Preparation,Electric And Magnetic Properties Of Polycrystalline And Single Crystalline Multiferroic Complex Oxides

As a kind of new multifunctional materials that simultaneously show ferroelectricity,ferromagnetism and/or ferroelasticity,multiferroics have attracted considerable attention in recent years due to their potential applications for advanced multifunctional devices and their intriguing physics.In this thesis work,the polycrystalline and single crystalline multiferroic complex oxides with possible magnetoelectric?ME?effect have been studied.On the one hand,we focused on the exploring the ME effect of polycrystalline and single crystalline hexaferrites.On the other hand,the typical multiferroic Fe-langasite Ba₃NbFe₃Si₂O₁₄ and netotype multiferroic indates Gd InO₃ were successfully grown by floating zone technology,and their physical properties were also characterized.The main results are summarized as follow.First,the cobalt-doping effect on the magnetic and electric properties of polycrystalline Y-type hexaferrites Ba₂Mg_2-xCo_xFe₁₂O₂₂?x=0.4,0.8,1.2,and 1.6?were investigated.Cobalt-doping can increase the dielectric constant,the magnetic phase transition temperatures,and the saturated magnetization.Two-dielectric-relaxation behavior is observed in the sample.The dielectric relaxation behavior at low temperatures is due to the Fe³⁺?Fe²⁺and Co²⁺?Co³⁺charge carriers,while at high temperatures it is on account of oxygen vacancies.The magnetodielectric was detected for x=0.4 sample.One single dielectric peak centered at zero magnetic field indicates that the transverse conical structure maintains down to zero magnetic field,hence this sample possesses possible ME effect.Then,Al-doping of polycrystalline Y-type hexaferrites Ba_0.5Sr_1.5Co₂(Fe_1-xAl_x)₁₂O₂₂?x=0,0.04,0.08,and 0.12?were studied.Al-doping has important effects on both the electric conductivity and magnetic properties.A very much enhanced resistivity?over 10 M??cm?and a high magnetic transition temperature?³46 K?have been obtained at a doping amount of x=0.04.Magnetic field dependences of dielectric and magnetoelectric properties of these samples were investigated.For x=0.04 and 0.08 samples,greatly enhanced ferroelectric polarization and abnormal magnetodielectric properties have been observed at a low applied magnetic field and they can be maintained down to zero field at 200 K,which may result from the occurrence of field-induced spin transverse cone structure.The ME coefficient increases from 500 ps/m to 1481 ps/m with increasing x value from 0.04 to 0.08.The results indicate that Al substitution is crucial for fine-tuning of the magnetic structure and can improve the ME properties of Co₂Y hexaferrites.In the meantime,the Ga doped polycrystalline Y-type hexaferrites BaSrCo₂Fe_12-x Ga_x O₂₂?x=0.0,0.5,1.0,and 1.5?were also studied.The charge carriers for relaxation and conductivity are oxygen vacancies,which are not changed by the Ga-doping.Ga-doping increases the saturation magnetization while has a little effect on the spatial magnetic structure transition temperature.For x=0.5 sample,grain and grain boundary effects are coexistent at low temperatures but the grain boundary effect dominates at high temperatures.The ME effect has been detected in this sample,and the magnetic field induced ferroelectric polarization can be reversed by changing the magnetic field sign,which indicates the clamping of magnetic and ferroelectric domains.On the basis studied of magnetic,electric and ME effect of polycrystalline Y-type hexaferrites,the single crystals of W-type SrCo₂Fe₁₆O₂₇ and X-type Sr₂Co₂Fe₂₈O₄₆ hexaferrites were chosen.Both single crystals have been successfully grown by floating zone method.X-ray diffraction and Raman spectra showed the high purity of hexaferrites phase.The magnetic order reorientation temperatures of these samples are all above the room temperature.Below the reorientation temperature,the crystals show magnetic anisotropy and with an easy cone magnetic structure.The magnetodielectric effect was first time reported in the X-type hexaferrite Sr₂Co₂Fe₂₈O₄₆.The single dielectric peak at zero magnetic field suggested the magnetic structure be transverse conical.Temperature dependence of dielectric constant??T?shows an intrinsic dielectric peak at³20 K,and the peak can be affected by applying a magnetic field.These phenomena indicate the coupling of electric and magnetic orders in X-type hexaferrite Sr₂Co₂Fe₂₈O₄₆.Finally,for typical multiferroic Fe-langasite Ba₃NbFe₃Si₂O₁₄,high quality single crystal has been grown by optimizing the floating zone method.A relaxation dielectric peak was found at around the Néel temperature,and electric polarization mainly along the c axis was detected below this temperature,which indicates the existence of the coupling between the magnetic and electric degrees of freedom.The neotype multiferroics indates GdInO₃ based single crystals have also been successfully grown by floating zone method.Gd InO₃ single crystal is transparent and colorless.When doping a little amount of Mn³⁺(GdIn_0.97Mn_0.03O₃),the crystal becomes blue color.The magnetic properties of these samples were studied,and both of them are antiferromagnetic and absence of magnetic order down to 2 K.Piezoresponse force microscopy image shows the vortex domain structures of Gd InO₃ sample,which indicates the ferroelectric property of Gd InO₃.The above work is significant for the studying of the indates family.