Properties of Magnetic Materials

Magnetic Materials – Steel is more suitable for permanent magnets, because it high coercivity meaning that it is not easily demagnetized by shaking. The fact that the remanence of iron is a little greater than that of steel is completely outweighed by its much smaller coercivity, which makes it very easy to demagnetize. On the other hand, iron is much more suitable for electromagnets, which have to be switched on and off, as in relays. Iron is also more suitable for the cores of transformers and the armatures of machines. Both of these go through complete magnetizing cycles continuously: transformer cores because they are magnetized by alternating current, armatures because they are turning round and round in a constant field. In each  cycle the iron passes through two parts of its hysteresis loop where the magnetizing field is having to demagnetize the iron. There the field is doing work against the internal friction of the domains. This work, like all work that is done against friction, is dissipated as heat. The energy dissipated in this way, per cycle, is less for iron than for steel, because iron is easier to demagnetize. It is called the hysteresis loss.

In a large transformer the hysteresis loss, together with the heat developed by the current in the resistance of the windings, liberates so much heat that the transformer must be artificially cooled. The cooling is done by circulating oil, which itself is cooled by the atmosphere: it passed through pipes which can be seen outside the transformer, running from top to bottom.

The higher the resistivity of a magnetic material, the less the eddy-currents in it when the flux through it is changed; and therefore the less the energy lost as heat.

Magnetic materials are a key component in a wide range of electrical devices. Electric generators, electric motors and transformers are just a few examples of devices that use magnetic materials.

Ferromagnetic materials can be divided into magnetically “soft” materials such as annealed iron and magnetically “hard” materials like alnico and neodymium-iron-cobalt magnets. These distinctions are based on the way that these materials respond to changing magnetic fields.

Permeability

The permeability of a material is its ability to permit magnetic field lines to pass through it. The higher the permeability, the easier it is to magnetize a material. This characteristic is one of the main differences between ferromagnetic and non-ferromagnetic materials, with the latter having lower permeability values.

Ferromagnetic metals like iron have very high relative permeabilities, often reaching over 100,000. This is due to the fact that the individual atoms in a ferromagnetic material contain electron spins, which can be aligned with an external magnetic field. This creates a magnetic flux density in the interior of the material, which can be many orders of magnitude stronger than the force needed to orient those electron spins. The relative permeability of the material is then defined as the ratio of this magnetic flux density to the magnetizing field strength H. This can be expressed as mr = B/H in the SI system of units (not the CGS units used by many technical publications).

Unlike the permeability of vacuum, which is a fixed value, mr varies with both the applied magnetizing field strength and temperature. This makes the permeability of a given material an important factor in determining the suitability of it for a particular application.

If a material has a low permeability, it will be very hard to magnetize and will lose its magnetic force easily. In contrast, a material with high permeability can be easily magnetized and will retain its strength even after the removal of the applied field.

If you see technical books that list the permeability of different materials, be careful to read the information carefully. In some cases, they may be referring to the normal or average susceptibility of the material, rather than its relative magnetic permeability. This can be misleading, as the two quantities are very different. Normal or average susceptibility is often listed as khv, while magnetic permeability is usually referred to as khm. To avoid confusion, you should always check that the units are correct before making a decision about which magnetic material to use in your project.

Coercivity

The coercivity of a magnetic material is the intensity of the external magnetic field required to reduce its magnetization to zero. This value is measured in oersteds and is often used to characterize the ferromagnetic properties of materials. A ferromagnet with a high coercivity is difficult to demagnetize and can be found in electric motors and magnetic recording media. A ferromagnet with

a low coercivity is easy to demagnetize and can be found, for example, in microwave devices and transformers. The coercivity of a magnet depends on the microstructure of the magnet and its crystalline structure. It is also influenced by previous heat treatments and microstructure modifications. In addition, the microstructure of a magnet can affect its extrinsic magnetic properties.

Coercivity is usually determined by measuring a hysteresis loop, which is a graph of the magnetization (M) as a function of the applied magnetic field. The data are acquired by passing the sample through a magnetization cycle, in which the sample is alternately magnetized and demagnetized. The value of the applied magnetic field where the data crosses zero is the coercivity.

A higher value of coercivity indicates that the ferromagnet has a more stable magnetization, and is therefore less susceptible to demagnetization under adverse conditions such as elevated temperatures or vibration. This characteristic is especially important in applications where the magnet will be exposed to radiation.

In general, the coercivity of a ferromagnet scales with its intrinsic magnetic anisotropy field, HA. However, the typical coercivity of commercial polycrystalline Nd-Fe-B sintered magnets is only about 0.2 HA. This is due to the thermal demagnetization effect.

The coercivity of a magnetic substance can be improved by decreasing its grain size, which decreases the intrinsic anisotropy field. In addition, the microstructure of ferromagnetic grains can have a significant impact on its coercivity. In particular, the shape of the grains, their equiaxed nature, and their uniform separation by non-ferromagnetic intergranular phases are all important factors.

A crystalline magnet has a higher coercivity than an amorphous one, because the atomic arrangement of its particles is more regular. The coercivity of a crystalline magnet is also influenced by the strength of exchange interactions between its particles.

Remanence

The remanence of magnetic materials is the magnetism that remains in a ferromagnetic material after the removal of an external magnetic field. This value is usually expressed in teslas or gauss, depending on the international system. The remanence is measured at the intersection of the hysteresis loop and the ordinate axis.

A material with high remanence is considered to be hard. In contrast, a paramagnetic material has low remanence. This is because paramagnetic materials have a tendency to align their magnetic moments in the direction of the external magnetic field. The remanence of a ferromagnetic material can be explained by the fact that atoms of the ferromagnetic material can align themselves along one of the direction vectors of the magnetic field, resulting in a large net magnetic moment. This phenomenon is also called spontaneous magnetisation.

Generally speaking, it is easier to demagnetize hard magnetic materials than soft ones. However, the exact reason for this is not known. It is probably due to the fact that the atoms of hard magnetic materials are more tightly packed than those of soft ones. In addition, the coercive force of hard magnetic materials is typically higher than that of soft magnetic materials.

Soft ferromagnetic materials can be magnetised easily, but they lose the magnetic force quickly. This property makes them useful in applications that require quick magnetisation and demagnetisation, such as magnetic storage. The low coercive force of these materials also leads to lower losses during operation, which can make them an attractive option for transformer cores and motor windings.

Remanence is an important factor to consider when selecting a magnetic material for your application. For example, a neodymium magnet’s remanence is significantly higher than that of a ferrite magnet. Therefore, neodymium magnets are often preferred for applications that require frequent reversal of the polarity. The remanence of a magnet can be limited by inserting an air gap in the core to prevent it from becoming demagnetised.

The remanence of a neodymium magnet is around 1-1.3 Tesla (10-13kGauss). Neodymium is a rare earth metal, which means its price is significantly more expensive than that of a ferrite magnet. Nevertheless, the higher remanence of neodymium is well worth it, as it provides superior magnetic properties and performance.

Magnetization

Magnetization is the property of a magnetic material that causes it to attract and repel magnetic fields. It is also the property that enables magnetic materials to store and release magnetic energy. Magnetization is determined by the magnetic moments of the individual atoms within the material. These moments can be aligned in a magnetic field to produce magnetism. It is important to understand that the magnetic moments are vectors with both direction and magnitude. The individual atomic moments cancel each other out in most materials, leading to diamagnetism. In contrast, some materials have a large number of unpaired electrons that can be aligned in one direction to produce a net magnetic field. This allows them to react much more strongly to a changing magnetic field than other materials. These are called ferromagnetic materials.

In a magnetic material, the magnetic moment is grouped into small regions of the material known as magnetic domains. The size of these domains is determined by the particle diameter. Larger domains tend to have more exchange and anisotropy energy, while smaller ones can have less.

The permeability of a magnetic material is influenced by the number of these magnetic domains and their arrangement. The permeability of a magnetic material increases with the number of domains and decreases as the particle diameter decreases. The permeability is also inversely proportional to temperature. This is known as the Curie law.

Magnetic materials are classified as either permanent or soft ferromagnetic. Permanent magnets are made from alloys of iron, aluminium, nickel and cobalt. These magnets can retain their magnetic properties indefinitely, unless they are demagnetised by an opposing magnetic field or raised above their Curie temperature. Soft ferromagnetic materials are able to be magnetised but will lose their magnetic force quickly.

A material’s magnetization can be measured by observing its hysteresis curve. This is a graph that shows the variation in B (magnetization) with h for different values of H (field strength). It should be noted that B will be higher during ‘charge up’ than during’relaxation’, due to energy losses in the internal friction of the material.