Study On Negative Magnetization Effect In Transition Metal Oxides With Different Structures

The negative magnetization effect,as a unique macroscopic physical phenomenon in magnetic materials,can be generated by the competition of complex magnetic interactions in the system.The magnetization state of the materials that exhibit negative magnetization can be manipulated by temperature and magnetic field,which is beneficial for potential applications in magnetic sensors,magnetic random storage and magnetic switching devices.In this dissertation,the variation tendency of negative magnetization effect has been systematically studied and analyzed based on the transition metal oxides with different types of structure.The main contents and results of the dissertation are summarized as follows:（1）The effect of doping in magnetism for the single phase Eu_1-xPr_xCrO₃polycrystalline ceramics has been investigated by the systematical measurements of magnetization as a function of temperature under both field cooling（FC）and zero field cooling（ZFC）modes.The results demonstrate that both the normal and inverse magnetic moments can coexist in the undoped sample,where the normal magnetic moment can be induced through the canted antiferromagnetic interaction of Cr sublattices,and the inverse magnetic moment can be provided by the paramagnetic moment of Eu³⁺ions under a negative internal field.The absolute value of inverse magnetic moment can become stronger than the normal magnetic moment by appropriate amount of Pr doping in the Eu position（0.1≤x<0.35）,resulting in the significant negative magnetization effect below the compensation temperature.Moreover,the compensation temperature can be increased from 82.4 K for x=0.1 sample to 155.5 K for x=0.3 sample.（2）The Mn doping effect on the magnetic properties for the YCr_1-xMn_xO₃polycrystalline system with perovskite structure has been experimentally studied and discussed systematically.The results show that the FC curve of the undoped sample tends to be saturated with the temperature decrease,while the dramatic drop of the magnetization can be observed in x=0.1 sample with the decrease of temperature after reaching the maximum value.With the further increase of doping concentration to x=0.15,the negative magnetization effect can be observed at the low temperature regime.This behavior of x=0.15 sample shows a significant difference with the undoped sample which has no negative magnetization effect in the whole temperature range.The observed phenomenon in the doped sample can be interpreted as the induced of the inverse magnetic moment,which can be generated by the Cr³⁺-O^2--Mn³⁺magnetic sublattices with the canted antiferromagnetic coulping.The net magnetic moments generated by the Cr³⁺-O^2--Mn³⁺magnetic sublattices are antiparallel to magnetic moments generated by Cr³⁺-O^2--Cr³⁺magnetic sublattice.As the doping concentration increases to x=0.15,the contribution of inverse magnetic moment generated by Cr³⁺-O^2--Mn³⁺magnetic sublattices can be stronger than the normal magnetic moment provided by Cr³⁺-O^2--Cr³⁺,and thus resulting in the negative magnetization effect at the low temperature regime for x=0.15sample.（3）The Fe doping effect on magnetic properties of（Nd Ca₂）(Cr_2-xFe_xNb)O₉ with perovskite structure has been studied experimentally.The analysis of the magnetization for the undoped sample indicates the coexistence of normal and inverse magnetic moments in the system.The normal magnetic moment can be generated by the canted Cr³⁺-O^2--Cr³⁺magnetic interaction,and the inverse magnetic moment can be provided by the paramagnetic contribution of Nd³⁺ions under a negative internal field which produced by the Cr sublattice.Apart from the Cr³⁺-O^2--Cr³⁺interaction,the antiferromagnetic interaction of Cr³⁺-O^2--Fe³⁺also can be introduced into the system with the Fe doping which antiparallel with the external field.Hence,the normal magnetic moment can be suppressed and the inverse magnetic moment plays the dominant role in the range of 0.2<x<0.6.Therefore,the negative magnetization effect can be induced in the doped samples.Meanwhile,the evolution of the magnetic sublattices of the doped samples with negative magnetization effect has also been analyzed based on the measurements of magnetization.（4）For NiCr₂O₄ with spinel structure,the effect on the magnetic properties due to Al³⁺doping on the Cr³⁺site has been experimentally investigated and analyzed.The results indicate that the FC curves in the low temperature regime gradually approache the temperature axis with the increase of doping concentration.Therefore,it can be concluded that two sets of magnetic sublattices of Ni O₄ tetrahedron and CrO₆ octahedron are coexistence in the NiCr₂O₄ system,and the octahedron sublattices CrO₆ can be divided into B₁ and B₂ sublattices according to the different Cr sites.The normal magnetic moment can be provided by the contributions of Ni and B₁ magnetic sublattices together,while the inverse magnetic moments can be provided by the B₂ magnetic sublattices which antiparallel with the applied field.The normal magnetic moment can be decreased gradually with the Al doping in the B₁ site,resulting in weaker ferromagnetism at a lower temperature.As the doping concentration increases to x=0.2,the normal magnetic moment provided by the Ni and the B₁ magnetic sublattice cannot offset the inverse magnetic moment provided by the B₂ magnetic sublattice due to the dilution effect by non-magnetic Al³⁺ions.As a result,the negative magnetization effect can be observed at the low temperature regime in the Al-doped sample.（5）The Ni₄Nb₂O₉ system opens up an potential possibility to observe the negative magnetization effect with the temperature decrease based on the analysis of the layered magnetic structure.Therefore,the high quality Ni₄Nb₂O₉ single-phase sample has been prepared and the detailed magnetic measurements have been performed systematically.The results demonstrate the transition from the paramagnetic to ferrimagnetic state at the magnetic ordering temperature of 76 K.The magnetization of Ni₄Nb₂O₉ drops sharply to zero after reaching the maximum value with the temperature decrease.Then the negative magnetization effect has been observed in the Ni₄Nb₂O₉ system with the further decrease of temperature.The rapid drop of magnetization and the negative magnetization phenomenon at the low temperature regime can be attribute to the enhancement of the inverse magnetic moment based on the hypothesis of the coexistence of the normal and inverse magnetic moments which generated from two antiparallel sublattices.（6）The magnetization state of the aforementioned negative magnetization materials can be tuned by the temperature and external field.The results show that the positive and negative magnetization states can be switched steadily and reversibly by merely changing the temperature or the magnitude of the field,displaying the stable magnetization state tuned and magnetic switching effect.