# Encyclopedia of electromagnetics

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# Magnetic materials

 Stan Zurek, Magnetic materials, Encyclopedia Magnetica, E-Magnetica.pl

Magnetic materials - a term commonly used for materials which exhibit strong magnetic properties, such as ferromagnetic or ferrimagnetic, further broadly classified as magnetically soft, hard, or semi-hard.1)

Magnetic hard drives rely on many types of magnetic and non-magnetic materials

However, in general all materials are “magnetic” but they respond in various ways to magnetic field, depending on their atomic structure and ambient conditions, or at least do not significantly obstruct magnetic field. In that sense, even vacuum is “magnetic” because magnetic field can propagate through it and by definition its magnetic permeability $μ_0$ is a universal physical constant in the SI system of units.2)

On the other hand, antiferromagnetic materials have internally ordered magnetic structure, but such that does not produce large values of susceptibility (i.e. appears to be “non-magnetic”). Yet, these materials find use in special magnetic applications such as magnetoresistive sensors in magnetic hard drives.3)

Magnetic field can be completely expelled from a superconducting body (Meissner effect) but this happens due to the induced surface electric currents, rather than magnetic response as such. However, from the outside, type I superconductors behave as if they were perfect diamagnets, with permeability of zero (comparing to vacuum which has permeability of unity). 4)

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Efficient energy transformation relies on soft magnetic materials Marvin L. Daniels, Public domain
Permanent magnets (hard magnetic materials) are capable of retaining a large amount of magnetic energy
Magnetic field penetrating “nonmagnetic” vacuum can bend the path of moving electrons by M. Białek, Wikimedia Commons, CC-BY-SA-3.0

Magnetic properties of all pure chemical elements of practical importance were measured, with at least an order-of-magnitude accuracy (see the large illustration with the periodic table below). The elements can be broadly classified into diamagnetic and paramagnetic (weak magnetic properties) and ferromagnetic (strong magnetic properties). At room temperature only three elements are ferromagnetic: iron, cobalt, and nickel.

However, pure elements are rarely used because of their magnetic properties (but they can be used for other reasons, like for instance copper for making electric wires or noble gases for providing protective chemical atmosphere).

From engineering viewpoint, metal alloys and chemical compounds, even made from non-ferromagnetic elements, can exhibit very strong magnetic effects, which need to be tailored, by many means: chemical composition, mechanical forming, thermal processing (annealing), with or without magnetic field.

Periodic table of elements, with magnetic properties5) (at very low temperatures, and also high pressure, many elements become superconducting and hence strongly diamagnetic)

There are several ways in which materials can respond, and these different types of response are described by various types of magnetism:

## Engineering magnetic materials

A wide range of materials (both “magnetic” and “non-magnetic”) are widely used in engineering applications.

### Soft

Soft magnetic materials (or rather “magnetically soft materials”) are used in energy generation and transformation, mechanical force generation, and signal processing such as sensing and transmission.

Type of material Main constituents Comments
Pure iron Fe Saturation up to 2.15 T6), high cost, used in DC applications such as electromagnet or flux guides for permanent magnet circuits
Mild steel Fe Saturation up to 2.15 T, lowest cost, produced in very high volume, used in DC applications such as electromagnet, non-critical relay, or flux guides for permanent magnet circuits
Non-oriented electrical steel Fe + Si Saturation up to 2.15 T, higher cost, produced in thin sheets (<1 mm) in very high volume, used predominantly in motors and smaller generators and transfomers, mostly operating at mains frequency
Grain-oriented electrical steel Fe + Si Saturation up to 2.1 T, higher cost, produced in very high volume, used in power transformers, large generators and motors operating at mains frequency
Thin-gauge electrical steel Fe + Si Produced in thinner sheets (<0.25 mm) to suppress eddy currents, used in motors and generators operating at higher operating frequencies, e.g. in motors of electric vehicles
Fe-based amorphous tape Fe + B Saturation up to 1.75 T, higher cost, produced in high volume, used in power transformers and sensing applications. Higher permeability than electrical steels but very brittle (very thin ribbon (<0.05 mm) with amorphous structure
Soft ferrite MnZn or NiZn Saturation up to 0.5 T, medium cost, used typically for high-frequency applications, mechanically brittle and difficult to machine, produced through sintering
Iron and iron-alloy powder Fe, Fe+Al Saturation up to 1.5 T, medium cost, used typically for chokes in power supplies up to 100 kHz
Co-based amorphous tape Co Very high permeability, very expensive, used in pulse applications
Garnet, spinel and hexagonal ferrites Various Saturation up to 0.5 T, used in GHz applications

For soft magnetic mateirals, the image below shows the summary of four types of characteristics: saturation, permeaiblity, frequency range, and cost.

Overview of properties of soft magnetic materials: saturation, permeability, frequency range, material cost7)

### Hard

Overview of properties of hard magnetic materials: remanence, coercivity, temperature range, material cost8)

### Semi-hard

Materials are classified as “semi-hard” mostly on the basis of their coercivity, with a range intermediate, between the soft (the lower the better) and hard (the higher the better).

However, from a wider perspective, it is the specific mode of use of a given material that defines the classification.9) There are three main types of applications for semi-hard magnetic materials:

#### Low-density information storage

Electromagnetic detection coils in an anti-theft system, installed by the entrance/exit of a shop

Retail of consumer products involves the risk of theft, especially with respect to smaller and expensive products such as consumer electronics or clothing. Electromagnetic phenomena are used in several technical implementations of anti-theft systems, which have to be of very low cost and reliable over significant distance to a detector (up to 2 m).10)

One such application involves semi-hard materials. Two strips of metal are packaged together in a low-cost tag, one is magnetically soft, the other is semi-hard, so that it can be magnetised without the use of excessive energy, and it will retain the magnetised state - therefore the information about the change of the state remains recorded.

The size of the strips of metal is such that they respond with a resonance, to a high-frequency electromagnetic field (e.g. 58 kHz). After the purchase, the checkout operator can run the tag over a strong magnetic field (magnet or electromagnet) and magnetise (or demagnetise) the semi-permanent strip. This alters the resonance point of the tag, such that the detection coils do not detect the resonance any more.11)

The tags must be cheap, because they are single-use only (and remain attached to the sold items), and their cost must be born by the retailer.

“Low-density” applies here, because effectively just one bit of information is stored - when tag was processed as “sold”.

#### High-density information storage

High-density information storage is used for digital applications in magnetic hard drives and magnetic tapes. In the past also magnetic cassettes (audio), floppy drives, and magneto-optic drives were used, but these were superseded by other solutions. Semi-hard materials are and were used in all such magnetic storage devices.

A layer of a semi-hard material is deposited on a suitable substrate (rigid for disks, flexible for tapes). The magnetic layer can be magnetised locally by a suitable recording head, and it is designed to retain the state of the local magnetisation, so that the reading head can read out the magnetic state and translate it into digital information (0 or 1).

The amount of information stored is directly proportional to the density of recording - the higher the density the more information can be stored with the same size of device, hence “high-density” is an appropriate name.

Currently, hard drive technology allows storing around 1 TB of data per 1 inch2 (or 6.5 cm2).

#### Mechanical force

Semi-hard materials can be also used to transfer or generate mechanical forces.

One of the most basic applications is the needle of an ordinary compass.12) The needle cannot be made from a “soft” magnetic material because it has to remain magnetised, even if medium magnetic fields appear for some time in the vicinity. On the other hand, the needle cannot be made from a very strong magnet, such that it is not attracted to weakly magnetic materials.

### Non-magnetic

In common language, “non-magnetic” are those materials which do not exhibit strong magnetic response - they have low relative permeability μr, very close to unity, or susceptibility close to zero (similar to vacuum).13)

Magnetic susceptibility14)15)16)
Diamagnetic
Nitrogen (N2) -0.005 × 10-6
Hydrogen (H2) -0.002 × 10-6
Water -9.1 × 10-6
Copper -9.7 × 10-6
Graphite -14 × 10-6
Silver -25 × 10-6
Gold -35 × 10-6
Bismuth -166 × 10-6
Pyrolytic graphite χ⟂17) −595 × 10-6
Paramagnetic
Air 0.36 × 10-6
Oxygen (O2) 1.9 × 10-6
Aluminium 22 × 10-6
Tungsten 88 × 10-6
Titanium 181 × 10-6
Permeabilities of magnetic materials (e.g. ferromagnets) are much higher than non-magnetic materials (e.g. vacuum, diamagnets and paramagnets)

#### Diamagnetic

All substances exhibit the diamagnetic effect due to orbital motion of electrons. They are always repelled from the applied magnetic field, but the effect is very small18), with the forces negligible in engineering applications. However, it can be demonstrated that a piece of diamagnetic pyrolytic graphite can levitate above a magnet.19)

##### Superconducting

Superconductors are said to be perfect diamagnets, but the mechanism is different.

#### Paramagnetic

The diamagnetic contribution is masked in some materials by the paramagnetic effect, which concentrates the applied field and produces force which always attracts.20)

The non-magnetic materials are