
Table of Contents
5.2. Metallic 3d Magnets: A Deep Dive into Magnetic Metals
Metallic 3d magnets include hard steels, uniaxial 3d - based alloys and alnico magnets. Rare - earth alloys are discussed separately in section 5.3. The three groups of materials are distinguished by their coercivity mechanism. Steels, now mainly of historical interest to the magnetism community were the materials for the archetypical bar and horseshoe magnets, where low coercivity due to small deviations from the ideal bcc crystal structure made it necessary to resort to cumbersome shapes to reduce the internal field \(H'\). Alnico magnets, which are still produced in significant quantities, exploit shape anisotropy in a two - phase nanostructure. Only the uniaxial alloys such as MnAl, CoPt or MnBi are true permanent magnets in the sense that the magnetocrystalline anisotropy field greatly exceeds the spontaneous magnetization, \(H_{\text{a}}\gg M_{\text{s}}\) (\(\kappa>1\)).
Iron, Cobalt, and 3d Alloys: Foundations of Magnetic Materials
Iron, cobalt and some of their alloys possess a large spontaneous magnetization. The greatest value of \(\mu_{0}M_{\text{s}}\) found at room temperature is 2.45 T, in bcc \(\text{Fe}_{65}\text{Co}_{35}\), sometimes known as permendur. Iron and cobalt form an extended solid solution \(\text{Fe}_{100 - x}\text{Co}_{x}\) when \(0\leq x\leq75\), but below 730 °C there is an order - disorder transition where a CsCl - structure (B2) intermetallic forms over an extended homogeneity range. The ordered alloy has a lattice parameter \(a_{0}\ 285.1\) pm, compared with 284.9 pm for the disordered alloy and 286.1 pm for bcc \(\alpha\) - Fe. The onset of strong ferromagnetism in Fe - Co alloys occurs at \(x = 35\). The atomic moment in \(\text{Fe}_{50}\text{Co}_{50}\) is \(2.4\ \mu_{\text{B}}/\text{atom}\), which is only slightly less than the value of \(2.5\ \mu_{\text{B}}/\text{atom}\) for \(\text{Fe}_{65}\text{Co}_{35}\).
The magnetocrystalline anisotropy in cubic iron - based alloys is fairly low; \(K_{1}\) is \(48\ \text{kJ}\ m^{-3}\ [4.8\times10^{5}\ \text{erg}\ cm^{-3}]\) in \(\alpha\) - Fe and it changes sign in bcc \(\text{Fe}_{100 - x}\text{Co}_{x}\) at \(x = 45\), where the alloys have a very high permeability. Likewise, in fcc \(\text{Fe}_{100 - x}\text{Ni}_{x}\) alloys, \(K_{1}\) changes sign at \(x = 70\). The magnetostriction is also very low near \(x = 80\), and alloys in this composition range, known as permalloy, have excellent permeability but rather small magnetization, \(\mu_{0}M_{\text{s}}\approx1.0\ T\). Another soft - magnetic composition with excellent permeability due to a combination of zero anisotropy and zero magnetostriction is sendust, \(\text{Fe}_{74.5}\text{Si}_{16}\text{Al}_{9.5}\). High permeability is combined with high spontaneous magnetization in bcc silicon steels, most notably \(\text{Fe}_{93.8}\text{Si}_{6.2}\) (3.2 wt% Si - Fe) which is manufactured in huge quantities as electrical sheet for use in transformers and electrical machinery.
Hexagonal cobalt has uniaxial anisotropy, but the \(c/a\) ratio of 1.632 is close to the ideal value of \(\sqrt{8/3}\) which corresponds to cubic symmetry of the cuboctahedron formed by atoms of the first coordination sphere. The value of \(K_{1}\), \(530\ \text{kJ}\ m^{-3}\), is an order of magnitude greater than for iron, but typical coercivities in bulk cobalt and iron are only \(1\ \text{kA}\ m^{-1}\) and \(0.1\ \text{kA}\ m^{-1}\), respectively, which are about a factor of 500 less than their anisotropy fields. To develop more coercivity it is necessary to find additives or heat treatments which impede the formation and propagation of reverse domains.
The iron - carbon phase diagram exhibits a celebrated eutectoid point at \(\text{Fe}_{97}\text{C}_{3}\) near 740 °C. Above this point, carbon dissolves interstitially in the non - magnetic \(\gamma\) - Fe (austenite). Below it, segregation occurs into bcc \(\alpha\) - Fe (ferrite) which can accommodate much less carbon and \(\text{Fe}_{3}\text{C}\) (cementite). Rapid quenching from above 730 °C does not allow the carbon sufficient time to diffuse to form cementite regions and a supersaturated tetragonally distorted metastable \(\alpha'\) phase, known as martensite, forms. The martensitic transition from \(\gamma\) to \(\alpha'\) occurs almost instantaneously as the austenite is undercooled. The fine - grained martensite crystallites are effective at impeding domain - wall motion, and have uniaxial anisotropy. They also make the steel mechanically hard. By contrast, pure \(\alpha\) - Fe is ductile and mechanically soft; hence the origin of the distinction between hard and soft magnets. With optimized composition
Iron, Cobalt and 3d Alloys
and heat treatment it is possible to improve the coercivity. Energy products of \(1.6\ \text{kJ}\ m^{-3}\) were achieved for suitably treated \(\text{Fe}_{96}\text{C}_{4}\). Tungsten additives improved this value, but tungsten steels were replaced by cheaper chromium steels with similar magnetic properties. Cobalt steels were developed by Honda from 1916, and with compositions such as \(\text{Fe}_{55}\text{Co}_{34}\text{Cr}_{6}\text{W}_{4}\) took the energy product up to \(8\ \text{kJ}\ m^{-3}\) [the 1 MGOe milestone]. Other Fe - Co - based alloys with additives such as Cr, Ni, Mo and V have been found which combine energy products up to \(30\ \text{kJ}\ m^{-3}\) with useful mechanical properties. For example, \(\text{Fe}_{36}\text{Co}_{52}\text{V}_{12}\) (Vicalloy) can be drawn into wires and strips for use in security tags. Vicalloy wire acts as bistable magnetic devices with a square hysteresis loop; a voltage may be generated in a pick - up coil as the domain wall propagates down the wire (the Wiegand effect). With special heat treatment under tensile stress, a coercivity of \(70\ \text{kA}\ m^{-1}\) and an energy product of \(60\ \text{kJ}\ m^{-3}\) are achieved. Some or all of the vanadium in Vicalloy may be replaced by chromium.
Powders
A different approach to achieving coercivity in iron, developed in France in the 1940s, was used until recently to prepare coercive Fe and Fe - Co powder composed of elongated crystallites whose transverse dimensions are in the 10 - 20 nm range. The crystallized single - domain magnets (Lodex) were made from Fe - Co crystallites electrodeposited in mercury. They are dispersed in a metal matrix, aligned and pressed or extruded. Finally, the bulk material was powdered, aligned and pressed into the required shape. Although no longer used for permanent magnets, acicular metal particles with length of about 100 nm and an aspect ratio of about 8 are used for magnetic tapes. The magnetization is reduced to about half the bulk value by an oxide surface layer and the coercivity is typically \(100 - 200\ \text{kA}\ m^{-1}\ [1.3 - 2.5\ \text{kOe}]\). The coercivity in acicular particles was discussed in section 3.4, and it is considered further in the next section on alnicos.
Thin Films
Sputtered thin films of hcp cobalt - based alloys such as \(\text{Co}_{80}\text{Cr}_{20}\) with additives of Ta or Pt are widely used in magnetic recording media on hard disks. The \(c\) - axis of the crystallites in the magnetic layer may be arranged to lie in - plane or perpendicular to the plane by using an appropriate seed layer such as Cr or Ti and controlling the substrate temperature during deposition. Coercivity depends not only on the texture of the film, but also on grain size and the degree of exchange decoupling of adjacent grains. Thin (20 nm) films with an in - plane texture have sufficient coercivity, about \(200\ \text{kA}\ m^{-1}\), and a fine grain size, about 20 nm, suitable for very high - density recording (of order 16 bit \(\mu\text{m}^{-2}\) [10 Gbit/inch2]).
Thicker films of hexagonal alloys such as \(\text{Co}_{80}\text{Pt}_{20}\) can be used to provide a bias field in thin - film devices such as magnetoresistive read heads. There again a texture is required with the \(c\) - axis in the plane of the film, which may

Figure 5.12. Ordered uniaxial layer structures: the face - centred tetragonal CuAu(I) structure and the hexagonal NiAs structure.
be achieved by using a chromium seed layer. Strong uniaxial anisotropies, associated with uniaxial crystal - field interactions at surfaces and interfaces, are found in various ultrathin film structures. These films are also being considered for future high - density recording applications.
Substantial magnetocrystalline anisotropy can be obtained in ordered 3d alloys with uniaxial crystal structures. Two examples of these structures are illustrated in figure 5.12.
Co - Pt
Equiatomic compounds such as CoPt or FePt can be produced in the ordered CuAu(I) structure, also known as the \(L1_{0}\) structure, by annealing the disordered fcc alloy at about 650 °C to produce tetragonally ordered domains with \(c/a = 0.98\) having their \(c\) - axes along different cube edges. There is strong \(c\) - axis anisotropy in this ordered structure, table 5.8, but there is no unique \(c\) - axis in a macroscopic sample so the magnets are isotropic. CoPt finds applications in a few niche areas such as surgery or precision instruments, where its exceptional corrosion resistance, ductility and good high - temperature performance can justify the high cost.
Mn - Al
A cheaper magnet with the same ordered CuAu structure is the metastable \(\tau\) phase of MnAl. It is produced from the disordered hcp \(\varepsilon\) phase which is stable above 880 °C. The \(\varepsilon\) phase of \(\text{Mn}_{x}\text{Al}_{100 - x}\) transforms to \(\tau\) on cooling a composition with \(51
Table 5.8. Intrinsic magnetic properties of ordered 3d alloys and typical permanent - magnet properties.
Alloy | Structure type | \(T_{\text{C}}\) (K) | \(\mu_{0}M_{\text{s}}\) (T) | \(K_{1}\) (MJ m-3) | \(\kappa\) | \(\mu_{0}M_{\text{r}}\) (T) | \(H_{\text{c}}\) (kA m-1) | \((BH)_{\text{max}}\) (kJ m-3) |
---|---|---|---|---|---|---|---|---|
FePd | CuAu(I)—tet | 760 | 1.38 | 1.8 | 1.1 | 0.95 | 160 | 64 |
FePt | CuAu(I)—tet | 750 | 1.43 | 6.6 | 2.0 | 1.04 | 390 | 160 |
CoPt | CuAu(I)—tet | 840 | 1.00 | 4.9 | 2.5 | 0.65 | 360 | 75 |
MnAl(C) | CuAu(I)—tet | 650 | 0.75 | 1.7 | 1.9 | 0.60 | 200 | 55 |
MnBi | NiAs—hex | 633 | 0.78 | 1.2 | 1.6 | 0.48 | 290 | 40 |
a typical value is \(\mu_{0}M_{\text{s}} = 0.75\ T\). The easy direction is along the tetragonal \(c\) - axis, but the Curie temperature is low. Anisotropic material produced by hot extrusion of the cast alloy is intermediate in cost and properties between ferrite and rare - earth magnets.
The manganese also forms NiAs - structure compounds with group V elements. The NiAs structure, illustrated in figure 5.12, is an ordered layer version of the hcp structure with stacking sequence ABAC.... Typical properties of MnBi are included in table 5.8. MnSb has easy - plane anisotropy and a spin reorientation at 520 K.
Alnicos
Alnicos are a family of heat - treated Fe - Co - Ni - Al alloys with possible additions of Cu, Ti... which were developed over a 30 - year period from 1932 (McCurrie 1982). There are two main groups: the isotropic alloys containing 0 - 20 wt% Co (Alnicos 1 - 4) and anisotropic alloys with 22 - 40 wt% Co (Alnicos 5 - 9). In 1932, Mishima reported \(H_{\text{c}} = 34\ \text{kA}\ m^{-1}\) in inhomogeneous alloys with approximate composition \(\text{Fe}_{60}(\text{AlNi})_{40}\), now known as Alnico 3. Shortly afterwards it was realized by research groups in the USA, Germany, UK and Japan that their magnetic properties are enhanced if some of the iron is replaced by cobalt. These discoveries marked the beginning of the development of alnico - type magnets, which are also sold under the names Alcomax, Hycomax, Koezlit and Ticonal. The intricate metallurgical microstructure of the alnico magnets is best understood from the ternary Fe - Ni - Al and Fe - Co - Al phase diagrams, which were investigated by Köster in 1933. The main feature of the relevant Fe - Co - AlNi pseudobinary is the tendency towards spinodal decomposition below about 900 °C. Figure 5.13 gives a schematic representation of this pseudobinary system. The two - phase region, area II in the figure, indicates the coexistence of the FeCo and AlNi regions whose sizes can be controlled by appropriate heat treatment. The iron - cobalt phase is the bcc solid solution \(\alpha_{1}\) whose magnetic properties are optimized around the composition

Figure 5.13. A schematic FeCo - NiAl phase diagram, showing (I) the solid solution region and (II) the two - phase region, \(\alpha_{1}+\alpha_{2}\). The full curve (solubility line) indicates phase equilibrium, whereas the dashed curve (spinodal line) refers to the possibility of metastable solid solution. Metastability occurs between the solubility and spinodal lines.
Table 5.9. Properties of some alnico - type magnets.
Year | Composition | \(\mu_{0}M_{\text{r}}\) (T) | \(H_{\text{c}}\) (kA m-1) | \((BH)_{\text{max}}\) (kJ m-3) |
---|---|---|---|---|
1932 | Alnico 3 \(\text{Fe}_{60}\text{Ni}_{27}\text{Al}_{13}\) | 0.56 | 46 | 10 |
1934 | Alnico 2 \(\text{Fe}_{55}\text{Co}_{18}\text{Ni}_{18}\text{Al}_{8}\text{Cu}_{1}\) | 0.72 | 45 | 14 |
1938 | Alnico 5 \(\text{Fe}_{50}\text{Co}_{21}\text{Ni}_{15}\text{Al}_{8}\text{Cu}_{6}\text{Nb}\) | 1.35 | 46 | 45 |
1955 | Alnico 8 \(\text{Fe}_{31}\text{Co}_{38}\text{Ni}_{11}\text{Al}_{9}\text{Cu}_{5}\text{Ti}_{7}\) | 0.88 | 120 | 42 |
\(\text{Fe}_{65}\text{Co}_{35}\), where \(\mu_{0}M_{\text{s}} = 2.45\ T\). NiAl is non - magnetic at all temperatures and crystallizes into the cubic CsCl structure \(\alpha_{2}\). Apart from these main phases, a number of minor phases are found in alnico - type magnets: their content and structure depends on details of composition and heat treatment. Typical atomic compositions are \(\text{Fe}_{53}\text{Co}_{10}\text{Al}_{19}\text{Ni}_{15}\text{Cu}_{3}\) (Alnico 2) and \(\text{Fe}_{47}\text{Co}_{21}\text{Al}_{17}\text{Ni}_{13}\text{Cu}_{2}\) (Alnico 5).
By applying a magnetic field of the order of 0.4 T during spinodal decomposition while cooling from 1200 °C, it is possible to grow elongated FeCo particles in an NiAl matrix which are aligned parallel to the field (figure 5.14). The FeCo regions make up around 60% of the volume. The magnetic properties are further improved by annealing the field - cooled samples at about 600 °C to improve the segregation of the ferromagnetic \(\alpha_{1}\) and non - magnetic \(\alpha_{2}\) phases. The outcome of this procedure is an anisotropic material, Alnico 5, with a maximum energy product of nearly \(50\ \text{kJ}\ m^{-3}\). Some further improvement is possible if the material is grain - oriented prior to the thermomagnetic and

Figure 5.14. The microstructure of anisotropic Alnico 8, showing Fe - Co needles embedded in a non - magnetic Ni - Al matrix (de Vos 1969).
annealing steps. The remanence of up to about 1.4 T can compete with that of rare - earth intermetallics, but the low coercivity, which barely exceeds 40 kA m-1, inhibits the development of a high energy product. Higher coercivities can be obtained by adding Ti, but this reduces the remanence. Typical properties of alnico magnets are given in table 5.9. Curie temperatures are in excess of 900 °C and the magnets can operate up to about 500 °C.
The main source of anisotropy is the shape anisotropy associated with the aspericity of the FeCo regions, hence the hardness parameter \(\kappa<0.5\). The acicular \(\alpha_{1}\) regions are about 15 nm in diameter and 120 nm in length, which hardens the intrinsically soft iron - cobalt regions (section 3.2.4.3). Furthermore, the non - magnetic matrix separates the Fe - Co regions magnetically, so that a single reversed Fe - Co region cannot initiate reversal throughout the magnet. The early Alnicos 1 - 4 were isotropic arrays of prolate iron - rich particles in an Ni - Al matrix. The last member of the alnico family is Alnico 9, where small additions of Ti, Nb and S are used to achieve energy products as high as 107 kJ m-3 in laboratory - scale magnets.
A related class of magnetic alloys produced by spinodal decomposition is based on Fe - Cr - Co. The iron - rich \(\alpha_{1}\) phase is dispersed in a Cr - rich \(\alpha_{2}\) matrix. Their properties are comparable to those of alnico, but the alloys are ductile and contain a lesser amount of expensive cobalt. Alignment can be achieved by hot deformation as well as by annealing in the presence of a magnetic field.
All the alnico magnets can be prepared by sintering the powdered alloys at about 1200 °C, followed by heat treatment appropriate for the composition. Although the magnetic properties of sintered alnicos are inferior to those of cast alnicos, powder metallurgy is very useful for making small magnets (50 mg - 100 g) with special shapes. Alnico powders can also be bonded in a polymer matrix.
From an industrial point of view, alnicos combine intermediate processing and raw material costs with useful magnet performance. In the middle of the 20th century, alnicos were widely used as general - purpose permanent magnets, but in recent decades inexpensive hexagonal ferrites and high - performance rare - earth magnets have ended their pre - eminence. Uncertainty in the supply and price of cobalt contributed to their relative decline. Nevertheless, there continues to be a steady demand for them in applications where thermal stability is important. Millions of alnico magnets are produced every year for watt - hour meters and the instrument panels of automobiles.