| Because of high saturation magnetization, great coercivty, excellent chemical stability and corrosion resistance,M-type strontium ferrite has been widely used as hard magnetic materials and high-density perpendicular magnetic recording media materials. Nanoparticle of this material are intensely studied recently due to their current and potential applications in biology and medicine like cells and biomolecules magnetics separation, drug delivery, contrast agents for magnetic resonance imaging and colloidal mediators for cancer magnetic hyperthermia. There are many methods of synthesizing ferrite nanopowder such as organic resin method, metal organic compound method, chemical coprecipitation method, hydrothermal synthesid, salt melt method, and sol-gel method. In this paper, strontium ferrite nanoparticles have been synthesized by sol-gel method, sol-gel combustion synthesis method and glycine-nitrate method. The effect of calcination temperature, ratio of Fe to Sr, ratio of complexing agents to metal ions, PH value and surfactant on the magnetic properties of strontium ferrite nanoparticles have been studied by means of X-ray diffraction, scanning electron microscopy, vibrating sample magnetometer and differential scanning calorimeter and main conclusions are as follows:1. Citrate sol-gel methodX-ray diffraction shows that the product consists of mainlyγ-Fe2O3 phase as well as small amount ofα-Fe2O3 and SrFe12O19 phase at the calcination temperature of 600℃. At 700℃,γ-Fe2O3 disappears and the product consists of mainly SrFe12O19 and small quantity ofα-Fe2O3 phase. At 800℃the product consists of only SrFe12O19 phase. So SrFe12O19 phase begins to form at 600℃ and fully forms at 800℃.When Fe/Sr=12, 11 amd 10, the product isn't pure SrFe12O19 phase. Single SrFe12O19 phase forms as Fe/Sr=9 and the temperature is above 800℃. Pictures of Scanning electron microscope show that with the increase of calcination temperature, the grain size of the powder increases. At 1000℃, nanorods appear. With increase the proportion of citric acid, grain size initially decreases and then increases. When C6H8O7/(Sr2++Fe3+)=2.0, the minimum size is 40 nm. Increase the PH value results in the increases of grain size. Reunion is less when PH=7.00 and adding surfactant can reduce it as well. Magnetic measurement shows that with increasing the calcinations temperature, remanent magnetization constantly increases while coercive force initially increases and then decreases. At 800℃, the coercive force gets the largest value. Magnetic properties of the product are in contact with the content ofα-Fe2O3 which is nonmagnetic phase. The lessα-Fe2O3 non-magnetic phase, the better magnetic properties. When Fe/Sr=9, magnetic properties are the best. At 900℃, specific remanent magnetization reaches 35.83 emu/g. Ratio of citric acid influences the coercive force greatly. With increasing the ratio of citric acid, the coercive force firstly increases and then decreases. When C6H8O7/(Sr2++Fe3+)=2.0, the maximum coercive force of 6565 Oe is atained at 800℃. Adjusting PH value of to 7 or adding suitable surfactant can improve the magnetic properties.2. Sol-gel combustion synthesis methodAt different calcination temperature, the magnetic properties of the product synthesized by sol-gel combustion synthesis method are better than that of those synthesized by sol-gel method. At different ratio of citric acid, specific remanent magnetization and maximum energy product of the product synthesized by sol-gel combustion synthesis method are better than those by sol-gel method. The magnetic properties of power samples are better than thin sheet samples, possibly because that the power samples are heated evenly. For sol-gel combustion synthesis method, when C6H8O7/(Sr2++Fe3+)=1.5, magnetic properties of the product are the best. Specific remanent magnetization is 41.75 emu/g and Specific saturation magnetization is 67.41 emu/g. Coercive force is 6185 Oe and the maximum energy product is 1.416 MGOe.3. Gycine-nitrate methodX-ray diffraction shows that at 600℃, the product consists of onlyγ-Fe2O3, being different from sol-gel method. At 700℃, large number of SrFe12O19 phase appears incombined with small amounts ofα-Fe2O3 phase. At 800℃, the product completely forms SrFe12O19 phase. When Fe/Sr=12, product doesn't form single SrFe12O19 phase. Single SrFe12O19 phase can forms at Fe/Sr=11, 10, 9, with calcinations temperature higher than 800℃. Scanning electron microscope shows that with increasing the calcination temperature, grain size of the product increases. Nanorods appear at 1000℃. With increasing proportion of glycine, grain size decreases. The minimum size of 45 nm is abtained as C2H5NO2/(Sr2++Fe3+)=4.0. Magnetic measurement shows that with increasing calcination temperature, pecific remanent magnetization constantly grows and coercive force initially increases and then decreases. With decreasing Fe/Sr ratio, pecific remanent magnetization and coercive force firstly increase and then decrease. At Fe/Sr=11 and calcinations temperature is 900℃, maximum pecific remanent magnetization of 35.32 emu/g and maximum coercive force of 5998 Oe are atained. Maximum energy product is 1.043 MGOe. With increasing the proportion of glycine, specific remanent magnetization firstly increases and then decreases subsequently. Coercive force constantly grows, the best magnetic properties are got at C2H5NO2/(Sr2++Fe3+)=3.0.
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