| The spinel oxides exhibit the rich and novel physical phenomena because of the competition and cooperation of charge,spin,orbit and lattice degrees of freedom.Moreover,it is an ideal material system for studying the various interactions including coordination and competition.Among them,CoCr2O4 has attracted much attention because of the multiferroicity at low temperatures and the reversal of polarization direction under the magnetic field.Howerver,there is no unified understanding of the magnetic structure of CoCr2O4 at low temperatures and the origin of the polarization.Whether the polarization originated from a special spiral magnetic structure,or the ion displacement caused by the local lattice distortion is not yet clear,and the local lattice structure may play a key role in understanding these problems.Note that different Jahn-Teller(JT)ions can cause the different local lattice distortions in CoCr2O4,i.e.,the presence of JT ion Ni2+would stretch the NiO4 tetrahedron,and the JT ion Cu2+would compress the CuO4 tetrahedron.Therefore,we can quantitatively modulate JT distortion and local lattice structure by the introduction of different type and content of JT ions at Cr and Cr sites in CoCr2O4,so as to systematically study the effect of JT ions on the magnetic and ferroelectric properties of CoCr2O4.In this dissertation,we selected CoCr2O4 as the research object,and investigated the crystal structure,magnetism,ferroelectricity,and magnetoelectric coupling effect of Co1-xNixCr2O4,Co1-xCuxCr2O4 and CoCr2-xMxO4 polycrystals.The main results are as follows:(1)we have studied the structural,magnetic and specific heat properties of Co1-xNixCr2O4(0?x?1)polycrystalline ceramics by introducing JT active Ni2+ions at the Co sites.The x<0.95 samples exhibit the cubic structure with the space group of Fd-3m at room temperature,and the x>0.98 samples exhite the tetragonal structure with space group of 141-amd.The cubic to tetragonal structural transition occurs at x=0.98.All the samples exhibit paramagnetic-ferrimagnetic transition,and the helical magnetic structure was suppressed and the antiferromagnetic interaction was strengthened with the increase of Ni content.Based on the results of magnetization and specific heat,we plotted the magnetic phase diagram of Co1-xNixCr2O4(0? x? 1).(2)We prepared Co1-xCuxCnO4(0?x?0.4)samples and studied their structural,magnetic and ferroelectric properties.All the samples exhibit the cubic structure with space group of Fd-3m at room temperature.The x=0.2 and 0.3 samples exhibit magnetization reversal behavior.Moreover,the remanent polarization of the x=0.3 sample(14?C/m2)is significantly enhanced with an order of magnitude compared with the x=0 sample.The results of variable temperature XRD and infrared spectroscopy indicate that the enhancement of polarization can be ascribed to the appearance of low-symmetry tetragonal distortion induced by the introduction of JT active Cu2+ions.(3)The doping effect of the JT ion Mn3+on the structural and magnetic propeties of CoCr2-xMnxO4(0?x?1.2)ceramic were studied.It is found that the x=1.2 sample undergoes a cubic-tetragonal transition.In addition,the x=0.6 sample exhibits magnetization reversal behavior,which originates from the competitive between the magnetic structure of the two sublattices that varies with temperature,which is in good agreement with the Neel theoretical model.It is found that Mn3+ions weakens the magnetic frustration,and regulates the exchange interactions,and inhibits the formation of helical spin ordering. |
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