis loop is in the4Kand300K, indicating that the compound at room temperature ferromagnetic andanti-ferromagnetic transition maybe above the room temperature. Permitivity datashow that the GdFe_0.5Mn_0.5O₃in the high temperature zone （300-500K） have takenplace in the ferroelectrid phase transition, suggesting the existenceof the magneticcoupling effect, also the measured dielectric peak at low temperatures move to highertemperature with increasing test frequency,which is characteristic of diffuse phasetransition of frequency dispersion phenomena, indicating a relaxor ferroelectric, andstill exist above the Curie temperature near the spontaneous polarization andferroelectric hysteresis loops. Which also futher proved that this material has goodroom temperature ferroelectricity. GdFe_0.5Cr（Mn）_0.5O₃is a very multiferroicmaterials of research value and application prospect.