The Study On The Magnetism And Exchange Bias In Oxide Doped By Transition Metal

The magnetic properties and exchange bias in oxides doped by thansition metals are hot project of current spintronics. The studies on the regulation of exchange bias effect and magnetism in oxides become particularly important, for which lay the foundations for developing practical applications of spintronic devices. In the thesis, our studies focus on the ferromagnetic metal (Ni, Cu) and non-magnetic metal (Cu) doped in antiferromagnetic base (CuO, NiO) and La0.4Sr0.6TiO3-δdiluted magnetic oxides (DMOs). The effects of transition metal doping on the exchange bias effect in composites system which composed of ferrites embedded in antiferromagnetic base and the room-temperature magnetism in La0.4Sr0.6TiO3-δdiluted magnetic oxides (DMOs) are studied. The studies provide experimental and theoretic basis for the further research of regulation of the room-temperature magnetism and exchange bias in these two types of materials. The main contents of this thesis can be summarized as follows:1. To better study the effects of the doping on the exchange bias in the FM/AFM system, the Cu1-xFexO nano-particle composites were synthesized by chemical co-precipitation and gol-gel method. The effects of different methods on the magnetism and exchange bias in compound particle system have been studied, and the right preparation processes for the future works have been explored. The XRD results indicate that the CuFe2O4 and other phases are detected in the sample synthesized by sol-gel method, while there is no the second phase in the sample synthesized by chemical co-precipitation method for the composition of x=0.05.The magnetic measurement results imply that the magnetism in samples by sol-gel method is larger than that by co-precipitation method due to the emergence of CuFe2O4 which dominate the magnetism in the sample. For the samples by the same method, the effect of different doping content on magnetism is much notable. For the samples propared by co-precipitation method, with the increase of Fe doping content, the magnetization increases, and when x>0.05, the Ms increase sharply because of the emergence of CuFe2O4 particles. These two methods have different impacts on the exchange bias of the composites. The chemical co-precipitation method is in favor of producing large exchange bias field.2. On the basis of previous works, a series of NiFe2O4/Ni1-xCuxO (0≤x≤0.1) bulk composites were synthesized by the chemical co-precipitation method to study the magnetic property and exchange bias. In the Cu doping nanocomposites, the exchange bias field and coercivity decrease with the increase of Cu doping implying the Cu doping did not increase exchange bias effect in this system. It indicates that although the non-magnetic Cu ions are doped in NiO matrix, the number of uncompensated spins is not enhanced at the interface due to the absence of domain state. Meanwhile, the pinned action force of the antiferromagnetic material is weakened by the non-magnetic Cu ions, thus the exchange bias field decreases.3. The MFe2O4(M=Cu, Ni)/Cu1-xNixO(x=0,0.03,0.06 and 0.1)nanocomposites were synthesized by the non-equal precipitation method. The results of microstructure and magnetic measurement show that there are two existed forms for the doping magnetic ions in the samples, most of which are doped in CuO matrix substituting for Cu ions, a few react with Fe to form into NiFe2O4 particles dominating the magnetic actions in the samples. With the increase of Ni doping content the exchange bias field increases gradually, implying that the domain state is formed in AFM, the uncompensated spins are increased, simultaneously the pinned action force of the antiferromagnetic material is enhanced too. The origin of the sharply increase of exchange bias field in the sample with x=0.10 is related to the multiphase such as NiO. Furthermore, the movement trend of coercivity dependent of the doping content also prove the phenomena.4. A series of La0.4Sr0.6Ti1-xNixO3-δ(x=0.01,0.02,0.04, and 0.08) bulk samples were synthesized by the sol-gel method. The XRD results indicate that there are NiO and Ti-Ni compounds in the samples with x>0.04 in contrast with no any other phase in samples with x<0.04. For the sample with x=0.02,the comparison of room-temperature magnetism in undoped and doped samples was especially studied while the effect of sintering ambience and sintering temperature on the magnetism was also studied. The results show that, the samples doped Ni exhibit good ferromagnetism and the others exhibit no ferromagnetism at room-temperature. The magnetization of the samples sintered in argon ambience is a factor of twenty larger than the samples sintered in air for the same sintering temperature. For the samples sintered in argon, with the increase of sintering temperature the magnetism first increases, then decreases, implying that the sintering temperature has an large impact on the magnetism of the samples. As to the origin of room-temperature ferromagnetism, in the framework of bound magnetic polaron theory, the oxygen defects in the samples induce the ferromagnetism.