Fe-doped Oxides Based Diluted Magnetic Semiconductors Synthesized By Mechanical Alloying

For the potential applications in spintronics and microelectronics,diluted magnetic semiconductors(DMSs)have being a research focus in materials science.Among them,oxides based DMSs(O-DMSs)have attracted considerable attention since the discovery of room-temperature ferromagnetism in Co-doped TiO₂.Many DMSs materials with Curie temperature under room temperature can not be used for application.And the origin of the ferromagnetism remains a very controversial topic. Therefore,the studies on O-DMSs are valuable on theory and application.The Fe-doped TiO₂,Fe-doped ZnO and Fe,Cu-codoped ZnO powders prepared by mechanical alloying were investigated in this work.The milling not only triggered the solid solution of Fe in TiO-2 lattice,but also induced the phase transformation in TiO₂.Fe-doped TiO₂ with different proportion of phases were obtained by controlling the milling times.The Fe and Fe oxides were not found in the powders.The doping of Fe induced the decrease of direct band gap and the increase of absorption in visible light region.With doping of Fe in TiO₂,the visible light photocatalytic activity for Methylene blue enhanced.All of the samples exhibited room-temperature ferromagnetism.The magnetism reached its maximum at the milling time of 3 h,and then decreased with the increasing of milling time.The magnetic moment per Fe atom in rutile is larger than that in anatase.And the contribution of Fe-doped srilankite to the magnetism is much more than the other two phases with the doping of Fe.The Fe-doped TiO₂ powders milled in different atmospheres presented pure rutile structure.The Raman band of E_g softened with the decrease of oxygen partial pressure (P_o2),while those of A_1gmode maintained a constant value.The iron ions in the powders were found to take Fe²⁺/ Fe³⁺ionic valences.The relative ratio of the Fe³⁺ increased with the increasing of P_o2.No trace of Fe²⁺ions was detected in the M(?)ssbauer spectrum of the sample milled in O₂.The absorption intensity in the visible region enhanced with the decreasing of P_o2.The visible light photocatalytic activity and magnetic moment per Fe atom enhanced with the decreasing of P_o2of the milling atmospheres.Fe-doped ZnO compounds fabricated by mechanical alloying showed single phase with wurzite structure when the Fe content below 9.89 at.%.The Fe atoms dissolved in the ZnO lattice in Fe²⁺and Fe³⁺state.Increasing the Fe content resulted in a decrease in the relative magnetization response.When Fe content is 9.89 at.%,the magnetism of the sample increased abruptly due to the appearance of Fe clusters.The Fe-doped ZnO powders milled in air and O₂ were the mixure of ZnO and FeO. The Fe-doped ZnO powders prepared by mechanical alloying were annealed in vacuum,air and hydrogen.The magnetic moment did not change in the vacuum-annealed sample.However,the air-annealed sample showed magnetization about half of the as-milled sample.It is noteworthy that the ferromagnetism increased by heat treated in hydrogen.And no secondary phase was detected in the sample.The magnetism of the powders annealed in H₂ decreased after reheated in air,suggesting it is a reversible process by hydrogenation and reheated in air.Fe,Co co-doped ZnO compounds were fabricated by mechanical alloying. Co-doping Cu enhanced the magnetic moment for some extent for Cu concentration below 2.0 at.%.The corresponding X-ray diffraction and M(?)ssbauer spectra showed that Fe clusters appeared when Cu content is 1.5 at.%.However the absent of secondary phase for Cu concentration below 1.5 at.%suggested that the increased magnetism is intrinsic.In the case of Zn_0.91Fe_0.07Cu_0.02O,majority of the Fe in the system went into Zn_1-xCu_xFe₂O₄(0≤x≤0.02),leading to an abrupt decrease of magnetism.