Indian Journal of Engineering & Materials Sciences Vol. 7, October-December 2000, pp. 364-369 Strontium ferrite permanent magnet-An overview a b Amitabh Verma , 0 P Pandeyb & Puneet Sharma b Thapar Institute of Engineering and Technology, Patiala 147 001, India a Materials Science Di vision, Thapar Centre for Industrial Research and Development, Patiala 147001 , India Received 25 February 2000; accepted 23 Novell/ber 2000 Among the permanent magnets hard ferrites, particularly M-type hexagonal ferrites (Sr-Ferrite and Sa-Ferrite) have special place by virtue of their low cost and reasonable magnetic properties. This paper gives a brief introduction of hard ferrites with special reference to strontium ferrite permanent magnet. Processing of, sintered and plastic - bonded magnet has been di scussed. The effect of composition of strontium ferrite powders and that of various additives, such as CaO, Si02, AI 20 ), etc. on the magnetic properties have also been outlined. Magnetic materials can be classified into two groups, per unit of available magnetic energy is low . i.e. magnetically soft (easy to magnetize and Comparative properties of M-type ferrite with that of demagnetize) and magnetically hard materials other magnets are given below in Table 11. (difficult to magnetize and demagnetize). Some Comparison shows that rare earth-based magnets have important hard magnetic materials are ferrites, very high magnetic properties than ferrites, but at Alnicos, rare earth - transition metal compounds. In present they cost more than ferrites. Presently, their the class of magnets, ferrites enjoy a unique position use is limited to these applications which call for due to their commercial importance. Ferrites can be miniaturization. described as a class of magnetic oxide which contain iron oxide as the principal component. Hard fen"ites Crystal Chemistry and Physical Properties have a hexagonal structure and can be classified in Barium and strontium ferrite have the same crystal M-, X-, Y- , W-, Z- type fen"ites and given by the structure as magnetoplumbite. The hexagonal unit cell formulae: of Sr-Ferrite is shown in Fig. 11 and schematic representation of Sr.Ferrite structure in Fig. 22 M-type - R Fel 201 9 R = Ba, Sr, Pb. Sr Ferrite has the Chemical Formula Sr" Fe"" 0 ',,, 2 2 2 W-type - R Me2 Fe1 60 27 Me =Fe +, Ni +, Mn + etc. The hexagonal unit cell of Strontium Ferrite contains 2 molecules or total 2x32 = 64 atoms. It is very lon g X - type - R Me Fe28 0 46 in c-direction (c=2.32nm and a=0.588 nm) . Sr" and 0 ' Y - type -R2 Me2 Fel 2 0 22 are both large and closely packed. The smaller Fe" Z - type -R3 Me2 Fe24 0 41 ions are located in the interstices. The structural unit is bui lt up of smaller units: a W- , X- , Y- , Z- type ferrites are not interesting cubic block having spinel structure and a hexagonal economically because of their relatively difficult block containing Sr" -ion. The unit cell is built up by processing, but the M-type ferrites, isostructural with stacking of layers of these atoms one on top of magnetoplumbite, are by far the most important another. Fig. 2 shows 10 layers of large ions (Sr",O') hexagonal fen"ites. M-type Ferrites are mainly used as permanent magnet materials, that have strong Table I- Magneti c properti es of Common permanent magnets resistance to demagnetizing field once they get S. Materi als ;Hc (B H)ma, Bf magnetized and have a domin ant pos ItI on in No. permanent magnet market. They are preferred over (mT) kAhn ( I-; J/m') Alnicos due to lo wer material and processing cost and I. Alnico 650 40-45 42-45 superi or coercivity. Sr-Ferrite and Ba-Ferrite are the 2. Ferrites 350-400 240-320 27 .5-31.5 two main material s in th e M-type ferrite fam il y. These 3. Sm Co 1050- 11 50 880- 1360 145- 160 ferrites have moderate mag netic properties and pri ce 4. Nd-Fe-B 1200- 1300 800- 1200 2 1 0-~50 VERMA el at: STRONTIUM FERRITE PERMANENT MAGNET 365 8 Cubic C A C ® H(!xogonol A C Cubic B A B ~ A Fi g. 2-Schematic layer wise representation of Sr-Ferrite struc­ ture Table 2-Primary and secondary properti es of Sr M I2 Primary Properti es Saturation magneti zati on, mT 475 Ani sotropic constant, kJ/m 3 360 Curie temperature, K 750 Secondary pro perti es Specific wall energy, J/m2 54.2xlO·-l Anisotropy fi eld , kNm 1506 Max. coercivit y, (Hc)max kA/m 1240 +C axis of the hexagonal cell. Of the 24Fe" ions per unit cell, 4 are in tetrahedral sites, 18 in octahedral and 2 in hexahedral. The intrinsic magnetic properties may be subdivided into primary and secondary. Saturati on magnetization (M,) and magnetocrystalline anisotropy constant (K.) are directly related to the magnetic structure. The secondary properties, sLlch as the anisotropy field strength (H.) and the specific domain 2 wall energy (Ym) are derived from the primary Fig. I- Hexagonal unit cell of Sr-Ferrite where, 0 represents 0 - ; 3 • represents and Sr2+ and . 00 are for Fe + ions at different crys­ properties. Primary and secondary magnetic tallographic positi ons properties of Sr M are given in the Table 21.2. with 4 ions per layer, eight of these are wholly Processing oxygen, while two contain one strontium ion each. In Sintered Magnets each hexagonal block, a Sr-ion substitutes for an Powder processing technique is adopted for making oxygen ion in the centre of the three layer and the sintered isotropic and anisotropic ferrite magnets. layers are stacked in the hexagonal sequence. Isotropic ferrites have uniform magnetic properties in The magnetism of SrFe"O" comes from the Fe" ion all directions, whereas anisotropic magnets have each carrying a magnetic moment of 5f.1l1' These are higher flux density in the orientation direction. The located in three crystallographically different kinds of properties of the magnets are largely dependent on the sites-tetrahedral, octahedral and hexahedral. The Fe" process parameters which, in turn, affect the grain ions have their moments normal to the plane of the size, shape, volume fraction of phases and their oxygen layers and thus parallel or anti parallel to the alignment. The schematic process flow diagram for 366 INDIAN J ENG. MATER. SCI., OCTOBER-DECEMBER 2000 Table 3--Nominal specification of -Fe203 powder (for Sr-M magnets) Compound wt. % u-Fe20 3 99.0 (min.) SiOz 0.30 - DAD Mixing CaO 0.05 (max.) MgO 0.05 (max.) MnO 0.05 (max. ) Pelletizing Alz0 3 0.20 (max.) Cl. 0.10 (max.) LO! 0.3-0.5 Calcination (1373- 1533 K) Moisture 0.1-0.3 Table 4--(:hemical composition of SrCOJ . Wl:I-Grilldillg Com~u~ wt. % SrCO) 98-99 BaCO) 1.0-2.5 I'rl:ssing without magncti c Na zO 0.01-0.03 field (isotropic) Pressing under magnotic CaO 0.01 -0.25 lI eld (Anisotropic) AI 20 3 0.01 -0.06 Fez0 3 0.005-0.0 1 LO! 0.25-0.60 Moisture 0.03-0.5 achievable in the sintered magnets. It is recommended that the average particle size of Fe,O, shall be kept small and size distribution within a narro w range. An average particle size in the range of 0.7-0.9 ~m and 4 standard daviation within 0.1 4-0.16 is recommended . Fi g. 3--Process fl ow diagram of sintered SrM (Strontium Ferrite) The powder shall be fin er then - 400 mesh. magnets Similarly, the chemical composition of SrCO, is specified in table 4. sintered magnets is given in Fi g. 3. The individual Composition process steps are further elaborated. The mole ratio of Fe,QJSrO is critical in achieving Raw Materials high remanence and coercivity. As per the chemical formula (SrFe"O,.,), this ratio shall be 6.0. However, it u -Fe,Q, (Rhombohedral) and SrCQ, are the major is observed that good magnets are never obtained raw material s used for the manufacture of SrM. Other when the Fe,O/SrO ratio is 6.0. This is due to th e fact oxides such as CaO, ALO ... SiO" Cr,O" etc are added th at Fe,Q, and SrCO, used are never 100% pure. The in small quantity to obtain or enhance certain specific common impurities, therefore, shall be accounted for properti es. u -Fe,O, is a byproduct of steel mills. while fixing the mole ratio of Fe,O, and SrO. The Pi ck ling of billets is done to remove the oxide scale in following condition shall be met for obtaining hi gh Br the steel mills. This leads to th e formation of FeCI,. and He values. 5 Fen-ic chloride is roasted in the oven to obtain u­ 5.57-0.25xlog (CI)-0.95xSi02<Fe20 3/(SrO+BaO)< Fe,Q,. By virtue of the process the Fe,Q, so obtained 5.81 - 0.25 x log (CI) - 0.65 x Si0 contains some impurities in the form of Silica (SiO,) 2 and chloride ions. The percentage of these impurities The above condition takes into account the shall be within the specified limits to obtain high common impurities, such as SiO" CI and BaCO, quality SrM. The general chemical composition of found in raw materials. If the mole ratio is less th an Fe,O, powder is specified in Table 3. the lower limit set by above inequality then Sr is in The powder size and powder size distribution too excess, and lower remanence and (B.H) ~ .