Banca de DEFESA: LAZARO LUIS DE LIMA SOUSA

Uma banca de DEFESA de DOUTORADO foi cadastrada pelo programa.
DISCENTE : LAZARO LUIS DE LIMA SOUSA
DATA : 30/07/2018
HORA: 14:00
LOCAL: AUDITÓRIO DO DFTE
TÍTULO:

STUDY OF DOUBLE PEROVISKITA $La_2MnFeO_6$ PREPARED BY SOLID STATE REACTION

PALAVRAS-CHAVES:

double-perovskite, solid state reaction, ferrimagnetism

PÁGINAS: 185
GRANDE ÁREA: Ciências Exatas e da Terra
ÁREA: Física
SUBÁREA: Física da Matéria Condensada
RESUMO:

In this work, three samples of $La_2MnFeO_6$  (LFMO) were prepared by solid state reaction, heat treated at 1200 $^{\circ}C$ at treatment times equivalent to 24, 48 and 96 $hrs$, each identified by name LFMO24, LFMO48 and LFMO96, respectively. The obtained structures are double-$perovskite$, monoclinic with spatial group $ P2_1 / n $ and lattice parameters given by $a$ = 5.52 Å, $b$ = 5.51 Å and $c$ = 7.81 Å. The sample crystallite sizes are equivalent to 361.0 $nm$ (LFMO24), 348.9 $nm$ (LFMO48) and 370.83 $nm$ (LFMO96). According to the specific heat (HC) measurements, the LFMO's have a ferrimagnetic phase transition of approximately 10 $K$, according to data adjustment using the mean field theory. Phase transitions due to charge location were observed, occurring at different temperatures depending on the degree of ordering of each system, in which the most disorganized state tends to have this transition tending to 140 $K$. An excess in HC is identified and modulated according to the sum of the defects of Frenkel and Schottky, where LFMO24 and LFMO48 presented smaller and greater amount of defects, respectively. The contribution of the crystalline lattice was stipulated using the Thirring Model, with Debye temperatures of the order of the expected for this type of material. DC, AC and magnetic hysteresis magnetization measurements confirm the existence of weak ferromagnetism behavior at room temperature in all LFMOs, in samples LFMO48 and LFMO96 it is assigned directly to the atoms of $Mn$, but in LFMO24, sample that presented higher value of magnetization at this temperature, there is an increase provided to the $Fe$ ions, in spite of the $M$ spectroscopy for the $^{57}Fe$, to show a doublet. The real part of the AC susceptibility, $\chi_{AC}^{'}$, complemented by different theoretical models, confirms a cluster-glass type phase, generated by agglomerates in a certain temperature range in the LFMO's, by about 30 $K$, where $\chi_ AC^{'}$ shows peaks that vary with frequency $f$. The strong presence of the Griffiths phase in the $1/\ chi_{AC}^{'} curves of LFMO48 and LFMO96 suggest that this phase is initiated at 65 $K$, occurring differently at LFMO24.

MEMBROS DA BANCA:
Externo à Instituição - FERNANDO LUIS DE ARAUJO MACHADO - UFPE
Externo à Instituição - GUSTAVO DE OLIVEIRA GURGEL REBOUCAS - UFERSA
Presidente - 350830 - JOSE HUMBERTO DE ARAUJO
Interno - 1674707 - MARCO ANTONIO MORALES TORRES
Interno - 1675199 - SUZANA NOBREGA DE MEDEIROS