Research On Mechanism Of Influence Of Doping On Magnetic Propetries Of M-type Strontium Hexaferrite

M-type strontium hexaferrite (SrFe₁₂O₁₉) is an important permanent magneticmaterial with better magnetic properties. Since it has abundant raw material, lowmanufacturing cost, stable properties and anti-oxidation, it has attracted extensiveinterests in past decades, and been widely used in telecommunication, magneticrecording medium, magneto-optics and microwave devices. However, in comparisonwith Nd₂Fe₁₇B, the magnetic properties of SrFe₁₂O₁₉(saturation magnetization of51emu/g and coercivity of4733Oe) are much lower, which seriously affects itsapplication. Therefore, improvement of the magnetic properties is very important forthe strontium hexaferrtie.M-type SrFe₁₂O₁₉is a permanent magnetic material with hexagonal ferrites anda magnetoplumbite structure, its magnetic properties originate from the Fe3+ions,which present a magnetic moment and magneto-crystalline anisotropy with singleeasy magnetization axis. Over years, many studies in this field have focused onvarious methods and techniques of element doping, by improving the intrinsicmagnetic moment and magnetic anisotropy of SrFe₁₂O₁₉to realize the improving ofmagnetization and coercivity. Such as: Zn-Sn co-substituted strontium hexaferrite issynthesized by sol-gel method, with the saturation magnetization value of81emu/g;Nd-Cd co-substituted strontium ferrite is prepared by self-propagating combustionmethod, with the saturation magnetization value of76emu/g; Zr-Cd co-substituted strontium ferrite is prepared by chemical co-precipitation method and its saturationmagnetization value reaches80emu/g; but the coercivity value only achieves1256Oe. And Nd-Co co-substituted strontium hexaferrite with the coercivity value of7120Oe is synthesized by sol-gel method; Zn-Zr co-substituted strontium ferrite isprepared by chemical co-precipitation method, and the coercivity value is1720Oe;The saturation magnetization and coercivity of La substituted strontium hexaferritereaches66emu/g and7000Oe, respectively etc. Althogh these cationic substitutionenhanced saturation magnetization, the coercivity of the M-type hexaferrites is notincreased obviously, and is still much smaller than that of Nd₂Fe₁₇B. Therefore,choosing the appropriate preparation methods and doping element to improve thecoercivity of M-type SrFe₁₂O₁₉have become a hot issue of concern. In addition tothe element doping methods, changing the thermodynamic process of SrFe₁₂O₁₉formation may also become an effective way to improve the coercivity of SrFe₁₂O₁₉.Such as: use of certain methods and techniques to adjust the intermediate phase inthe formation process, or improve the internal stress of SrFe₁₂O₁₉. However, researchin this area has not been reported yet.This paper mainly concerns the key scientific question of the generally lowcocervity of strontium hexaferrite, by taking the appropriate preparation methods andtechnical to realize the regulation of the thermodynamic processes of SrFe₁₂O₁₉, andselect suitable doping elements and doping methods to realize SrFe₁₂O₁₉doping. Aseries of research works have carried out to improve the coercivity of SrFe₁₂O₁₉, andhave achieved the following results:The research work for preparation, characteristic and magnetism performanceof M-type SrFe₁₂O₁₉were developed in conventional sintering and mechanical alloymethod. The research results show: in the mechanical alloying process, firstly, theFe₃O₄with spinel structure is fabricated by mechanochemical reaction of α-Fe₂O₃and SrCO₃with molar ratio of3:1in high energy ball milling, the120h-milledmixture consists of Fe₃O₄and Sr existing in the grain boundary. According to thechange of TG curve of120h-milled mixture, it is found that the SrFe₁₂O₁₉is synthesized between500℃and800℃. In the conventional sintering process, theα-Fe₂O₃reacts with SrCO₃to form an intermediate phase of SrFeO2.97first, and thenthe SrFeO2.97reacts with α-Fe₂O₃to form SrFe₁₂O₁₉at the sintering temperature of1000℃, implying that the reaction temperature is much higher than that of the120h-milled mixture. The saturation magnetization and coercivity of SrFe₁₂O₁₉which is prepared by mechanical alloying method is60.65emu/g and5481.1Oe,respectively, its coercivity is much larger than that of SrFe₁₂O₁₉which is prepared byconventional sintering method (3932.4Oe). In the two preparation methods, theexperiencing of different intermediate phase leads to the difference of formationtemperature, magnetic properties and sample phase composition of SrFe₁₂O₁₉.Mechanical alloying method is conducive to the formation temperature reduction,the improvement of the coercivity and the obtaining of single-phase samples ofSrFe₁₂O₁₉.Sr(OOCCH₃)₂·0.5H₂O, C₁₅H₂₁FeO₆and Polyvinylphrrolidone, withM.W.13000000are the initial raw material, use sol-gel method to prepare thesolution. By using the spin-coated method, the preferred orientation thin films ofSrFe₁₂O₁₉are fabricated on sapphire (006) substrate. The VSM measurementindicates that the coercivity of SrFe₁₂O₁₉films which prepared in the differentmatching of Sr:Fe=1:12and Sr:Fe=1:10and annealed in air at900℃, is6486.3Oeand6279.9Oe, respectively, much larger than that of SrFe₁₂O₁₉bulk materials, whichis significant for the study of strontium ferrite thin films.A series of strontium hexaferrite compounds SrAl_xFe_12-xO₁₉(0≤x≤4) have beensynthesized by glycin-nitrate method and subsequent heat treatment has been takenin the temperature ranging from700℃to1250℃. It is found that the Al in thestarting mixture almost all incorporated into SrFe₁₂O₁₉by substituting for Fe to formSrAl_xFe_12-xO₁₉compounds. The content of Al in the SrAl_xFe_12-xO₁₉increases withincreasing heating temperature and the nominal Al content. Since the nonmagneticAl³⁺ion only occupies the Fe³⁺ions site at2a and12k with spinning upward in theSrAl_xFe_12-xO₁₉and has smaller ionic radius comparing with Fe³⁺ion, the latticeconstant decreases and the magneto-crystal anisotropy of2a and12k sites increaseswith increasing the Al content doped in the SrAl_xFe_12-xO₁₉, hence, resulting in reduction in the saturation magnetization of the SrAl_xFe_12-xO₁₉and enhancement inthe coercivity with increasing heating temperature and the nominal Al content.However, when the4Fe³⁺ions are almost replaced by Al³⁺ions, the net magneticmoment will be closed to zero, that will weaken the exchange interaction betweenFe³⁺ions, resulting in the decrease of coercivity. The largest coercivity obtained inthe SrAl4Fe8O19sintered at1000℃is17570Oe, which is much larger than the valueof the SrFe₁₂O₁₉(5356Oe) and exceeds the coercivity of Nb₂Fe₁₇B (15072Oe).For the SrAl_xFe_12-xO₁₉compound that formed at sintering temperature of1100℃, a series of measurements about the magnetization changes with temperature(M-T curve) have been carried out, the results show that: The M-T curve of undopedSrFe₁₂O₁₉shows obvious P-type ferrimagnetic characteristics, magnetization firstincreases with the increasing temperature to reach44K, meanwhile themagnetization reaches its maximum, this temperature is called the transitiontemperature, when the temperature is higher than the transition temperature, themagnetization decreases with the increasing temperature; Once Al was doped intoSrFe₁₂O₁₉,the transation temperature decreased with the increasing of Al dopingcontent; when the nominal Al doping content is higher than x=3, no transitontemperature was observed in the measurement, which means P-type ferrimagneticcharacteristics disappeared. According to Neel theory, the P-type ferrimagneticcharacterstics of undoped SrFe₁₂O₁₉can be attributed to its Fe³⁺that distributed infive different lattice sites, and the number of Fe³⁺with spinning upwards is muchmore than spinning downwards. Due to the difference in magnetization variationwith temperature between Fe³⁺sublattices with spinning upwards and downwards,the SrFe₁₂O₁₉shows M-T curves of P-type ferrimagnetic characteristics. Once Alwas doped in the SrFe₁₂O₁₉, non-magnetic Al³⁺ions substitute for spinning upwardsFe3ions, resulting in the trend of quantity balance between Fe³⁺spinning upwardsand downwards, thus for the Al doped samples, with the increase of Al dopingcontent, transiton temperature reduces and the magnetization decreases gradually.When Al reaches the doping content of x=3, the transition temperature disappearedcompletely. When the doping content of Al reaches x=4, the number of Fe³⁺spinningupwards and downwards is equal, the sample realized transition from ferrimagneticto antiferromagnetic gradually.Preparation and characterization of rare earth element La, Dy and transitionelement Co substituted M-type strontium hexaferrite which prepared by glycin-nitrate method have been carried out. The results show that: For the Ladoping strontium hexaferrite, since the ionic radius of La3+ion is much smaller thanthat of Sr2+ion, which made the symmetrical characteristic change at2b site, resultsin the increasing of coercivity with the increase of La doping. However, La3+ionsubstitution of Sr2+ion make part of Fe³⁺ion change to Fe²⁺ion, which lead to theslightly decrease of saturation magnetization. For the Dy doped M-type strontiumhexaferrite, Dy also substitutes for Sr2+ion, but the effect of substitution is weaker,the saturation magnetization and coercivity of Dy doped SrFe₁₂O₁₉is lower than thatof SrFe₁₂O₁₉without Dy doping. For the Co doped M-type strontium hexaferrite,when the content of Co reaches x=0.5, the single phase SrCo_0.5Fe_11.5O₁₉can besynthesized at1100℃, its saturation magnetization is71.23emu/g, which is close tothe theoretical value of SrFe₁₂O₁₉, but the coercivity decrease to2520.7Oe. Thus itcan be seen that Co doping has a great help on the improvement of the saturationmagnetization of SrFe₁₂O₁₉.