Preparação e caracterização de partículas magnéticas dos sistemas Mn1-xZnxCr2O4 e Cu1-xNixCr2O4

In this dissertation we present the synthesis of magnetic nanoparticles of Mn1-xZnxCr2O4 e Cu1-xNixCr2O4 (0 x 1,0) compounds prepared by coprecipitation method. These compounds were characterized by X-ray diffraction and scanning electron microscopy measurement at room temperature. The CuCr2O4 and NiCr2O4 chromites, produced by solid state reaction method, were studied by ac susceptibility and magnetization as a function of temperature and magnetic field. The nanometric powders chromites have lattice parameters, which are consistent with those reported for the bulk materials in the literature. The average sizes of the particles were calculated from the Scherrer's equation and compared with the size obtained by scanning electron microscopy. We noticed an increase in the average size of particles with increasing temperature calcination. The variation of the lattice parameters in the Cu1-xNixCr2O4 system doped with Ni (0 ≤ x ≤ 1) allows identifying two regions, being a tetragonal and another cubic. The CuCr2O4 and the Cu1-xNixCr2O4 system display tetragonal symmetries, due to the Jahn-Teller effect in oxygen tetrahedral, however the cubic is NiCr2O4 at room temperature. The x-ray diffraction results in Mn1-xZnxCr2O4 system associated with Rieltveld refinement analyses, show that the samples with (0 ≤ x ≤ 1) have x-ray diffraction pattern similar (cubic symmetric and space group Fd3m), confirmed by Rietveld refinement, thus indicating that the main phase of the system as a whole remains unchanged with the replacement of Mn by Zn. Magnetic measurements were carried out with a SQUID magnetometer (Superconducting Quantum Interference Device) display a ferrimagnetic behavior for the CuCr2O4 bulk compound with a transition temperature TC = 122 K. The magnetic field dependence of the magnetization for the CuCr2O4 indicates an average magnetic of de 0.15 mB /per unit formula at 2 K which is smaller than the expected value for ferrimagnetic arrangement of Cr3+ and Cu2+ ions. We can explain the low value of saturate moment ...