These are materials exhibiting ferromagnetism. The magnetic properties of all materials make them respond in some way to a magnetic field, but most materials are diamagnetic or paramagnetic and show almost no response.
The materials that are most important to magnetic technology are ferromagnetic and ferrimagnetic materials. Their response to a field H is to create an internal contribution to the magnetic induction B proportional to H, expressed as B = ||H, where |i, the permeability, varies with H for ferromagnetic materials. Ferromagnetic materials are the elements iron, cobalt, nickel, and their alloys, some manganese compounds, and some rare earths. Ferri-magnetic materials are spinels of the general composition MFe2O4, and garnets, M3Fe5 O12, where M represents a metal.
Ferromagnetic materials are characterized by a Curie temperature above which thermal agitation destroys the magnetic coupling giving rise to the alignment of the elementary magnets (electron spins) of adjacent atoms in a crystal lattice. Below the Curie temperature, ferromagnetism appears spontaneously in small volumes called domains. In the absence of a magnetic field, the domain arrangement minimizes the external energy and the bulk material appears unmagnetized.
Magnetic materials are further classified as soft or hard according to the ease of magnetization. Soft materials are used in devices in which change in the magnetization during operation is desirable, sometimes rapidly, as in AC generators and transformers. Hard materials are used to supply a fixed field either to act alone, as in a magnetic separator, or to interact with others, as in loudspeakers and instruments. Both soft and hard materials are characterized by their magnetic hysteresis curve (Figure M.4).
These two broad groups — 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.
These materials are characterized by their low loss and high permeability. There are a variety of alloys used with various combinations of magnetic properties, mechanical properties, and cost (Table M.5). There are seven major groups of commercially important materials: iron and low-carbon steels, iron-silicon alloys, iron-aluminum-silicon alloys, nickel-iron alloys, iron-cobalt alloys, ferrites, and amorphous alloys.
The behavior of soft materials is controlled by the pinning of domain walls at heterogeneities such as grain boundaries and inclusions. Thus, the common goal in their production is to minimize such heterogeneities. In addition, eddy-current loss is minimized through alloying additions that increase the electrical resistivity. Initial permeability, important in electronic transformers and inductors, is improved by minimizing all sources of magnetic anisotropy, for example, by using amorphous metallic alloys and by using zero magnetostrictive alloys. A high maximum permeability, which is necessary for motors and power transformers, is increased by the alignment of the anisotropy, for example, through development of crystal texture or magnetically induced anisotropy.
The class of alloys used in largest volume is by far iron and 1 to 3.5% silicon-iron for applications in motors and large transformers. In these applications the cost of the material is often the dominant factor, with losses and excitation power secondary but still important. Thus, the improvment over the years in these alloys has been in developing lower losses without increases in cost.
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.
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 100° C. Chromium-manganese-antimonide, CrxMn2xSb, is such a material. Chromium manganese alone is ferromagnetic, but the anti-monide 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 weight is 3.2 g/cm3. It has high coercive force, and has such high electrical resistance that it may be considered as a nonconductor. 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 127,200 A/m, with initial force to 206,700 A/m, high electrical resistivity, high resistance to demagnetization, and light weight, with specific gravity from 4.5 to 4.9. Ferrimag and Cromag are ceramic magnets. Strontium carbonate is superior to barium carbonate for magnets but is more costly. Lodex magnets are extremely fine particles of iron-cobalt in lead powder made into any desired shape by powder metallurgy.
Some Properties of Selected Soft Magnetic Materials
Permanent magnets, or hard magnetic materials, strongly resist demagnetization once magnetized (Table M.6). They are used, for example, in motors, loudspeakers, meters, and holding devices, and have coercivities Hc from several hundred to many thousands of oersteds (10 to over 100 kA/m). The bulk of commercial permanent magnets are of the ceramic type, followed by the Alnicos and the cobalt-samarium, iron-neodymium, iron-chromium-cobalt, and elongated single-domain (ESD) types in decreasing sequence of usage. The overall quality of a permanent magnet is represented by the highest-energy product (BH)m; but depending on the design considerations, high Hc, high residual induction Br (the magnetic induction when H is reduced to zero), and reversibility of permeability may also be controlling factors.
To understand the relation between the resistance to demagnetization, that is, the coer-civity, and the metallurgical microstructure, it is necessary to understand the mechanisms of magnetization reversal. The two major mechanisms are reversal against a shape anisotropy and reversal through nucleation and growth of reverse magnetic domains against crystal anisotropy. The Alnicos, the iron-chromium-cobalt alloys, and the ESD Lodex alloys are examples of materials of the shape anisotropy structure, whereas barium ferrites, the cobalt-samarium alloys, and the iron-neodymium-boron alloys are examples of the crystal anisotropy-con-trolled materials.
Neodymium-iron-boron is a powerful magnetic material that works best at room temperature. It begins losing its properties at higher temperatures, and at 312°C it loses its magnetism completely.
An investigation into the use of magnets in a Stirling engine to power deep space probes is under way. Stirling engines have a sealed cylinder in which hot gases move two pistons back and forth. By placing magnets on the ends of the pistons, and surrounding the cylinder with wire coils, the magnets would induce a current flow.
Research is being conducted whereby a magnetic material that would work at elevated temperature is being produced in particles that each have two different compositions — one at the outer edge to resist demagnetization, and another at the core to retain magnetic power at higher temperatures. The "functionally graded" material would essentially have the outside material protecting the inside material.
The new magnets would be attractive for a variety of applications on Earth as well — cars, electronics, computers, and power tools.
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 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.
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 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 593°C, is used in wire form for permanent magnets for miniature apparatus. It has a coercive force of 39,750 A/m. Hipernom is a high-permeability nickel-molybdenum magnet alloy containing 79% nickel, 4% molybdenum, and the balance iron. It has a Curie temperature of 460°C and is used for relays, amplifiers, and transformers.
Representative Permanent-Magnet Properties
Note: Oe = oersteds, T = teslas, G = gauss, MGOe = megagauss-oersteds.
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 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 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 55,650 A/m, or ten times that of a plain tungsten magnet steel. Alnico 8 has 35% cobalt, 34% iron, 15% nickel, 7% aluminum, 5% titanium, and 4% copper. The coercive force is 115,275 A/m. It It has a hardness of Rockwell C59. The magnets are cast to shape and finished by grinding. Hyflux AlNico 9, of the same coercive force, has an energy product of 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 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.
Cobalt-platinum, as an intermetallic rather than an alloy, has a coercive force above 341,850 A/m, and a residual induction of 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 is a similar alloy that retains 90% of its magnetization flux up to 343°C. It is used for miniature relays and focusing magnets. Ultramag is a platinum-cobalt magnet material with a coercive force of 381,600 A/m. The Curie temperature is about 500 °C, and it has only slight loss of magnetism at 350°C, whereas cobalt-chromium magnets lose their magnetism above 150°C. The material is easily machined. Alloy 1751 is a cobalt-platinum intermetallic with a coercive force of 341,850 A/m, or of 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.
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 garnets (YIG) and yttrium-aluminum garnets (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 resisistant.
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 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 ten times that 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 are chemical compounds, SmCo5 and CeCo5. These magnets have intrinsic coercive forces up to 2.2 million A/m. The magnetooptic magnets for memory systems in computers are made in thin wafers, often no more than a spot in size. These are ferromagnetic ceramics of’ europium-chalco-genides. Spot-size magnets of europium oxide only 4 |im in diameter perform reading and writing operations efficiently. Films of this ceramic less than a wavelength in thickness are used as memory storage mediums.
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.