The Structure And Magnetic Properties Of Spinel Ni_1-xCo_xCr₂O₄ And Perovskite Oxides

In this thesis we have performed extensive studies on spinel oxide Ni1-xCoxCr2O4,perovskite oxides La1-xGdxCoO3 and La2CoMn06,and investigated the structure,optical properties and magnetic properties.The major interesting results can be summarized as follows.Polycrystalline samples of Ni1-xCoxCr2O4 were prepared by solid-state reaction.The NiCr2O4 has the tetragonal structure with the space group I41/amd,while the structure of Ni1-xCoxCr2O4 with x ? 0.05 is cubic with the space group Fd-3m.The UV-vis bands of Ni1-xCoxCr2O4 with x>0 show the characteristic absorption of the Co ions at the tetrahedral(A)sites,and the Cr ions locate in the octahedral(B)sites.From the weak UV-vis absorption at about 415 cm-1,the FT-IR absorption peak at about 600 cm-1,and the broad Raman band at about 640 cm-1,it is deduced that some Co ions enter the B sites.It is obvious that Co ions are present at not only A sites,but B sites in the Ni1-xCoxCr2O4 samples.The origin of the position disorder of the Co ions in the spinel structure is probably due to the similar ionic radii of the Co ion(0.65 A)and the Cr ion(0.62 A)at B sites.In the FT-IR spectra,the band at about 510 cm-1 shifts towards the high frequency side with the increasing cobalt content.It is resulted from the reduction of the M-O distance in the octahedron by the replacement of the Ni2+ by the Co2+ ions.The Curie temperatures(Tc)are 75 K and 90 K for the compounds with x = 0.2 and 0.8,respectively.La1-xGdxCoO3 for x ? 0.2 crystallize in the rhombohedral structure,and it is orthorhombic for x ?0.2.The analysis of the Co 2p3/2 XPS spectra indicates that the Co ions are at both +3 and +8/3 valence states.The bending vibration of the Co-O bond linearly shifts from 429 cm-1 to 482 cm-1 with the increasing x,and it is coincident with the increasing Co-O-Co angle.The shoulder peak at approximately 575 cm-1 becomes obviously wide with the increasing doping Gd3+ content owing to the magnetic interactions of the Gd3+ ions with the surrounding CoO6 octahedra.The energy band gap slightly increases from about 2.40 eV to about 2.75 eV as the structure transforms from the rhombohedral to the orthorhombic structure.The magnetic properties are ferrimagnetic with x = 0.05,0.3 and 0.7.However,the calculated Curie-Weiss temperature and constant of La0.3Gd0.7CoO3 are not reasonable,it is probably due to a nonlocalized electronic state.La2CoMn06 were prepared by the solid state reaction(SSR)and the sol-gel method(SGM).Compared to SSR,the SGM sample has a smaller lattice.The XPS spectra of O ls core level of SGM show an extra peak at 533.16 eV,owing to the oxygen deficient region.The forbidden band gap energies are 1.78 eV and 1.93 eV for the SGM and the SSR samples,respectively.At low temperature,both the samples have multiple magnetic phases.Both of them have one ferromagnetic(FM)transition at 153 K and one antiferromagnetic(AFM)transition at 123 K in common,and a spin glass state below 75 K is confirmed.Furthermore,SSR has one extra FM transition at 223 K,while SGM does not have.The existence of the AFM clusters and the coupling between the AFM cluster and the FM or the spin glass domains are confirmed by the low magnetization at 70 kOe,the high coercive field of over 10 kOe,and the existence of the magnetic exchange bias.