Studies On Synthesis And Magnetic Properties Of Rare Earth (Sm, Gd) Doped TiO2 Semiconductor

As a branch of Spintronics,diluted magnetic semiconductor(DMS) is to combine magnetic irons into traditional semiconductors and to replace the lattice position of the semiconductor.The material will have ferromagnetic performance in macroscopic.DMS takes advantage of both charge and spin properties of electrons, and will play an important role in the integrated circuit and other material with memory.As we all know,diluted magnetic semiconductors(DMS) combine two properties,semiconductor nature and ferromagnetic behavior,however,most of DMS materials typically suffer from a low Curie temperatures(T_c) or weak ferromagnetism.After the discovery of room temperature ferromagnetism(RTFM) in Co-doped anatase TiO₂ thin film by Matsumoto et al.,relative research works of RT ferromagnetic DMS oxide on preparations and theories of oxide semiconductors were accelerated.Now the study of RT ferromagnetic DMS oxides mainly concentrates on the various transition-metal-ions,such as Mn,Cr,Fe,V or Ni,doped ZnO,SnO₂ or TiO₂ thin film or nanoparticles.Rare-earth(RE) doped materials display usage in a wide variety of applications. Up to date,a variety of semiconductor materials have been investigated as the host lattice for RE cations for photoluminescence including ZnO,ZrO₂,SnO₂ and TiO₂. However,Utill now,magnetism of RE ions doped DMS oxide at room temperature has not been studied yet.TiO₂ is a good candidate to be used as the host material of RE with relatively wide band gaps,high refractive indices and lower phonon energy. To comprehend its magnetic mechanism more deeply in theory and get better recovery properties,in this letter,we prepared rare earth(Sm,Gd) doped TiO₂ DMS oxide nanocrystalline powders by a sol-gel method.The effect of dopant content on the structural and magnetic properties of samples was investigated,and the room temperature ferromagnetism was observed.We prepared nanocrystalline powders with nominal composition of Ti_1-xSm_xO₂ (0.02≤x≤0.06),Ti_1-xGd_xO₂(0.02≤x≤0.06) by a sol-gel method.These powders were then annealed at different temperatures ranging from 500 to 750℃for 2 h in air.The microstructure analysis and phase identification were carried out by X-ray diffraction(XRD).For the sample annealed at 500℃all the diffraction peaks correspond to a single phase with tetragonal anatase structure.The rutile phase began to appear as the annealing temperature increases to 650℃for Sm-TiO₂ samples and 600℃for Gd-TiO₂ samples.No impurity phases are observed for all the samples. Based on the Scherrer's method,the average grain size of the samples was calculated. With increasing temperature,the grain size increases due to the growth of grains.The Raman spectra of TiO₂ nanocrystalline powders with various Sm content were carried out.All the observed bands are identified from the tetragonal anatase phase.The presence of any other vibration mode from secondary phase,such as metallic Sm,was not detected in any of these spectra.X-ray Photoelectron Spectroscopy was carried out to determine the valence of Sm element.The Sm 3d spectra indicated the chemical states of Sm to be Sm³⁺.Both the peak position and the energy different between Sm 3d^5/2 and Sm 3d^3/2 indicate that Sm element in our sample exists in the ion form.Magnetic properties were measured using an Alternating Gradient Magnetometer (AGM).The samples clearly exhibit ferromagnetic behavior at room temperature.We also studied the Ti_1-xSm_xO₂å'ŒTi_1-xGd_xO₂(0≤x≤0.06) nanocrystalline powders samples with different RE content.There is an optimal Sm content for obtaining the maximal saturation magnetization.RE dopants can enhance room-temperature FM at low dopants.The possible mechanisms of FM in RE doped TiO₂ were discussed.To this point there is no doubt that the magnetism is strongly sensitive to the oxygen vacancies.The Sm-TiO₂ nanocrystaIline powders annealed at 500℃in vacuum.The present sol-gel powders annealed in the air,thus containing little oxygen vacancies compared to the vacuum-annealed sample,exhibited small ferromagnetic behavior.So the existence of oxygen vacancies is believed to be crucial for achieving the room-temperature ferromagnetism in the Sm-doped powders.With the increase of sintering temperature,the saturation magnetization of ferromagnetic phase decreases.With the decrease of temperature,the size of a crystal is reduced and the number of atoms at the crystal surface compared to the total number of atoms in the crystal increases.Therefore the higher concentration of oxygen vacancies can be generated at low annealing temperature,which induce a ferromagnetic coupling.The Ms for powders annealed at a high temperature becomes lower due to the decrease of oxygen vacancy concentration.So the observed room temperature ferromagnetism is strongly connected with the oxygen vacancies.