Influence of the iron content on the formation process of substituted Co-Nd strontium hexaferrite prepared by the citrate precursor method

S. E. JACOBO; C. HERME; P. G. BERCOFF

Buenos Aires

Simposio; 15th International Symposium on Metastable, Amorphous and Nanostructured Materials (ISMANAM); 2008

It has been reported that the magnetic properties of M-type ferrites fine particles were enhanced by the substitution of La-Co ions [1]. As hematite seems to be the main secondary phase when rare earth ions substitution increases [2], a deficient iron formulation can be proposed in order to improve the results. In this paper, the formation process of substituted strontium hexaferrites (Sr1-xNdx Fe12-xCoxO19 and Sr1-xNdxFe12 (1-x)CoxO19 for x=0.3) through the citrate precursor method is shown by following the thermal decomposition pathway of the as-burnt precursors. Both structural and magnetic properties of samples are investigated and the influence of Fe3+ is particularly emphasized. Samples of composition SrFe12O19 (S0), Sr0.7Nd0.3Fe8.4Co0.3O19 (SL) and Sr0.7Nd0.3Fe11.7Co0.3O19 (SS) were prepared by the self-combustion method. The powders were heat treated in air at 500, 800 and 1100ºC for two hours. The phase identification of the different powders was performed using XRD. Samples S0 and SS treated at 500ºC contain many phases, such as hematite, maghemite, SrCO3 and small amounts of hexaferrite. In sample SL only maghemite and a small amount of SrCO3 are present. Only sample SL heat-treated at 1100ºC shows well-defined peaks of the hexaferrite phase and no extra peaks related to secondary phases. At 800ºC the main peaks of the M-hexaferrite phase are present in all the samples and no secondary phases are noticed. Table 1 shows the magnetic properties of the samples treated at different temperatures. Nd-Co substitution increases saturation magnetization (MS) and coercivity (Hc) in samples treated at different temperatures. Hc increases in substituted samples treated at 800ºC and then decreases as particles size increases. Sample SL -with low Fe3+ content- shows the best magnetic properties with no secondary phase segregation, indicating that the best results for applications of this ferrite will be obtained with an iron deficiency in the stoichiometric formulation. Table1:  Magnetic parameters (Hc, Ms) of the powders heated at various temperaturesSample  T [ºC]  Hc[Oe] M(Max-15kOe)[emu/g]S0          500        216          38.7                  800       2080         30.4            1100       3127         72.2SL          500         427         53.2              800         4800        58.2             1100        4613        74.2SS           500          483        31.9                800        4712        37.5               1100       4613        60.0 [1] H. Taguchi, Y.Minachi, K.Masuzawa, H.Nishio, Eighth Proceedings of the international Conference on Ferrites (2000) 405. F. Kools, A. Morel, R. Grössinger, J. M. Le Breton, P. Tenaud, J. Mag. Mag. Mater.242-245 (2002) 1270.[2] L.Lechevallier, J.M.Le Breton, A.Morel, P.Tenaud, J. Mag. Mag. Mater.316 (2007) e109.