About Magnetic Materials
Magnetic materials encompass a wide variety of materials, which are used in a diverse range of applications. Magnetic materials are utilised in the creation and distribution of electricity, and, in most cases, in the appliances that use that electricity. They are used for the storage of data on audio and video tape as well as on computer disks. In the world of medicine, they are used in body scanners as well as a range of applications where they are attached to or implanted into the body. The home entertainment market relies on magnetic materials in applications such as PCs, CD players, televisions, games consoles and loud speakers.
It is difficult to imagine a world without magnetic materials and they are becoming more important in the development of modern society. The need for efficient generation and use of electricity is dependent on improved magnetic materials and designs. Non-polluting electric vehicles will rely on efficient motors utilising advanced magnetic materials. The telecommunications industry is always striving for faster data transmission and miniaturisation of devices, both of which require development of improved magnetic materials.
Magnetic materials are classified in terms of their magnetic properties and their uses. If a material is easily magnetised and demagnetised then it is referred to as a soft magnetic material, whereas if it is difficult to demagnetise then it is referred to as a hard (or permanent) magnetic material. Materials in between hard and soft are almost exclusively used as recording media and have no other general term to describe them. Other classifications for types of magnetic materials are subsets of soft or hard materials, such as magnetostrictive and magnetoresistive materials.
Magnetic Units & Terminology
In the study of magnetism there are two systems of units currently in use: the mks (metres-kilograms-seconds) system, which has been adopted as the S.I. units and the cgs (centimetres-grams-seconds) system, which is also known as the Gaussian system. The cgs system is used by many magnets experts due to the numerical equivalence of the magnetic induction (B) and the applied field (H).
When a field is applied to a material it responds by producing a magnetic field, the magnetisation (M). This magnetisation is a measure of the magnetic moment per unit volume of material, but can also be expressed per unit mass, the specific magnetisation (s). The field that is applied to the material is called the applied field (H) and is the total field that would be present if the field were applied to a vacuum. Another important parameter is the magnetic induction (B), which is the total flux of magnetic field lines through a unit cross sectional area of the material, considering both lines of force from the applied field and from the magnetisation of the material. B, H and M are related by equation 1a in S.I. units and by equation 1b in cgs units.
B = ”_{o} (H + M) |
Equ.1a |
B = H + 4 p M |
Equ.1b |
In equation 1a, the constant ”_{o} is the permeability of free space (4p x 10^{-}^{7}^{ }Hm^{-}^{1}), which is the ratio of B/H measured in a vacuum. In cgs units the permeability of free space is unity and so does not appear in equation 1b. The units of B, H and M for both S.I. and cgs systems are given in table 2. Note that in the cgs system 4pM is usually quoted as it has units of Gauss and is numerically equivalent to B and H.
Another equation to consider at this stage is that concerning the magnetic susceptibility (c), equation 2, this is the same for S.I. and cgs units. The magnetic susceptibility is a parameter that demonstrates the type of magnetic material and the strength of that type of magnetic effect.
Equ.2 |
Sometimes the mass susceptibility (c_{m}) is quoted and this has the units of m^{3}kg^{-}^{1} and can be calculated by dividing the susceptibility of the material by the density.
Another parameter that demonstrates the type of magnetic material and the strength of that type of magnetic effect is the permeability (m) of a material, this is defined in equation 3 (the same for S.I. and cgs units).
Equ.3 |
In the S.I. system of units, the permeability is related to the susceptibility, as shown in equation 4 and can also be broken down into m_{o} and the relative permeability (m_{r}), as shown in equation 5.
m_{r} = c + 1 |
Equ.4 |
m = m_{o} m_{r} |
Equ.5 |
Finally, an important parameter (in S.I. units) to know is the magnetic polarisation (J), also referred to as the intensity of magnetisation (I). This value is effectively the magnetisation of a sample expressed in Tesla, and can be calculated as shown in equation 6.
J = m_{o} M |
Equ.6 |
Quantity |
Gaussian |
S.I. Units |
Conversion factor |
---|---|---|---|
Magnetic Induction (B) |
G |
T |
10^{-}^{4} |
Applied Field (H) |
Oe |
Am^{-}^{1} |
10^{3} / 4p |
Magnetisation (M) |
emu cm^{-}^{3} |
Am^{-}^{1} |
10^{3} |
Magnetisation (4pM) |
G |
- |
- |
Magnetic Polarisation (J) |
- |
T |
- |
Specific Magnetisation (s) |
emu g^{-}^{1} |
JT^{-}^{1}kg^{-}^{1} |
1 |
Permeability (”) |
Dimensionless |
H m^{-}^{1} |
4 p . 10^{-}^{7} |
Relative Permeability (”_{r}) |
- |
Dimensionless |
- |
Susceptibility (c) |
emu cm^{-}^{3} Oe^{-}^{1} |
Dimensionless |
4 p |
Maximum Energy Product (BH_{max}) |
M G Oe |
k J m^{-}^{3} |
10^{2} / 4 p |
Table 1: The
relationship between some magnetic parameters in cgs and S.I. units.
(Where: G = Gauss, Oe = Oersted, T = Tesla)
Intrinsic Properties of Magnetic Materials
The intrinsic properties of a magnetic material are those properties that are characteristic of the material and are unaffected by the microstructure (e.g. grain size, crystal orientation of grains). These properties include the Curie temperature, the saturation magnetisation and the magnetocrystalline anisotropy.
Saturation Magnetisation
The saturation magnetisation (M_{S}) is a measure of the maximum amount of field that can be generated by a material. It will depend on the strength of the dipole moments on the atoms that make up the material and how densely they are packed together. The atomic dipole moment will be affected by the nature of the atom and the overall electronic structure within the compound. The packing density of the atomic moments will be determined by the crystal structure (i.e. the spacing of the moments) and the presence of any non-magnetic elements within the structure.
For ferromagnetic materials, at finite temperatures, M_{S} will also depend on how well these moments are aligned, as thermal vibration of the atoms causes misalignment of the moments and a reduction in M_{S}. For ferrimagnetic materials not all of the moments align parallel, even at zero Kelvin and hence M_{S} will depend on the relative alignment of the moments as well as the temperature.
The saturation magnetisation is also referred to as the spontaneous magnetisation, although this term is usually used to describe the magnetisation within a single magnetic domain. Table 3 gives some examples of the saturation polarisation and Curie temperature of materials commonly used in magnetic applications.
Material |
Magnetic Structure |
J_{S} at 298K |
T_{C} |
Fe | Ferro |
2.15 |
770 |
Co | Ferro |
1.76 |
1131 |
Ni | Ferro |
0.60 |
358 |
Nd_{2}Fe_{14}B | Ferro |
1.59 |
312 |
SmCo_{5} | Ferro |
1.14 |
720 |
Sm_{2}Co_{17} | Ferro |
1.25 |
820 |
BaO.6Fe_{2}O_{3} | Ferri |
0.48 |
450 |
SrO.6Fe_{2}O_{3} | Ferri |
0.48 |
450 |
Fe 3wt% Si | Ferro |
2.00 |
740 |
Fe 4wt% Si | Ferro |
1.97 |
690 |
Fe 35wt% Co | Ferro |
2.45 |
970 |
Fe 78wt% Ni | Ferro |
0.70 |
580 |
Fe 50wt% Ni | Ferro |
1.55 |
500 |
MnO.Fe_{2}O_{3} | Ferri |
0.51 |
300 |
Magnetic Anisotropy
In a crystalline magnetic material the magnetic properties will vary depending on the crystallographic direction in which the magnetic dipoles are aligned. Figure 4 demonstrates this effect for a single crystal of cobalt. The hexagonal crystal structure of Co can be magnetised easily in the [0001] direction (i.e. along the c-axis), but has hard directions of magnetisation in the <1010> type directions, which lie in the basal plane (90° from the easy direction).
A measure of the magnetocrystalline anisotropy in the easy direction of magnetisation is the anisotropy field, Ha (illustrated in figure 4), which is the field required to rotate all the moments by 90° as one unit in a saturated single crystal. The anisotropy is caused by a coupling of the electron orbitals to the lattice, and in the easy direction of magnetisation this coupling is such that these orbitals are in the lowest energy state.
The easy direction of magnetisation for a permanent magnet, based on ferrite or the rare earth alloys, must be uniaxial, however, it is also possible to have materials with multiple easy axes or where the easy direction can lie anywhere on a certain plane or on the surface of a cone. The fact that a permanent magnet has uniaxial anisotropy means that it is difficult to demagnetise as it is resistant to rotation of the direction of magnetisation.
Figure 3: The magnetocrystalline anisotropy of cobalt.