Magnetic alloys

6.8.4 Magnetic alloys

The work done in moving a domain boundary depends on the energy of the boundary, which in turn depends on the magnetic anisotropy. The ease of magnetiza- tion also depends on the state of internal strain in the material and the presence of impurities. Both these latter factors affect the magnetic ‘hardness’ through the phenomenon of magnetostriction, i.e. the lattice constants are slightly altered by the magnetization so that a directive influence is put upon the orientation of magnetization of the domains. Materials with inter- nal stresses are hard to magnetize or demagnetize, while materials free from stresses are magnetically soft. Hence, since internal stresses are also responsible for mechanical hardness, the principle which governs the design of magnetic alloys is to make permanent magnetic materials as mechanically hard and soft mag- nets as mechanically soft as possible.

Magnetically soft materials are used for trans- former laminations and armature stampings where a high permeability and a low hysteresis are desirable: iron–silicon or iron–nickel alloys are commonly used for this purpose. In the development of magnetically soft materials it is found that those elements which form interstitial solid solutions with iron are those which broaden the hysteresis loop most markedly. For this reason, it is common to remove such impurities from transformer iron by vacuum melting or hydrogen annealing. However, such processes are expensive and, consequently, alloys are frequently used as ‘soft’ mag- nets, particularly iron–silicon and iron–nickel alloys (because silicon and nickel both reduce the amount of carbon in solution). The role of Si is to form a

-loop and hence remove transformation strains and also improve orientation control. In the production of iron–silicon alloys the factors which are controlled include the grain size, the orientation difference from one grain to the next, and the presence of non-magnetic inclusions, since all are major sources of coercive force. The coercive force increases with decreasing grain size because the domain pattern in the neigh- bourhood of a grain boundary is complicated owing to the orientation difference between two adjacent grains. Complex domain patterns can also arise at the free surface of the metal unless these are parallel to a direc- tion of easy magnetization. Accordingly, to minimize the coercive force, rolling and annealing schedules are adopted to produce a preferred oriented material with

a strong ‘cube-texture’, i.e. one with two h1 0 0i direc- tions in the plane of the sheet (see Chapter 7). This procedure is extremely important, since transformer material is used in the form of thin sheets to mini- mize eddy-current losses. Fe–Si –B in the amorphous

state is finding increasing application in transformers.

The iron–nickel series, Permalloys, present many interesting alloys and are used chiefly in communica- tion engineering where a high permeability is a neces- sary condition. The alloys in the range 40–55% nickel are characterized by a high permeability and at low field strengths this may be as high as 15 000 compared with 500 for annealed iron. The 50% alloy, Hyper- nik , may have a permeability which reaches a value of 70 000, but the highest initial and maximum perme- ability occurs in the composition range of the FeNi 3 superlattice, provided the ordering phenomenon is sup- pressed. An interesting development in this field is in the heat treatment of the alloys while in a strong mag- netic field. By such a treatment the permeability of Permalloy

65 has been increased to about 260 000. This effect is thought to be due to the fact that dur- ing alignment of the domains, plastic deformation is possible and magnetostrictive strains may be relieved.

Magnetically hard materials are used for applica- tions where a ‘permanent magnetic field is required, but where electromagnets cannot be used, such as in electric clocks, meters, etc. Materials commonly used for this purpose include Alnico (Al –Ni –Co) alloys, Cunico (Cu–Ni –Co) alloys, ferrites (barium

and strontium), samarium–cobalt alloys (SmCo 5 and Sm 2 ⊲ Co, Fe, Cu, Zr⊳ 17 ) and Neomax ⊲Nd 2 Fe 14 B⊳. The Alnico alloys have high remanence but poor coerciv- ities, the ferrites have rather low remanence but good coercivities together with very cheap raw material costs. The rare-earth magnets have a high performance but are rather costly although the Nd-based alloys are cheaper than the Sm-based ones.

In the development of magnetically hard materials, the principle is to obtain, by alloying and heat treat- ment, a matrix containing finely divided particles of

a second phase. These fine precipitates, usually dif- fering in lattice parameter from the matrix, set up coherency strains in the lattice which affect the domain boundary movement. Alloys of copper–nickel –iron, copper–nickel –cobalt and aluminium–nickel –cobalt are of this type. An important advance in this field is to make the particle size of the alloy so small, i.e. less than a hundred nanometres diameter, that each grain contains only a single domain. Then magneti- zation can occur only by the rotation of the direc- tion of magnetization en bloc. Alnico alloys con- taining 6–12% Al, 14–25% Ni, 0–35% Co, 0–8% Ti, 0–6% Cu in 40–70% Fe depend on this fea- ture and are the most commercially important perma- nent magnet materials. They are precipitation-hardened alloys and are heat-treated to produce rod-like pre- cipitates (30 nm ð 100 nm) lying along h1 0 0i in the bcc matrix. During magnetic annealing the rods form along the h1 0 0i axis nearest to the direction of the field, when the remanence and coercivity are markedly

increased, Sm 2 ⊲ Co, Fe, Cu, Zr⊳ 17 alloys also rely on the pinning of magnetic domains by fine precipitates. Clear correlation exists between mechanical hardness

and intrinsic coercivity. SmCo 5 magnets depend on the very high magnetocrystalline anisotropy of this

192 Modern Physical Metallurgy and Materials Engineering compound and the individual grains are single-domain

features of anti-ferromagnetism are similar in many particles. The big advantage of these magnets over the

respects to ferromagnetism, and are summarized as Alnico alloys is their much higher coercivities.

follows:

The Heusler alloys, copper–manganese–aluminium, are of particular interest because they are made up

1. In general, the magnetization directions are aligned from non-ferromagnetic metals and yet exhibit ferro-

parallel or anti-parallel to crystallographic axes, e.g. magnetic properties. The magnetism in this group of 2C in MnI and CoO the moment of the Mn and Co 2C

ions are aligned along a cube edge of the unit cell. dently because of the presence of manganese atoms.

alloys is associated with the compound Cu 2 MnAl, evi-

The common directions are termed directions of

The compound has the Fe 3 Al-type superlattice when

anti-ferromagnetism.

quenched from 800 °

2. The degree of long-range anti-ferromagnetic order- netic, but when the alloy is slowly cooled it has

C, and in this state is ferromag-

ing progressively decreases with increasing temper-

a -brass structure and is non-magnetic, presumably ature and becomes zero at a critical temperature, because the correct exchange forces arise from the lat-

T n , known as the N´eel temperature; this is the anti- tice rearrangement on ordering. A similar behaviour is

ferromagnetic equivalent of the Curie temperature. found in both the copper–manganese–gallium and the

3. An anti-ferromagnetic domain is a region in which copper–manganese–indium systems.

there is only one common direction of anti- The order–disorder phenomenon is also of magnetic

ferromagnetism; this is probably affected by lattice importance in many other systems. As discussed pre-

defects and strain.

viously, when ordering is accompanied by a structural change, i.e. cubic to tetragonal, coherency strains are

The most characteristic property of an anti- set up which often lead to magnetic hardness. In FePt,

for example, extremely high coercive forces are pro-

a maximum as a function of temperature, as shown in duced by rapid cooling. However, because the change

Figure 6.34a. As the temperature is raised from 0 K the in mechanical properties accompanying the transfor-

interaction which leads to anti-parallel spin alignment mation is found to be small, it has been suggested that

becomes less effective until at T n the spins are free. the hard magnetic properties in this alloy are due to the

Similar characteristic features are shown in the resis- small particle-size effect, which arises from the finely

tivity curves due to scattering as a result of spin dis- laminated state of the structure.

order. However, the application of neutron diffraction techniques provides a more direct method of study-

6.8.5 Anti-ferromagnetism and

ing anti-ferromagnetic structures, as well as giving

ferrimagnetism

the magnetic moments associated with the ions of the metal. There is a magnetic scattering of neutrons in

Apart from the more usual dia-, para- and the case of certain magnetic atoms, and owing to the ferromagnetic materials, there are certain substances

different scattering amplitude of the parallel and anti- which are termed anti-ferromagnetic; in these, the

parallel atoms, the possibility arises of the existence net moments of neighbouring atoms are aligned in

of superlattice lines in the anti-ferromagnetic state. opposite directions, i.e. anti-parallel. Many oxides

In manganese oxide MnO, for example, the param- and chlorides of the transition metals are examples

eter of the magnetic unit cell is 0.885 nm, whereas the including both chromium and ˛-manganese, and

chemical unit cell (NaCl structure) is half this value, also manganese–copper alloys. Some of the relevant

0.443 nm. This atomic arrangement is analogous to the

Figure 6.34 (a) Variation of magnetic susceptibility with temperature for an anti-ferromagnetic material, (b) neutron diffraction pattern from the anti-ferromagnetic powder MnO above and below the critical temperature for ordering (after Shull and Smart, 1949) .

The physical properties of materials 193 structure of an ordered alloy and the existence of mag-

Molecular polarization occurs in molecular materials, netic superlattice lines below the N´eel point (122 K)

some of which contain natural dipoles. Such materials has been observed, as shown in Figure 6.34b.

are described as polar and for these the influence of an Some magnetic materials have properties which

applied field will change the polarization by displacing are intermediate between those of anti-ferromagnetic

the atoms and thus changing the dipole moment (i.e. and ferromagnetic. This arises if the moments in one

atomic polarizability) or by causing the molecule as a direction are unequal in magnitude to those in the

whole to rotate to line up with the imposed field (i.e.

other, as, for example, in magnetite, Fe 3 O 4 , where the

orientation polarizability). When the field is removed

these dipoles may remain aligned, leading to perma- occupy their own particular sites. N´eel has called this

ferrous and ferric ions of the FeO.Fe 2 O 3 compound

nent polarization. Permanent dipoles exist in asymmet- state ferrimagnetism and the corresponding materials

rical molecules such as H 2 O, organic polymers with are termed ferrites. Such materials are of importance

asymmetric structure and ceramic crystals without a in the field of electrical engineering because they are

centre of symmetry.

ferromagnetic without being appreciably conducting; eddy current troubles in transformers are, therefore, not so great. Strontium ferrite is extensively used in

6.9.2 Capacitors and insulators

applications such as electric motors, because of these properties and low material costs.

In a capacitor the charge is stored in a dielectric mate- rial which is easily polarized and has a high electrical

resistivity ¾10 11 VA 1 m to prevent the charge flow-

6.9 Dielectric materials

ing between conductor plates. The ability of the mate- rial to polarize is expressed by the permittivity ε and