| Perovskite-type rare-earth orthogonal ferrites(RFeO3,R=rare-earth ion)is an important class of perovskite-type oxide materials,which has multiple degrees of freedom(spin,lattice,charge and orbital).The coupling between them shows wonderful magnetic,dielectric and ferroelectric properties,and has broad application prospects in the fields of spintronic devices,energy conversion and storage,etc.,which has caused strong interest of researchers in the past few decades.On the other hand,as a typical strongly correlated electronic system material,the rare-earth orthogonal ferrites not only have the super-exchange interaction between Fe3+-O2-Fe3+and the double-exchange interaction between Fe2+-O2-Fe3+,but also the interaction between the 3d orbital electrons of Fe ions and the 4f orbital electrons of R ions.These complex interactions determine the magnetoelectric properties of the material.Therefore,in addition to the practical application,the internal physical mechanism of the material is also the focus of the researchers.Compared with other rare-earth ions with a valence of+3 only,Ce3+ions with a valence of+3 are easily oxidized to Ce4+ions with a valence of+4,causing changes in the size and valence of the R-site ion.In the perovskite-type rare-earth orthogonal ferrite RFeO3 synthesized with Ce element,the change in the valence state of Ce ions will cause the change in the valence state of Fe ions,triggering changes in physical properties of the material.Therefore,this thesis takes CeFeO3 as a representative material,using simple solid-phase reaction method to prepare polycrystalline perovskite-type rare-earth orthogonal ferrite RFeO3 ceramic material.And a series of methods such as self-doping and hetero-doping are used to adjust the structure and electromagnetic properties of the RFeO3 material.The related physical mechanisms are explored to provide theoretical and experimental basis for future practical applications.The specific research content and results are as follows:1.The pure CeFeO3 ceramic sample was prepared by a solid-phase reaction method,and the CeFeO3 ceramics showed room temperature multiferroicity by adjusting the oxygen content.Through X-ray diffraction and X-ray photoelectron spectroscopy technology,it was found that the change of oxygen content caused changes in the valence states of Ce and Fe ions and the vacancies of Ce and Fe ions,which led to the transformation of the crystal structure in the material from an orthogonal structure to a distorted tetragonal structure with the local lattice distortion.Therefore,the sample annealed in oxygen exhibits spontaneous ferroelectric polarization,and its residual polarization is relatively high(Pr?0.8 ?C/cm2).At the same time,by testing the hysteresis loop at room temperature,it is found that as the oxygen content increases(the valence state of Ce ion increases),the saturation magnetization of the sample continues to increase.This is due to the enhancement of the ferromagnetic double exchange interaction between Fe2+-O2--Fe3+ ions ascribed to the increase in the bond angle of Fe2+-O2---Fe3+ caused by the transformation of the crystal structure.These experimental results show that CeFeO3 ceramic material is a new room temperature multiferroic material.2.In-depth understanding of the mechanism of spin reorientation in RFeO3 materials.It is found that in the CeFeO3 material with high oxygen content,for the magnetic sublattice of Fe3+ions,a new type of double spin-reorientation transition?4 ??41??21 is exhibited.That is:in addition to the reported first-order spin-reorientation transition ?4??1 at the transition temperature TSR1=230 K induced by the Ce3+-Fe3+ferromagnetic interaction,it is also observed a new spin-reorientation transition ?4??2 at the transition temperature TSR2=126 K.Combining magnetic test and valence state analysis,it is found that the new phase transition is induced by the ferromagnetic double-exchange interaction between Fe2+-Fe3+ ions in the Fe sublattice.Therefore,it is shown that the two ferromagnetic interactions of R3+-Fe3+and Fe2+-Fe3+in the RFeO3 material will affect the value of the anisotropy K2.In other word,the combination and competition of the two ferromagnetic interactions determine the value of K2 to take a positive or negative value,and whether the material exhibits the second-order spin-reorientation phase transition(?4??2)or the first-order spin-reorientation phase transition(?4??1).3.The effect of Sm doping at the R site on the crystal structure,dielectric and magnetic properties of CeFeO3 ceramics was systematically studied.It is found that the doping of Sm3+ions can significantly improve the dielectric properties of CeFeO3 ceramics.A giant dielectric response at room temperature is observed in Ceo.5Sm0.5FeO3 compounds,which shows high dielectric constant and low dielectric loss.Combining X-ray photoelectron spectroscopy and magnetic measurements,it is found that the increase in dielectric constant can be attributed to the increase in the content ratio of Fe2+/Fe3+ ions caused by the charge transfer between Ce3+ and Fe3+ions.But the enhanced ferromagnetic double-exchange interaction between Fe2+ and Fe3+ ions is disadvantages for the reduction of dielectric loss.On the other hand,it is found that the composite spin configuration of ?4 and ?1 and/or ?2 state for Fe3+ ions at room temperature induced by the doping of Sm3+ ions not only affects the magnetic properties of the material,but also is beneficial for the reduce in low-frequency dielectric loss.These results provide a different view for the regulation of dielectric properties and related physical theory of the applications.4.The effects of rare-earth ions on the structural and magnetic properties of RFe0.5Cr0.5O3(R=Ce,Pr,Nd,Sm)ceramics are systematically studied,including magnetization reversal and exchange-bias effect.Based on the model containing the paramagnetic R sublattice and the canted antiferromagnetic Fe/Cr sublattice,by fitting the temperature-dependent magnetizations of the samples,it is found that the magnetic moments of the polarized Pr3+ and Nd3+ ions are antiparallel to the moment of Fe3+/Cr3+ions,which lead to the presence of magnetization reversal in NdFe0.5Cr0.5O3 or the suppressed magnetization reversal in PrFe0.5Cr0.5O3 due to the weak anisotropy,respectively.In addition,the exchange-bias effect is clearly observed in R=Nd and Sm materials,which is attributed to the "pinning effect" of Fe/Cr sublattice by R.At the same time,it is suggested that an intrinsic correlation exists between the magnetization reversal and the exchange-bias effect through the magnetic interaction between R and Fe/Cr sublattices.It is also found that the positive and negative exchange-bias effect field is attributed to the competition between the Zeeman energy of the R3+ ion spin and the coupling energy between the Fe/Cr and R sublattices.These results contribute to the research on the practical application and physical theory of RFe0.5Cr0.5O3 ceramics.Based on all the experimental results in this paper,it can be found that the perovskite-type rare-earth orthogonal ferrites have rich magnetoelectric properties and have potential application in multifunctional magnetoelectric devices.At the same time,the magnetoelectric properties can be adjusted conveniently through changing the valence state or content of the ions.It is hoped that the control method of the structure and physical properties of the RFeO3 materials can be further broadened,so as to realize its industrial application in multifunctional devices. |
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