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High Performance Ferrite Magnets -From the Perspective of Powder Technologyt

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High Performance Ferrite Magnets -From the Perspective of Powder Technologyt Review High Performance Ferrite Magnets -From the Perspective of Powder Technologyt- Hitoshi Taguchi TDK Materials Research Center* Abstract Coercivity in hexa-ferrite magnets originates in the magnetic behavior of so-called "single domain particles" with uniaxial magneto-crystalline anisotropy. The critical diameter, below which a single domain is stable, is estimated at around 1 micrometer (l(r6 m) forM-type Sr-ferrite, which means that control of grain size to under a micron is significant for achieving high coercivity. For this reason studies were performed on a process for obtaining sub micron powder and sintered grains, as well as on techniques for obtaining high orientation under magnetic fields when using submicron powder. For analytical purposes, particle size distribution, magnetic properties, and lattice defects of Sr-ferrite particles were investigated to explain the observed behavior. 1. Introduction magnet material called "Ferroxdure" [1]. Since then, In general, the crucial properties of permanent mag­ much basic research has been directed at improving nets are not only their high energy product (BH)max, the properties and productivities of these materials. which allows volume reduction, but also high reliabil­ Typical and successful examples are the discovery of ity (chemical and magnetic stability) and low price. Sr-ferrite with larger magnetic anisotropy (Kl), and Selection of magnet materials should be based on the development of pressing technology under a mag­ these criteria. Over the past few decades, many mate­ netic field, which doubles the residual flux density rials have been studied and proposed, but only ferrite, (Br). For this reason anisotropic M-type Sr-ferrite is NdFeB, SmCo, and some AlNiCo magnets are still in now dominant in mass production. use today. At 32-40 k]/m3, ferrite magnets have a con­ siderably lower (BH)max than NdFeB-magnets with 2. General Description of Ferrite Magnets 320-400 k]/m3, but ferrite magnets account for an estimated 95% of world production by weight, which 2.1 Crystal Structure of Ferrite Magnets testifies to the unsurpassed cost efficiency of ferrite Fig. 1 shows a magnetoplumbite (M) type crystal magnets. Another advantage is the chemical stability structure. Oxygen ions that form hexagonal closed of these oxides, which makes them environmentally safe, further establishing their continued use. The first "ferrite magnet" was Co-ferrite with a spinel structure (OP-magnet) invented by Prof. Y. 0 0'- ion Kato and Prof. T Takei of Tokyo Institute University in 1932. The OP-magnet is now considered a land­ ~Fe"+ (12k) mark of scientific progress, although it was produced Octahedral "E $Fe"+ (4£2) on a very small scale due only to its poor magnetic site properties and weak mechanical strength. Later ]. ]. $Fe"- (2a) Went et a!. (Philips N.V.) systematically studied cP Fe'· (4£1) Tetrahedral Ba-ferrites with hexagonal structures and in 1952 s1te ~ Fe"· (2b) Trigonal Bi­ released the M-type Ba-ferrite as a new permanent pyramidal site * 570-2, Matsugashita, Minamihatori Narita, Chiba 286-8558 japan Tel (0476)37-1642 Fax. (0476)37-1648 e-mail: [email protected] ' Received 12 October, 1998 Fig. 1 Perspective illustration of M-type Ba-ferrite 116 KONA No.16 (1998) on the line between M and S (spinel) as shown in Fig. 2. Table 1 shows the fundamental properties of these hexa-ferrites. M-type Sr-ferrite (SrM) has a 10% larger anisotropic constant (K1) than M-type Ba-ferrite (BaM). Magnetization of W-type ferrites is lO'lf; greater than M-type ferrite, but its K1 is relatively small in general. Despite a great number of studies concerning cation substitution in the M-type struc­ ture, very few attempts to increase magnetization have succeeded. Similar to spinel ferrites, the saturation magnetiza­ tion of M-type ferrites will increase when non-mag­ netic ions such as Zn occupy the tetrahedral 4fl sites; 3+ BaO Fe at 4fl sites has down-spins. Recently the partial S : MeFe20 4 (spinel structure) substitution of La3+ and Zn 2+ for Sr2+ and Fe3+ L : BaFe40 7 (hypothetical structure) B: BaFe20 4 achieved magnetization that was 5% higner than con­ ventional M-type Sr-ferrite, while retaining the K1 Fig. 2 Ba0-Me0-Fe20 3 system, showing the relationships of chemical compositions among ferrimagnetic hexagonal value of conventional M-type Ba-ferrite, thereby compounds The symbol Me represents a divalent ion. resulting in the highest (BH)max value of 41 kJ/m3, as shown in Table 2 [7]. packing are partially substituted by strontium ions (Sr2+). Ferrous ions (FeH) lie between the oxygen 2.2 Theory of Single Domain Particles ions and are magnetically connected with each other. It is assumed that coercivity in ferrite magnets These generate uniaxial magnetic anisotropy along originates in the magnetic behavior of "single domain the c-axis in the M-structure, which is the basic rea­ particles" [8]. In general, magnetic materials form a son why M-type ferrite is a permanent magnet. Other "magnetic domain structure" in order to reduce static than M-type ferrite, W-and X-type ferrites also have magnetic energy. Boundaries between two domains uniaxial anisotropy, which means they have the poten­ are magnetic walls where magnetic moments gradu­ tial to be permanent magnets. Their compositions lie ally change direction. When reducing the particle or Table 1 Fundamental magnetic properties of hexa-ferrite magnet materials crs Tc HA Kt Symbol Chemical formula Reference x1Q-G(Hm2/kg) (K) (MA/m) 0/m") SrM Sr0-6Fez0:; 1.14 733 1.59 0.35 SrFe2W SrO · 2Fe0 · 8Fe203 1.23 783 1.55 [2] SrZn2W SrO ·2Zn0 ·8Fe20 3 1.25 643 [3] SrZnX 2(Sr0 · ZnO · 7Fez03) 1.21 703 0.98 0.26 [4] BaM Ba0-6Fe,03 1.08 723 1.35 0.32 [5] BaFe2W BaO · 2Fe0 · 8Fe20 3 1.19 773 1.4 [2] BaZn2W BaO · 2Zn0 · 8Fe203 1.23 643 1 0.26 [6] Table 2 Magnetic properties of newly developed Sr-ferrite Fundamental properties II Sintered samples Js Kt Tc Br He] Ir/Is Hk/Hc] (BH)max Density Composition Reference (T) 0/m3) (K) (T) (kA/m) (%) (%) (k]/m3) (Mg/m:~) SrFe1z019 0.465 0.35 728 0.44 320 97.5 91.5 37 5.00 [11] Sro7L<l<J.3 0.485 0.33 693 0.46 207 98.3 94.4 41 5.06 [7] Fen.7Zno.3019 KONA No.16 (1998) 117 I BaC03 or SrC03 Additives Static magnetic energy Ect = (2rr/9!1olJ ,2R" R: radius of the particle Dry crushing (a) Single domain state Wetmilling 1 Additives Domain wall energy Drying Adjustment of slurry content Ow= 4vAK I Granulating I Deagglomeration i with binder Pressure filtration ' Dry pressing in I (wer pressing) in I Dry pressing I magnetic field 1 magnetic field Isotropic magnets Anisotropic magnets Anisotropic magnets (b) Double domain state Fig. 3 Domain structure of one particle with uniaxial anisotropy grain size of magnetic materials, the "non magnetic wall state" (i.e., single domain state) becomes ener­ getically more stable as shown in Fig. 3. And once this single domain state is realized, coercivity will be Fig. 4 Conventional production technology of sintered ferrite magnets maximized because magnetic reversal mechanisms are limited only to spin rotation. such as water in a magnetic field. Although the flota­ tion of the particles in the solvent facilitates their rota­ 2.3 Conventional Process for Sintered Ferrite tion and orientation, dewatering takes a longer time. Magnets Si02 and CaC03 are usually added at the pulverizing Fig. 4 shows the conventional manufacturing process stage in order to promote densification and inhibit of ferrite magnets. The raw materials strontium car­ grain growth at the next sintering stage. Compacts bonate (or barium carbonate) and iron oxide are are sintered at 1200-1250aC in air. mixed and calcined at 1250-1300aC in air. Especially in the case of anisotropic ferrite production, ferrite for­ 2.4 A Concept for High-Performance Ferrite mation must be concluded at this stage because it is Magnets necessary to orient the ferrite particles at the press­ From the perspective of materials research, resid­ ing stage. For this reason, higher calcination tempera­ ual flux density (Br) and intrinsic coercivity (He]) are tures (higher than 1250aC) have been conventionally the most important properties. Both Br and HcJ con­ employed, causing grain growth. The calcined prod­ sist of factors originating in the crystal structure and ucts, which are usually hard pellets of more than a micro- or nano-structure, as shown in Fig. 5. All few mm in size, are crushed and pulverized. For the these factors should be improved simultaneously in production of anisotropic magnets, calcined pellets order to improve magnetic properties. are first dry-pulverized and then wet-milled to submi­ Br is determined by the product of density, the cron order. Ferrite magnets can be classified into two groups Crystal structure Microstructure according to their manufacturing process: isotropic Br = Magnetization Us) X Orientation (%) x density (%) magnets and anisotropic magnets. Anisotropic mag­ nets are mainly manufactured by pressing in a mag­ HcJ = Anisotropic field X Ratio of single domain (HA=2K /Js) particles (%) netic field. There are two methods for pressing, wet 1 and dry. Wet-pressing is a process for compacting a Fig. 5 Factors of magnetic properties (Br and HcJ) in ferrite slurry of fine-milled powder mixed with a solvent magnet 118 KONA No.16 (1998) degree of orientation, and the saturation magnetiza­ synthesis, or by glass crystallization are too small tion Gs). M-type Sr-ferrite has a Js value of about (under 0.1 flm) to be oriented in a magnetic field. Our 0.465 T The density and the degree of orientation research therefore centered on conventional ceramic have upper limits of about 98% for sintered magnets, methods for obtaining submicron-sized primary parti­ which afford the highest values.
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