Study On The Magnetoelectric Properties Of La_1.67Sr_0.33NiO₄ And Ca₂CoSi₂O₇

Multiferroic material is a kind of multifunctional material with two or more properties of ferroelectric,ferromagnetic and ferroelastic.It has become one of the important research objects in the field of material physics and condensed matter physics.Materials with magnetic and electrical properties are widely found in transition metal oxides.Through sample preparation,characterization and analysis,this paper focused on the possible magnetoelectric coupling effects in La_1.67Sr_0.33NiO₄(LSNO)and Ca₂CoSi₂O₇(CCSO).The mechanism of magnetic and electrical properties is discussed.1.The first chapter introduces the research foundation of dielectric materials and multiferroic materials.The response mechanism of dielectric in electric field,the mechanism of spin-related polarization and the source of magnetoelectric coupling effect in materials are briefly discussed.The research status of La2-_xSr_xNiO₄ and M₂CoSi₂O₇(M=Ca,Sr and Ba)systems was briefly reviewed.2.The optical floating zone furnace,pulsed magnetic field experimental equipment and low temperature dielectric measurement system are introduced.The La_1.67Sr_0.33NiO₄ single crystal and Ca₂CoSi₂O₇ polycrystalline samples were prepared by optical floating zone method and solid state method respectively.X-ray diffraction data and retiveld structure refinement results show that the samples are all single phase,and the crystal structure is analyzed.3.The dielectric response of La_1.67Sr_0.33NiO₄ single crystal with the frequency,temperature and magnetic field was investigated.The experimental results show that there is no magnetization saturation under 58 T pulsed magnetic field.The dielectric constant in the high temperature region reached 10⁴,and the dielectric behavior in the low temperature region confirms to the variable-range-hopping(VRH)model.The temperature of the dielectric anomaly induced by magnetic field is consistent with that of the change of charge order(CO),which indicates that the charge-ordered compound La_1.67Sr_0.33NiO₄ may have magnetoelectric coupling and the CO formed by Ni³⁺and Ni²⁺ions may be one of the main reasons for magnetic dielectric anomaly.4.The magnetic properties of multiferroic material Ca₂CoSi₂O₇ were studied in detail.The results show that a magnetization plateau appears under the pulsed high magnetic field.At a temperature of 5.6 K and a magnetic field of 0.1 T,the sample developed a phase transition from paramagnetic to antiferromagnetic,as the phase transition from antiferromagnetic to ferromagnetic the transition occurs when the magnetic field is higher than 1 T.When the temperature is lower than 5.6 K,the applied magnetic field significantly inhibited the dielectric constant of the sample,indicating that the sample has a strong magnetoelectric coupling effect.The structural phase transition at a temperature of 5.6 K and the change of the hybridization direction between Co²⁺3d and O^2-2p orbitals may be the reasons for the magnetic and electrical anomalies.The results of specific heat and magnetocaloric effects also support the above conjecture.