Banca de DEFESA: JOAO BATISTA DE AZEVEDO FILHO
Uma banca de DEFESA de DOUTORADO foi cadastrada pelo programa.DISCENTE : JOAO BATISTA DE AZEVEDO FILHO
DATA : 29/07/2016
HORA: 14:00
LOCAL: a definir
TÍTULO:
STRUCTURAL PROPERTIES AND MAGNETIC OF DOUBLE PEROVSKITES La2NiMnO6, La2CoMnO6 AND La2FeMnO6
PALAVRAS-CHAVES:
synthesis in the solid state; double perovskite; magnetic characterization; ab initio calculation
PÁGINAS: 105
GRANDE ÁREA: Ciências Exatas e da Terra
ÁREA: Química
SUBÁREA: Físico-Química
ESPECIALIDADE: Química Teórica
RESUMO:
This work investigated in theoretical and experimental form the double La2NiMnO6 (LNMO), La2CoMnO6 (LCMO) and La2FeMnO6 (LFMO). Samples with monoclinic crystal system and space group P21/n, were produced by the modified ionic coordination method. The calculation ab initio using the density functional theory (DFT) and the method of the local spin density approximation (LSDA+U) with the correction of Hubbard were successfully applied to investigate the structural, electronic and magnetic effects of these materials. The calculation of the geometry optimization corroborates with the experimental results of FT-IR that show the distortion in the octahedral sites of ions Ni/Mn, Co/Mn and Fe/Mn, which in turn causes the crystal network in a non-linear way. The analysis by differential scanning calorimetry (DSC) and termogravimetric analysis (TGA) revealed the stability of the compound in which occurred variations of only 1.54, 3.42 and 3.16 % for LNMO, LCMO e LFMO, respectively, which are attributed to the loss of adsorbed water on the surface of the material. Measures of magnetization for the sample LNMO show ferromagnetic phase with TC ≈ 220 K and irreversibility, a typical behavior of spin-glass phase at 80 K. The calculation of the electronic structure and density of states allowed to observe the semiconductor state of the material, which presents gap spin up = 0.6 and gap spin down = 1.7 eV and the covalent character of the Ni-O-Mn bonds, being that their states of valence correspond to Ni2+(3d8) : t32g ↑ t32g ↓ e2g ↑ (S = 1) and Mn4+(3d3) : t32g ↑ e0g (S = 3/2), respectively. These states reveal ferromagnetic coupling between the Ni2+ and Mn4+ ions, resulting in a ferromagnetic material. The magnetization measurements for the sample LCMO show ferromagnetic phase with TC ≈ 210 K and irreversibility at 180 K. With the calculation of the electronic structure and the density of states, it was possible to observe the condition of the semiconductor material, which presents gap spin up = 0.5 and gap spin down = 1.4 eV, as well as the covalent character of the Co-O-Mn bonds, being that their respective states of valence correspond Co2+(3d7) : t32g ↑ t22g ↓ e2g ↑ (S = 3/2) and Mn4+(3d3) : t32g ↑ e0g (S = 3/2), respectively. These states reveal ferromagnetic coupling between the Co2+ and Mn4+, ions, resulting in a ferromagnetic material. In the sample LFMO, the magnetization curves can be interpreted as the combination of two different magnetic phases. An exchange bias (EB) effect was observed at temperatures below 20 K, indicating a coupling between a hard with a soft magnetic phase. The temperature of irreversibility of the magnetization curves and AC susceptibility measurements suggest the existence of a spin-glass phase type cluster-glass with Tg ≈ 100 K. The formation of clusters in double perovskites is a known phenomenon in the literature and interpreted as an antisite disorder. The exchange bias effect loses its action at temperatures above 20 K, suggesting that the coupling between the hard magnetic phase and the soft one decreases as the temperature increases T > 20 K. With the calculation of the electronic structure and density of states, it was possible to observe the condition of the semiconductor material, which presents gap spin up = 0.6 and gap spin down = 2.6 eV and the covalent character of the Fe-O-Mn bonds, being that their states of valence correspond to Fe3+(3d5) : t32g ↑ e2g ↑ (S = 5/2) and Mn3+(3d3) : t32g ↑ e1g (S = 2), respectively. These states reveal antiferromagnetic coupling between the Fe3+ and Mn3+, ions, resulting in a ferromagnetic material.
MEMBROS DA BANCA:
Presidente - 350633 - ADEMIR OLIVEIRA DA SILVA
Externo ao Programa - 1959889 - DAVI SERRADELLA VIEIRA
Externo à Instituição - JOSE ALZAMIR PEREIRA DA COSTA - UERN
Externo ao Programa - 350830 - JOSE HUMBERTO DE ARAUJO
Externo à Instituição - WAGNER DE MENDONÇA FAUSTINO - UFPB