MAGNETIC MATERIALS-Magnet steels-chromium magnet steels
Metallic and ceramic materials that become magnetized when placed in a magnetic field. All magnetic materials can be classified into two broad groups—soft magnetic materials and hard magnetic materials. Soft magnetic materials, sometimes called electromagnets, do not retain their magnetism when removed from a magnetic field. Hard magnetic materials, sometimes referred to as permanent magnets, retain their magnetism when removed from a magnetic field. Cobalt is the major element used for obtaining magnetic properties in hard magnetic alloys.
Common soft magnetic materials are iron, iron-silicon alloys, and nickel-iron alloys. Irons are widely used for their magnetic properties because of their relatively low cost. Common iron-silicon magnetic alloys contain 1, 2, 4, and 5% silicon. There are about six types of nickel-irons, sometimes called permeability alloys, used in magnetic applications. For maximum magnetostriction the two preferred nickel contents are 42 and 79%. Additions of molybdenum give higher resistivities, and additions of copper result in higher initial permeability and resistivity.
Magnet steels, now largely obsolete, included plain high-carbon (0.65 or 1%) steels or high-carbon (0.7 to 1) compositions containing 3.5 chromium—chromium magnet steels; 0.5 chromium and 6 tungsten—tungsten magnet steel; or chromium, tungsten, and substantial cobalt (17 or 36)—cobalt magnet steels. They were largely replaced by ternary alloys of iron, cobalt, and molybdenum, or tungsten. Comol has 17% molybdenum, 12 cobalt, and 71 iron. Indalloy and Remalloy have similar compositions: about 20% molybdenum, 12 cobalt, and 68 iron. Chromindur has 28% chromium, 15 cobalt, and the remainder iron, with small amounts of other elements that give it improved strength and magnetic properties. In contrast to Indalloy and Remalloy, which must be processed at temperatures as high as 2280°F (1250°C), Chromindur can be cold-formed.
Some cobalt magnet steels contain 1.5 to 3% chromium, 3 to 5 tungsten, and 0.50 to 0.80 carbon, with high cobalt. Alfer magnet alloys, first developed in Japan to save cobalt, were iron-aluminum alloys. MK alloy had 25% nickel, 12 aluminum, and the balance iron, close to the formula Fe2NiAl. It is age-hardening and has a coercive force of 520 Oe (41,340 A/m) and maximum energy product of 1.35 X 106 G • Oe (10,746 T • A/m). Oerstit 400, used by the Germans during the Second World War because it gave high coercive force in proportion to weight, contained 22% cobalt, 16 nickel, 8 aluminum, 4 copper, and the balance iron. Cunife is a nickel-cobalt-copper alloy that can be cast, rolled, and machined. It is not magnetically directional like the tungsten magnets and thus gives flexibility in design. The density is 0.300 lb/in3 (8,304
kg/m3), the electric conductivity is 7.1% that of copper, and it has good coercive force. Cunife 1 contains 50% copper, 21 nickel, and 29 cobalt. Cunife 2, with 60% copper, 20 nickel, and 20 iron, is more malleable. This alloy, heat-treated at 1100°F (593°C), is used in wire form for permanent magnets for miniature apparatus. It has a coercive force of 500 Oe (39,750 A/m). Hipernom, of Westinghouse Electric Corp., is a high-permeability nickel-molybdenum magnet alloy containing 79% nickel, 4 molybdenum, and the balance iron. It has a Curie temperature of 860°F (460°C) and is used for relays, amplifiers, and transformers.
In the Alnico alloys, a precipitation hardening occurs with AlNi crystals dissolved in the metal and aligned in the direction of magnetization to give greater coercive force. This type of magnet is usually magnetized after setting in place. Alnico 1 contains 21% nickel, 12 aluminum, 5 cobalt, 3 copper, and the balance iron. The alloy is cast to shape, is hard and brittle, and cannot be machined. The coercive force is 400 Oe (31,800 A/m). Alnico 2, a cast alloy with 19% nickel, 12.5 cobalt, 10 aluminum, 3 copper, and the balance iron, has a coercive force of 560 Oe (44,520 A/m). The cast alloys have higher magnetic properties, but the sintered alloys are fine-grained and stronger. Alnico 4 contains 12% aluminum, 27 nickel, 5 cobalt, and the balance iron. It has a coercive force of 700 Oe (55,650 A/m), or 10 times that of a plain tungsten magnet steel. Alnico 8, of Crucible Steel Co., has 35% cobalt, 34 iron, 15 nickel, 7 aluminum, 5 titanium, and 4 copper. The coercive force is 1,450 Oe (115,275 A/m). It has a Rockwell C hardness of 59. The magnets are cast to shape and finished by grinding. Hyflux Alnico 9, of the same coercive force, has an energy product of 9.5 X 106 G • Oe (75,620 T • A/m). The magnets of this material, made by Indiana General Corp., are cylinders, rectangles, and prisms, usually magnetized and oriented in place. The Alnicus magnets, of U.S. Magnet & Alloy Corp., are Alnico-type alloys with the grain structure oriented by directional solidification in the casting which increases the maximum energy output. Ticonal, Alcomax, and Hycomax are Alnico-type magnet alloys produced in Europe. Various other alloys of high coercive force have been developed for special purposes. Silmanal, with 86.75% silver, 8.8 manganese, and 4.45 aluminum, has a coercive force of 6,300 Oe (500,850 A/m), but a low flux density. Platinax, with 76.7% platinum and 23.3 cobalt, has a coercive force of 2,700 Oe (214,860 A/m). Bismanol, developed by the Naval Ordnance Laboratory, is a bismuth-manganese alloy with 20.8% manganese. It has a coercive force of 3,600 Oe (286,560 A/m), but oxidizes easily. Cobalt-platinum, as an intermetallic rather than an alloy, has a coercive force above 4,300 Oe (341,850 A/m) and a residual induction of 6,450 G (0.645 T). It contains 76.8% by weight of platinum and is expensive, but is used for tiny magnets for electric wristwatches and instruments. Placovar, of Hamilton Watch Co., is a similar alloy that retains 90% of its magnetization flux up to 650°F (343°C). It is used for miniature relays and focusing magnets. Ultra-mag is a platinum-cobalt magnet material with a coercive force of 4,800 Oe (381,600 A/m). The Curie temperature is about 932°F (500°C), and it has only slight loss of magnetism at 662°F (350°C), whereas cobalt-chromium magnets lose their magnetism above 302°F (150°C). The material is easily machined. Alloy 1751, of Engelhard Corp., is a cobalt-platinum intermetallic with a coercive force of 4,300 Oe (341,850 A/m), or of 6,800 Oe (540,600 A/m) in single-crystal form. The metal is not brittle and can be worked easily. It is used for the motor and index magnets of electric watches.
Soft magnetic ceramics, also referred to as ceramic magnets, ferromagnetic ceramics, and ferrites (soft), were originally made of an iron oxide, Fe2O3, with one or more divalent oxides such as NiO, MgO, or ZnO. The mixture is calcined, ground to a fine powder, pressed to shape, and sintered. Ceramic and intermetal types of magnets have a square hysteresis loop and high resistance to demagnetization, and are valued for magnets for computing machines where a high remanence is desired. A ferrite with a square loop for switching in high-speed computers contains 40% Fe2O3, 40% MnO, and 20% CdO. Some intermetallic compounds, such as zirconium-zinc, ZrZn2, which are not magnetic at ordinary temperatures become ferromagnetic with properties similar to ferrites at very low temperatures and are useful in computers in connection with subzero superconductors. Some compounds, however, are the reverse of this, being magnetic at ordinary temperatures and nonmagnetic below their transition temperature point. This transition temperature, or Curie point, can be arranged by the compounding to vary from subzero temperatures to above 212°F (100°C). Chromium-manganese-antimonide, CrxMn2xSb, is such a material. Chromium-manganese alone is ferromagnetic, but the antimonide has a transition point varying with the value of x.
Vectolite is a lightweight magnet made by molding and sintering ferric and ferrous oxides and cobalt oxide. The density is 0.114 lb/in3 (3,156 kg/m3). It has high coercive force and has such high electrical resistance that it may be considered a nonconductor. It is very brittle and is finished by grinding. Magnadur was made from barium carbonate and ferric oxide and has the formula BaO(Fe2O3)6. Indox and Ferroxdure are similar. This type of magnet has a coercive force to 1,600 Oe (127,200 A/m), with initial force to 2,600 Oe (206,700 A/m), high electrical resistivity, high resistance to demagnetization, and light weight, with specific gravity from 4.5 to 4.9. Ferrimag, of Crucible Steel Co., and Cromag are ceramic magnets. Strontium carbonate is superior to barium carbonate for magnets but is more costly. Lodex magnets, of General Electric Co., are extremely fine particles of iron-cobalt in lead powder made into any desired shape by powder metallurgy.
Ceramic permanent magnets are compounds of iron oxide with oxides of other elements. The most used are barium ferrite, oriented barium ferrite, and strontium ferrite. Yttrium-iron garnet (YIG) and yttrium-aluminum garnet (YAG) are used for microwave applications.
Flexible magnets are made with magnetic powder bonded to tape or impregnated in plastic or rubber in sheets, strip, or forms. Magnetic tape for recorders may be made by coating a strong, durable plastic tape, such as a polyester, with a magnetic ferrite powder. For high-duty service, such as for spacecraft, the tape may be of stainless steel. For recording heads the ferrite crystals must be hard and wear-resistant. Ferrocube is manganese zinc. The tiny crystals are compacted with a ceramic bond for pole pieces for recorders. Plastiform is a barium ferrite bonded with rubber in sheets and strips. Magnyl, of Applied Magnetic Corp., is vinyl resin tape with the fine magnetic powder only on one side. It is used for door seals and display devices.
Rare-earth magnetic materials, used for permanent magnets in computers and signaling devices, have coercive forces up to 10 times those of ordinary magnets. They are of several types. Rare-earth-cobalt magnets are made by compacting and extruding the powders with a binder of plastic or soft metal into small precision shapes. They have high permanency. Samarium-cobalt and cesium-cobalt magnets are cast from vacuum melts and, as made by Bell Laboratories, are chemical compounds, SmCo5 and CeCo5. These magnets have intrinsic coercive forces up to 28,000 Oe (2.2 X 106 A/m). The magnetooptic magnets produced by IBM for memory systems in computers are made in thin wafers, often no more than a spot in size. These are ferromagnetic ceramics of europium-chalcogenides. Spot-size magnets of europium oxide only 157 (xin (4 (xm) in diameter perform reading and writing operations efficiently. Films of this ceramic less than a wavelength in thickness are used as memory storage media. Neodymium-iron-boron magnets, which are used in computer disk-drive systems, retain useful performance at temperatures up to 248°F (120°C). They also maintain their room-temperature energy at —452°F ( — 269°C). Praseodymium-iron-boron magnets can generate greater energy, appreciably greater at — 452°F
Magnetic fluids consist of solid magnetic particles in a carrier fluid. When a magnetic field is applied, the ultramicroscopic iron oxide particles become instantly oriented. When the field is removed, the particles demagnetize within microseconds. Typical carrier fluids are water, hydrocarbons, fluorocarbons, diesters, organometallics, and polyphenylene ethers. Magnetic fluids can be specially formulated for specific applications such as damping, sealing, and lubrication.

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