Single Crystal Growth, Magnetic Property And Magnetoelectric Coupling In Rare Earth Transition Metal Oxides

Multiferroics, in which the ferroelectric（or antiferroelectric） and ferromagnetic（or antiferromagnetic） order coexist have become one of the hottest disciplines of condensed matter physics and materials science, because of its huge potential applications in the new information storage devices and advanced multifunctional magnetoelectric devices. The coexistence of the two order parameters may result in the coupling interaction between them. In detail, the ferroelectric polarization may change the magnetic property by redistributing the spin order, correspondingly, the fluctuation of the spin order may induce the dielectric anomaly or the ferroelectric relaxation through the magnetostrictive effect or electron-phonon interaction. The dielectric anomaly at the magnetic transition temperature observed in experiment is indicative of the inherent magnetoelectric（ME） coupling in multiferroics. Furthermore, the application of an external magnetic field will induce the dielectric change. The perovskite structure of rare-earth orthoferrites RFe O₃（R= rare earth） showing variety of magnetic properties have always the hot spot of research. In recent years, the spin reorientation transition in rare-earth orthoferrites has attracted great renewed attention because it is closely related to the ultrafast photomagnetic effect and the magnetic induced ferroelectricity. The theory and the origin of spin change in the magnetic phase transition has important significance to explore the physical mechanism of multiferroic.on the other hand, strong ME effects up to room temperature in some hexaferrites possessing heliconical spin orderings have been reported. Which lead the study of hexaferrites into a new field and become one of hot research topic. In this dissertation the novel magnetic properties and for perovskite oxides Ho Fe O₃, Ho_0.5Pr_0.5Fe O₃ and Y type hexaferrite Ba_2-x Srx Co₂Fe_12-y Aly O₂₂ were studied. The full text is divided into six chapters:The first chapter, we outline the lattice structure and magnetic structure of RFe O₃ and Y type hexaferrite. Based on them, we further introduce the development and reaserch situation of spin reorientation and multiferroics in RFe O₃, as well as the magnetoelectric effect in the Y type hexaferrite.The second chapter, we introduce the main principles and methods in our experiment. Including preparation of the experimental samples（solid-state reaction method for ceramic samples and floating zone method for single crystal growth）, structure properties analyses（X-Ray Diffractomer and Laue camera） and physical properties measurements（physical property measurement system, vibrating sample magnetometer, high impedance electrometer）.The third chapter, we studied the magnetic properties of Ho Fe O₃ and Ho_0.5Pr_0.5 Fe O₃ single crystal. The Ho Fe O₃ expressed unusual spin reorientation, at low temperature for Γ₂ state, from 34 K to 45 K undergoing Γ₂ to Γ₁₂ phase spin orientation, from 45 K to 60 K take place Γ₁₂ to Γ₄ phase spin orientation. Ho_0.5Pr_0.5 Fe O₃ has taken place the Γ₂ to Γ₄ spin orientation between 75 K and 90 K. Moreover, At lower temperature, the magnetization along a-axis shows an abrupt jump, which occurs only in the fieldcooling process, and is sensitive to external applied magnetic field. By analyzing the interaction between Fe and R ions and the direction of the spin, we conclude that it is caused by the spin reversal of the rare earth ions.The fourth chapter, we studied the magnetic and magnetoelectric coupling properties of Y type hexaferrite Ba_2-x Sr_x Co₂Fe₁₂O₂₂（x=0.5, 1, 0.5）. With the increase of Sr doping, the lattice parameters gradually reduce, the transition temperature from spiral magnetic structure to linear magnetic structure increasing gradually, the coercive force and saturation magnetization of samples enhanced. The magnetic field induced electric polarization were observed in Ba_0.5Sr_1.5Co₂Fe₁₂O₂₂ sample, electric polarization temperature of, the temperature were up to 150 K and the magnetic field can control the magnitude and the sign of the electric polarization.The fifth chapter, We studied the magnetic and the magnetoelectric coupling property of Y-type hexaferrite Ba_0.5Sr_1.5Co₂Fe₁₁ Al O₂₂. Polycrystalline samples were prepared by solid-state reaction method and confirmed by X-ray diffraction（XRD） at room temperature. Temperature dependences of magnetization（M-T） measurement suggest that the transition temperature from spiral to collinear magnetisms is at about 350 K. Magnetization hysteresis measurements of the samples show ferrimagnetic behavior from 10 K to 350 K. Under external magnetic fields, the magnetic field induced electric polarization is observed up to 200 K. The magnitude and the sign of the electric polarization can be controlled by the applied magnetic field. The maximum of the electric polarization is 120 μC/m².The sixth chapter, we summarize the present thesis, and point out the future research direction and application prospect of our research field.