About Magnetism

Permanent magnetic material made of iron, aluminium, nickel and cobalt. May also contain other admixtures. Produced by sintering or casting.

A physical property which, in case of permanent magnets, is responsible for differences in the amount of magnetic energy required to magnetise a magnet in different directions. Anisotropic permanent magnets are made so that they only have one axis along which they can be easily magnetised. Remanence of permanent magnets is greatly increased by anisotropy. Magnet production utilises two main types of anisotropy: magnetocrystalline anisotropy and shape anisotropy.

Loss of magnetic moment over time. Usually only a fraction of a percent over several (sometimes many) years.

A transitional temperature above which ferro- or ferrimagnetic materials lose their spontaneous magnetisation and become paramagnetic.

Term explanation

Also called coercive force. The ability of a permanent magnet to resist demagnetisation by an external magnetic field as well as its own demagnetising field.

There are two types of coercivity:

1. "True" coercivity, simply referred to as "coercivity" – magnetic field in which the total induction in the magnet is zero.
2. "Intrinsic" coercivity – field in which the total polarisation is zero (polarisation vectors of individual magnetic domains cancel each other out).

A circuit where magnetic flux passes through a closed path, e.g. through magnetic steel.

A number lower than or equal to one which determines reduction of magnetic induction value. The result is magnetic induction which the magnet actually "perceives". It is solely a function of the shape of the magnet and determines the slope of the working line in the second quadrant of the magnetisation curve. When the demagnetising factor equals zero, there is no demagnetisation and the induction value is equal to the magnetic remanence.

The second quadrant of the hysteresis loop. From the technical point of view, this section of the loop contains the most important characteristics of permanent magnets.

Ferromagnetic particles or objects pass through a coil with an induced alternating magnetic field, usually with a frequency of 50 Hz. The geometry of the coil or set of coils is designed in such a way that the amplitude size of the field gradually decreases. When the material leaves the coil, at a point with a very weak magnetic field, it has already passed through several smaller and smaller BH cycles. In the end, the material is completely demagnetised.

A direction along which an anisotropic magnet is magnetised. Every anisotropic magnet usually has one easy axis, while isotropic magnets have none (or all axes are "easy").

Sintered permanent magnet, made of iron oxide and (most commonly) barium carbonate or strontium carbonate. Ferrite is the cheapest common permanent magnet and a typical representative of ferrimagnetic materials.

A material exhibiting spontaneous magnetisation. Contains one or more kinds of magnetic atoms in its crystal lattice with the same orientation of magnetic moment (as compared to ferrimagnetic materials, where the magnetic moments are in opposition). The result is a large net magnetic moment. Alnico and Re/Co-based materials are ferromagnetic.

A device for measuring magnetic flux. Its basic principle relies on either a moving coil (sensor) with known cross-section or a static coil which is used to electronically integrate changes in the magnetic field. It is an accurate device, often used in instruments for measuring hysteresis loops.

A material exhibiting spontaneous magnetisation. Contains at least two kinds of magnetic atoms in its crystal lattice with unequal magnetic moments in opposing directions, meaning one kind of atoms outweighs the other (as opposed to anti-ferromagnetic materials, where the magnetic moments completely cancel out). The result is a net magnetic moment, similar to ferromagnetic materials.

A CGS unit of magnetic induction and magnetisation/polarisation. 1 G (gauss) = 10^-4 T (tesla).

A semiconductor device for measuring magnetic fields. When certain semiconductors (most commonly InAs) are subjected to a magnetic field and an electric current flows in a direction perpendicular to the magnetic field, voltage in a direction perpendicular to the direction of both the field and the current is created on the contacts of the semiconductor. In the ideal case, the dependency of the voltage on the magnetic field is linear.

In reality, however, there are mechanisms (such as magnetoresistance) that cause deviations from linearity. Corrections can be done electronically. The Hall probe is much more versatile than a fluxmeter, but not as accurate. Manufacturers state the error of an individual probe within a few percent based on probe quality.

A system of two coils wound in the same direction and axially separated by a certain distance. It is used to create a homogeneous magnetic field for various purposes. Often used, for instance, in combination with a fluxmeter to measure the average polarisation of permanent magnets.

A curve of magnetic induction or magnetisation/polarisation on the ordinate versus magnetising field on the abscissa. The magnetising field cycles from zero to positive maximum, then to negative maximum and back to the positive maximum. It thus passes through five quadrants. Hysteresis loop of materials with non-linear permeability shows non-linear characteristics. Curves are usually symmetrical around the origin of the coordinate system. The hysteresis loop characterises the basic properties of permanent magnets.

Partial or full loss of magnetisation. It can occur when a magnet is subjected to high temperatures or a demagnetising field. Magnetisation can be restored spontaneously by magnetising provided the structure of the material has not changed.

I.e. when magnetic properties are the same in all directions. Materials with cubic symmetry are magnetically isotropic. Magnetic materials in which the grains are randomly oriented can also be magnetically isotropic. Isotropic materials have lower coercivity than anisotropic materials and therefore also have a lower energy product.

Demagnetising magnetic field which causes magnetisation/polarisation to drop to zero.

Magnetic flux which does not follow the intended magnetic circuit path. Leakage is common in magnetic circuits, unlike electric circuits, because no insulators (materials with zero magnetic conductivity) exist for a static magnetic field. Only a perfect superconductor can block a magnetic field.

A representation of a magnetic field expressed in Am^-1  (ampere/metre) [SI] or Oe (oersted) [CGS] units.  1 Am^-1 = 4π10^-3 Oe. 

a.k.a. magnetic flux density. Representation of the magnetic field expressed in T (tesla) [SI] or G (gauss) [CGS] units. 1 T (tesla) = 1 Wb.m^-2 (weber per square metre).

See this video that explains the relation between the flux density and the separation result. 

A process in which a magnetically susceptible material is extracted from a mixture using a magnetic force.

A small region within a magnetic material where magnetic moments of the atoms are aligned in a single direction, hence making the region spontaneously magnetised. In the absence of an external magnetising field, individual domains are randomly oriented.

The number of domains aligned in one direction depends on the magnitude of the external magnetic field. Domains are separated by domain walls. The walls are formed by a certain number of atomic layers in which the directions of magnetic moments change direction from domain to domain. The magnetic moments of domains are reoriented by movement of domain walls.

The flow of a magnetic field through a surface area. The unit of magnetic flux is Wb (weber).

Total magnetic moment of a material. It is a volume integral of all elementary (domain) magnetic moments. After applying a sufficiently strong magnetising field, magnetisation no longer grows, since all moments are already aligned in the same direction. Corresponds to polarisation.

A curve representing the change of characteristics a material undergoes when under the effects of a magnetising field. It is plotted in a similar way as the hysteresis loop. The magnetisation curve is distinctly non-linear for materials with non-linear permeability. For soft magnetic materials, the so called "virgin" curve is important. It characterises a material from a completely demagnetised state. The virgin curve is also important for magnetising permanent magnets. However, the second quadrant of the magnetisation curve, i.e. the demagnetisation curve, is more important for operating permanent magnets.

A magnet in which the magnetic material (ferrite or Re/Co, i.e. rhenium or cobalt) is distributed in a plastic matrix. The purpose is to create flexible magnets or magnets that are easier to machine.

A device for magnetising permanent magnets. The most commonly used instruments for this purpose are DC electromagnets or pulse coils. A pulse coil can be powered by a generator, a flywheel, a transformer or a capacitor. Capacitors have become the most popular option in recent times as they allow magnetising materials that have the highest coercivities.

The point maximum product of B and H on the demagnetisation curve. It determines the optimal working point of a permanent magnet. A magnet operating in at this point provides the highest output per unit of volume.

A permanent magnetic material containing neodymium, iron and boron. It has the highest energy product. The basic composition ratio is 2:14:1. Other elements can be added to improve its properties, primarily Dy (dysprosium) for higher coercivity and Co (cobalt) for increasing the Curie point.

A circuit where a portion of the magnetic flux path is closed by air.

A CGS unit of magnetic field strength. 1 Oe (oersted) = 1000.(4π)^-1 A.m^-1.

Ratio of magnetic induction and magnetic field strength. It can be thought of as the ability of a material to "conduct" magnetic flux (magnetic conductivity). This "total" permeability is related to the permeability of vacuum via a factor called "relative" permeability. The value of permeability of vacuum in SI units is  4π.10^-7 N.A^-2.

The slope of the working line. It is determined by the geometry of the magnetic circuit, or, in other words, by the overall demagnetisation in the magnetic circuit.

For all useful intents and purposes, polarisation is equivalent to magnetisation. See magnetisation. Polarisation, J, is a term imposed by the Kennelly system of units and is measured in "tesla", while magnetisation, M, belongs to the Sommerfeld system (both systems are SI) and is measured in A.m^-1. Polarisation is almost invariably used in technical applications while magnetisation is more often encountered in scientific circles. Generally speaking, the terms can be viewed as interchangeable.

Average permeability of the recoil loop.

A small hysteresis loop resulting from sudden change in the course of the magnetising field and subsequent return of the course back to the original position while measuring the hysteresis loop.

Residual magnetisation after the magnetising field has been removed. Corresponds to the ratio of domain moments that remain aligned in the same direction.

Maximum possible magnetisation value for a particular material.

A permanent magnet produced via powder metallurgy from samarium and cobalt. There are two types of commercial Sm-Co materials, with the ratio of their compounds being either 1:5 or 2:17. These materials are characteristic by having the highest intrinsic coercivity values.

Resistance of magnets to change of magnetic moment over time (see Ageing) or due to temperature changes. Magnets are sometimes artificially stabilised by exposure to temperature cycles or demagnetising cycles.

An SI unit of magnetic induction. 1 T (tesla) = 10⁴ G (gauss).

Defined on the magnetisation curve by the working line.

Line with negative slope with origin at [0;0] of the coordinate system of the demagnetisation curve. It intersects the demagnetisation curve in the working point of the magnetic circuit. The slope of the line is determined by the shape of the magnet and of the magnetic circuit.

An SI derived unit of magnetic flux.1 Wb = 1 m².kg.s^-2.A^-1,1 or also 1 V.s or 1 T.m².