Effects Of La/Nb/Al Element Doping On The Structure And Magnetic Properties Of M-Type Strontium Ferrite

M-type ferrites are widely used as permanent magnets in microwave devices,magneto-optic,magnetic recording media and high frequency devices because of their high saturation magnetization,high coercive force,high resistivity,excellent corrosion resistance,excellent chemical stability and relatively low price.In order to further improve its magnetic properties（coercive force）,strontium ferrite was prepared by solid phase sintering with Fe₂O₃ and Sr CO₃ as the main raw materials and with La,Nb and Al oxide as additives.By means of TG/DSC,XRD/Rietveld Refinement,FTIR,SEM-EDS and VSM detection and analysis method,the optimum synthesis process of M-type strontium ferrite was researched.The effects of La,Nb and Al doping on the phase,morphology and magnetic properties of strontium ferrites with different structures were mainly investigated.The solid-phase sintering test results of SrFe₁₂O₁₉ ferrite show that at the sintering temperature of 1240℃and holding time of 2.5h,the magnetic properties of the sample are the best,and the saturation magnetization（Ms）,remanence（Mr）and coercive force（Hc）are71.68 emu/g,33.51 emu/g and 3622.9 Oe,respectively.Sr_1-xLa_xFe₁₂O₁₉ and Sr Fe_12-xNb_xO₁₉ferrites were prepared by doping La element at Sr site and Nb element at Fe site respectively under the optimum sintering process.It was found that single-phase and multi-domain structures（Mr/Ms<0.5）was prepared when La doping amount（x）<0.09 and Nb doping amount（x）<0.06;The changes of saturation magnetization（Ms）and remanence（Mr）of strontium ferrite doping La and Nb are not so much than that of strontium ferrite without doping La and Nb,but the coercive force（Hc）is obviously improved.The highest coercive force of single-phase strontium ferrite is 4277.2 Oe（La doping x=0.09）and 4037.9 Oe（Nb doping x=0.06）respectively.Sr_1-xLa_xFe_12-xNb_xO₁₉ and Sr_0.88La_0.12Fe_11.88–xNb_0.12Al_xO₁₉ strontium ferrites were respectively prepared by La-Nb and La-Nb-Al co-doping.The results show that strontium ferrite with single phase,hexagonal block grain is formed when the doping amount of La-Nb element（x）<0.03;and with the increase of x,the second phase（Fe₂O₃）disperses out;when x=0.12,the saturation magnetization decreases by about 5 emu/g,the coercive force of prepared Sr_0.88La_0.12Fe_11.88Nb_0.12O₁₉ ferrite sample significantly increased to 4516.6 Oe;and Sr_0.88La_0.12Fe_11.88Nb_0.12O₁₉ strontium ferrites with La,Nb and Al co-doping were prepared by further doping Al element based on the prepared Sr_0.88La_0.12Fe_11.88Nb_0.12O₁₉ferrite.It was found that when Al doping amount（x）=1.0,Ms and Hc of the prepared Sr_0.88La_0.12Fe_11.88Nb_0.12O₁₉ ferrite were respectively 47.46 emu/g and 7453.3 Oe,and coercive force increased 3830.4 Oe compared with the pure Sr Fe₁₂O₁₉ without doping.The effect mechanism of multi-element of La-Nb-Al synergy on strontium ferrite structure and magnetic properties was also clarified when the high coercive strontium ferrite was successfully prepared,which provided theoretical basis for the production process of strontium ferrite from Bayan Obo super iron concentrate（containing rare earth and niobium）.