Magnetic Properties And Exchange Bias Effect In The Ferrite-based Composites

The magnetic coupling properties in limited magnetic systems, such as the couplingbetween charge and spins of electron, the interfacial coupling betweenferromagnetism(FM) and antiferromgnetism(AFM), the magnetoelectric coupling betweenferroelectrics and ferromagnetism et al., have recently stimulated a sharply increasingnumber of research activities for their scientific interest and significant technologicalpromise in the spintronics devices. In this thesis, our studies focus on the compositessystem of ferrite embedded an antiferromagnetic oxide matrix. The exchange bias effectand the magnetic properties in these composites system are investigated, which couldprovide experimental and theoretic basis for the further research of exchange bias. Themain contents of this thesis can be summarized as follows:1. An introduction of spintronics has been presented. More attentions have beenpaid on exchange bias effect and magnetic properties of magneto electronics materials,which are considered as the most promising applied value. On the basis of the review ofexperimental research status and theoretical model on the exchange bias effect in FM/AFM composites system, the basic thinking of subject selection and investigationsignificance is put forward.2. Base on the different properties of the material, the special experimental methodshave been designed respectively. In this way, the ferromagnetic phases successfullydisperse at the grain boundaries of antiferromagnetic granular systems and theirdistribution can be controlled effectively, which has a key role on the exchange biasbehavior of the composites.3. A series of Ni_1-xFe_xO (x=0, 0.015, 0.03, 0.05, and 0.1) bulk samples wassynthesized by the chemical concentration-precipitation method. Phase compositionanalysis was carried out, which showed that trace amounts of ferromagnetic phaseNiFe₂O₄ could not be detected by x-ray diffraction in these bulk samples with x＜0.03. When x≥0.03, NiFe₂O₄ ferrite is detected easily. The magnetic properties of all the bulksamples were investigated by measuring their magnetization as a function of temperatureand magnetic field. The results indicated that all the bulk samples sintered in air exhibitedlarge room-temperature ferromagnetic behavior ascribed to a ferromagnetic impurityphase. Simultaneously, an exchange bias and training effect were also observed in all thebulk samples, suggesting the possibility of the existence of a strongferromagnetic/antiferromagnetic exchange coupling in this kind of compound. Specifically,the exchange bias field could be tuned by changing the concentration of the Fe dopant.4. A granular system composed of ferrimagnetic (Ferri) NiFe₂O₄ micronparticlesembedded in an AFM NiO matrix has been synthesized by a high-temperature phaseprecipitation method from Fe-doped NiO matrix. In order to allow a comparison, aNiO/10% NiFe₂O₄ mixture was also prepared by the same way. Exchange bias andmagnetic properties of NiFe₂O₄ and AFM NiO bulk composites have been investigated.Large coercivities and shifted hysteresis loops are still observed for all samples after fieldcooling. But the vertical magnetization shifts are not observed. In comparison with thebulk samples, a NiO/10% NiFe₂O₄ nanocomposite was also prepared via direct mixture, inwhich both the horizontal and vertical shift in the hysteresis loops are observed at10K.The phenomena observed are explained in terms of interfacial exchange interactionbetween the two phases and the finite size effect, respectively.5. Polycrystalline nanoparticles with nominal compositions NiFe₂O₄/Li-NiO weresynthesized by a sol-gel technique in air. We investigated mainly the exchange bias effectof the nanocomposites Ni_0.7Fe_0.2Li_0.1O and Ni_0.6Fe_0.2Li_0.2O. The' exchange bias field andcoercivity in the nanocomposites, where NiO was doped with Li, are not enhanced, andbut weakened. The results show that although some uncompsensated spins are produced inthe antiferromagnetic material which is doped with the nonmagnetic ion, simultaneouslythe pinned action force of the antiferromagnetic material are weakened. So, there needs anoptimized dopant content to tune the exchange bias of the nanocomposites, and the concliusions are needed to been investigated further.6. The 0.1MFe₂O₄/0.9BiFeO₃ (M=Co, Cu, Ni) nanocomposite samples weresynthesized by the chemical method from water soluble inorganic salts. Phasecomposition analysis was carried out, which showed that these bulk samples werecomposed of a ferrimagnetic MFe₂O₄ (M=Co, Cu, Ni) and a ferroelectric antiferromagnet(FEAF) BiFeO₃ phases, respectively. The magnetic properties of all the samples wereinvestigated by measuring their magnetization as a function of temperature and magneticfield. The results indicated that all the 0.1CuFe₂O₄/0.9BiFeO₃ samples sintered in airatmosphere at 550℃for 3 h exhibited a negative shift and an enhanced coercivity at lowtemperature ascribed to strong ferromagnetic/antiferromagnetic exchange couplingbetween the BiFeO₃ and CuFe₂O₄ grain boundary. In view of these results, we tend tothink the CuFe₂O₄/BiFeO₃ nanocomposite system may be a useful multifunctionalmaterial.