Negative Magnetization Effect And Room-temperature Multiferroic Properties In Spinel Materials

Recently,ferrimagnetic materials with spinel crystal structure have attracted much attention.Because of the complex magnetic structure,some novel magnetic effects may be probably induced,which leads to the potential application prospect and important significance on basic research.The main research contents in this paper are respectively negative magnetization effect and the room-temperature multiferroic properties in ferrimagnetic materials with spinel structure.On the one hand,studies on negative magnetization effect are mainly focused on the perovskite materials up to now.This special behavior in ferrimagnetic materials with spinel structure is rarely reported.On the other hand,the spinel materials possess some magnetic advantages such as easy saturation and the high saturation magnetization.However,they still cannot realize the room-temperature multiferroic properties because of the weak electric performance.In view of the above problems,negative magnetization effect and room-temperature multiferroic properties are explored with several representative ferrimagnetic materials with spinel structure in this paper.The systematic studies include material preparation process,physical experiment measurements and the regular summary of results.The main research results and innovations are as follows:(1)Negative magnetization and the related experiment researches are studied in spinel Co0.8Cu0.2Cr2O4 sample.Results suggestion that the sample presents evident negative magnetization effect below 50 K.With further study,the effect can strongly depend on the applied magnetic field.Based on the comprehensive analysis,negative magnetization behavior can be attributed to the reduction of positive magnetic moments with incorporation of Cu2+ ions.Moreover,the sample magnetization can be switched between negative and positive values by changing the value of magnetic field.At the same time,the switch phenomenon is stable and easily controlled.These results indicate that negative magnetization materials have huge potential application prospects on magnetic switch devices and storage devices.(2)The effect of different contents of Cu2+ ions on magnetic behavior in Co1-xCuxCr2O4 samples is explored experimentally.It is found that negative magnetization effect only appears with x ranging from 12.5%to 25%.In addition,negative magnetization effect varies regularly with changing content of Cue2+ ions.Through analyzing,we consider that the introduction of Cu2+ ions can weaken the ferromagnetic interaction of CoO4 crystal lattices,reducing the value of positive magnetization.When Cu2+ content exceeds 0.125,the value of positive magnetization will be less than the absolute value of negative magnetization,which results in the appearance of negative magnetization.Remaining the original crystal structure,influence of annealing temperature on negative magnetization effect is studied in Coo.775Cu0.225Cr2O4 sample because of its excellent negative magnetization behavior.Based on the results,physical mechanism of negative magnetization effect is further discussed.(3)Solid solution xBi0.5Na0.5TiO3-(1-x)NiFe2O4 samples with perovskite Bi0.5Na0.5TiO3 and spinel NiFe2O4 are prepared through sol-gel method.The room-temperature multiferroic properties are studied.Results present that the xBi0.sNa0.5TiO3-(1-x)NiFe2O4 samples simultaneously exhibit both magnetic and ferroelectric properties at room temperature.What is more,the two kinds of properties can be coupled with each other.Based on comprehensive analysis,the regular change tendency of multiferroic properties with different components in the samples is due to the dilution effect between the ferroelectric phase Bi0.5Na0.5TiO3 and the magnetic phase NiFe2O4.In addition,the magnetoelectric coupling effect in the samples can be related to the interaction of magnetostrictive effect in magnetic phase and the piezoelectric effect in ferroelectric phase.Furthermore,the influence of magnetic field annealing on multiferroic properties is explored in the samples.Compared with the samples by conventional annealing process,xBi0.5Na0.5TiO3-(1-x)NiFe2O4 samples annealed with magnetic field display evident improvement in ferroelectric properties and magnetoelectric coupling effect,though the effect of magnetic field annealing on crystal structure and micromorphology can be ignorable.Thus,the enhancement behavior can come from the orderly growth of magnetic domains and electric domains during the magnetic field annealing process.(4)(1-x)(0.7BiFeO3-0.3Bi0.5Na0.5TiO3)-xCoFe2O4 samples are successfully synthesized with the coexistence of spinel and perovskite phases through sol-gel method.Experimental results indicate that the room-temperature multiferroic properties,dielectric properties and impedance properties exhibit regular change tendency with the variation of CoFe2O4 content.Moreover,the introduction of CoFe2O4 phase greatly improves the room-temperature magnetoelectric coupling properties of the samples.