MAGNETIC PROPERTIES OF La-Co SUBSTITUTED M-TYPE Sr FERRITE ULTRA FINE POWDERS PREPARED FROM THE CITRATE PRECURSORS

摘要

Magnetic properties of La-Co substituted M-type Sr ferrite powders were studied. Samples were prepared from the citrate precursors. High purity reagent of Sr(NO_3)_2, Fe(NO_3)_3·9H_2O, Co(NO_3)_2·6H_2O and La_2O_3 were used as starting materials. They were dissolved in distilled water with citric acid. The nominal molar ratio of the mixed solutions were 1 : 5 of metal : citric acid. Prepared aqueous solution was heated at 120°C for dehydration and gelling. Thermal pyrolysis was carried out by heating the gel at the temperature range between 300°C and 350°C for 1h in air. The obtained precursor powders were ground with an alumina mortar and compacted by uniaxial pressing into disk specimens, 20 mm in diameter and 1 mm thick, and then heated at temperature range between 750°C and 900°C in air. Phase identification and determination of lattice parameters were carried out by powder X-ray diffraction (XRD). Scanning Electron Microscope (SEM) was utilized to investigate the microstructure of the polycrystalline ferrites. Magnetic properties were discussed by magnetization measurements by using a vibration sample magnetometer (VSM) at a maximum applied field of 16 kOe. Magnetization and coercive force were measured at room temperature. Temperature dependence of residual magnetization was measured for determine the Curie temperature. The La-Co substituted SrM ferrite powders prepared in this study showed typical magnetic hysterisis of hard ferrite. Extreme enhancement of coercive force was performed by La-Co substitution. Maximum value of coercive force achieved in this study is 8.0 kOe. This enhancement of coercive force is realized by mainly increase of magnetocrystalline anisotropy caused by La-Co substitution.