Preparation And Transport Properties Of Room Temperature Ferromagnetic Sr 3 YCo 4-x Mg X O 10.5+δ (0≤x≤0.5) System Materials

The tetragonal phase Sr₃YCo₄O_10.5+δ is the based phase.when the Co ions are replaced by different ions（usually transition metal ions）,the crystal structure,lattice constant,the valence state and the spin state of Co ions are changed.And have impacts on heat,magnetic and other transport properties.In this thesis,Sr3YCo4-xMgxO10.5 + δ（0<x<0.5）polycrystals and epitaxial films were prepared by solid-state reaction method and PLD respectively.The effects of Mg doping on crystal structure,thermoeletricity transport properties and magnetic transport properties was studied.Sr₃YCo_4-xMg_xO_10.5+δ（x = 0,0.05,0.1,0.2,0.3,0.4,0.5）series polycrystals bulks were prepared by solid-state reaction in the air with sintering process of 1180℃/24h+1180℃/24h.The lattice constant increases with increasing the Mg doping amount,and the ion radius of Mg²⁺（0.66A）is larger than that of Co^3+/4+（0.53-0.61A）.Polycrystals bulks was the semiconductor electrical transport behavior,and the resistivity decreases with increaseing the doping amount.The thermoelectric power decreases with increasing the doping amount,mainly due to part of the Co³⁺ is replaced by Mg²⁺.In order to maintain the charge balance,the substituted Co³⁺ is converted to Co⁴⁺,and the carrier is only supplied by a small number of Co⁴⁺,resulting in an increase in the carrier concentration.When the doping content is 0,0.05,0.1,0.2,the Tc is about 330K.With increasing the doping amount,the Tc moves to the high temperature region,and the Hopkinson peak appears obviously.The magnetization of the undoped component reaches 1.15 eum/g,but the 0.3 component is only 0.1eum/g.It is due to Co³⁺ ions in the high spin state and Co⁴⁺ is in a low spin state,and ion radius of Co⁴⁺ ions is smaller than Co³⁺,more easily distributed in CoO4.5 layer.With Mg²⁺ the doping amount increasing,Co³⁺(Co³⁺(HS,t_2g⁴e_g²,S=2)turn to Co⁴⁺(LS,t_2g⁵e_g⁰,S=1/2).The saturation magnetic moments of Co³⁺ with high spin and Co⁴⁺with low spin are 4μB and 1μB,respectively,and the saturation magnetic moment of Co ions is reduced.Doping amount increaseing lead to the ferromagnetism suppressed.The tetragonal Sr₃YCo_4-xMg_xO_10.5+δ（x = 0,0.05,0.1,0.2,0.3,0.4,0.5）were used as the targets for LaAl03（001）single crystal at 0°,5°,10°.The（008）（400）diffraction peaks appeared on the XRD pattern of the films at 770 ℃/1 00Pa/300mJ/4Hz + annealed at 770 ℃/104Pa/20min,indicating that the films were tetragonal.With increasing the doping amount,the peaks position shifted left and the lattice constant increased,which was consistent with the XRD results of polycrystalline.The 20-scan and 0-scan XRD patterns of the thin films（400）on the LAO（001）substrate with tilted 5° and 10° show that thin films with different tilt angles are epitaxial growth in the c-axis direction,and the tilt angle is 10° has a lower epitaxy than 5°.The tetragonal phase Sr₃YCo_4-xMg_xO_10.5+δ films was semiconductor transport behavior.With the increasing of doping amount,the resistance of the films decreases gradually,and the regularity is consistent with the polycrystals.The magnetization of applied magnetic field was perpendicular to direction of the surface of the film was higher than when the applied magnetic field was parallel to the films.That is,the film parallel to the magnetic field Co ion spin rotation required more energy,the film perpendicular to the magnetic field Co ion spin rotation required less energy.The crystal structure of the Sr₃YCo₄O_10.5+δ film grown at 790 ℃/100Pa/300mJ/4Hz + 790 ℃/104Pa/20min was tetragonal and no LIV signal.The crystal structure of Sr₃YCo₄O_10.5+δ film grown at 760℃/100Pa/300mJ/4Hz + 760℃/104Pa/20mins was cubic cubic structure with LIV signal response.As the doping amount increases,the LIV signal gradually decreases.