About Magnets

History of Permanent Magnets

Lodestones

The history of permanent magnets goes back to ancient times. Records from early Greek, Roman and Chinese civilisations make reference to rare and mysterious stones called lodestones. These lodestones could attract each other and also small pieces of iron in what seemed a magical way and when suspended from a thread, they always pointed in the same direction. We now know that lodestones contain magnetite, an oxide of iron and that they are a naturally occurring magnet having the composition Fe3O4.

Although lodestones were considered an intriguing phenomenon by scientists of the day, they were not really utilised in any constructive way until around 1200 AD with the introduction of the mariners (magnetic) compass. The mariners compass is a device housing a pivoting magnetised needle, which freely and consistently points towards magnetic north. This enables travellers to consistently and safely navigate their way from one place to another.

Many exciting discoveries involving magnets and relating to electricity have been made over the 800 years since the invention of the mariners (magnetic) compass, however strong permanent magnets as we know them today, are only a very recent invention.

You may think that we would have to go back 150 or even 200 years to look at the development of strong permanent magnets, but that is just not the case. In actual fact the history of magnets as we know them today only goes back as far as 1940.

Modern research into magnetism is heading in many different directions involving a vast array of materials, however at this time there are only four types of magnets that are commonly used throughout the world. They are Alnico, Ferrite, Samarium Cobalt and the most recently invented, Neodymium Iron Boron.

Alnico Magnets

The first in our group of modern magnets was Alnico. These magnets came onto the scene in the 1940’s and they started a revolution in the use of permanent magnets. For the first time it was now possible to replace electro magnets with permanent (Alnico) magnets. This provided a design flexibility that had previously been unthinkable. They were used in devises such as generators, electric motors, microphones and loud speakers just to name a few.

Alnico magnets are made from Aluminium, Nickel and Cobalt. They have an excellent temperature tolerance of up to 550o C and are relatively corrosion resistant. Alnico’s biggest downfall is its poor resistance to demagnetisation. This instability sometimes makes Alnico unsuitable for certain demanding engineering requirements. Although they are still widely used today, Alnico Magnets are gradually becoming priced out of the market with the availability of newer, high tech and lower priced magnets that are now common place.

Ferrite Magnets

In the early to mid 1950’s came the creation of Ferrite magnets. Often referred to as ceramic magnets, Ferrites quickly became the preferred choice over Alnico due to their low cost and their strong resistance to demagnetisation. The availability of Ferrite magnets rapidly increased the momentum in the abundance of inventions and improvements to existing products using magnets. Ferrite magnets are used in all manner of products including electronic sensing devices, electric motors, loud speakers, lifting devices, magnetic separators etc.

Ferrite magnets are low cost and they have excellent corrosion resistance. They are very hard and as such they are very brittle. This also makes them difficult to machine or drill holes. They have a very good temperature tolerance with relative magnetic stability at temperatures of up to 250o C.   

Samarium Cobalt Magnets

1970 saw the entrance of Samarium Cobalt magnets (Sm-Co) into the market place. In a similar way that Ferrite and Alnico magnets had done before, Samarium Cobalt magnets brought a whole range of new characteristics, applications and possibilities to the industry.

Samarium Cobalt magnets have several features that made them far superior to any magnet previously seen. They have a magnetic strength that is several times greater than either Alnico or Ferrite magnets, while at the same time they offer a very high temperature stability. Temperatures of 300o C are seen as no problem for Samarium Cobalt magnets and in addition to this they also demonstrate excellent corrosion resistance. The two disadvantages of Samarium Cobalt magnets are that they are very brittle and therefore extremely fragile and they are a very expensive magnet to produce.

Samarium Cobalt magnets properties make them a perfect choice for high strength and high temperature applications such as stepper motors and furnace sensors etc. Their corrosion resistance opens up numerous opportunities in marine environments and also in chemical industries.  

Neodymium Iron Boron Magnets

1983 saw the discovery of Neodymium Iron Boron Magnets (NdFeB). The invention of this new magnet was announced almost simultaneously by two separate companies, (General Motors and Sumitomo Special Metals) who were working independently of each other on very different production methods of a very similar product.

This announcement created a huge amount of interest as magnet technology had now taken another leap forward to produce an entirely new magnetic material. Neodymium Iron Boron Magnets, or “Rare Earth” magnets as they are more commonly known, are even stronger than Samarium Cobalt magnets and their manufacturing process is considerably cheaper, therefore providing the market with an incredibly strong, but cost effective product. Unfortunately Rare Earth magnets do have two disadvantages. They have poor corrosion resistance and they are more temperature sensitive than other magnets. However new high temperature tolerant grades of Rare Earth magnets  are being created and released on a regular basis. Rare Earth magnets offer high magnetic energy and good stability making them the logical choice for many electronics and engineering projects.

Rare Earth magnets are used in all manner of ways in almost every sector of the magnetics industry such as mining, health, construction, electronics, education and the food industry. They have become the industry standard when size, strength and efficiency are required.

Research and Development

Since the discovery of Neodymium Iron Boron Magnets there has been countless hours of research and development throughout the world in the never ending pursuit to produce even stronger magnets with better corrosion resistance and with a higher temperature capacity. Unfortunately this research has so far not resulted in any significant announcements that have changed, added to, or further enhanced the history of magnets.

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Glossary of Magnetic Terms

Quick link - click on the first letter of the word you want to find:
A - B - C - D - E - F - G - H - I - J - K - L
M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

Air Gap

A low permeability gap in the flux path of a magnetic circuit. Often air, but inclusive of other materials such as paint, aluminum, etc.

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Anisotropic Magnet

A magnet having a preferred direction of magnetic orientation, so that the magnetic characteristics are optimum in one preferred direction.

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Bipolar Magnets

Both poles are on one side

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Closed Circuit

This exists when the flux path external to a permanent magnet is confined within high permeability materials that compose the magnet circuit.

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Coercive Force, Hc

The demagnetising force, measured in Oersteds, necessary to reduce observed induction, B, to zero after the magnet has previously been brought to saturation.

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Curie Temperature, Tc

The temperature at which the parallel alignment of elementary magnetic moments completely disappears, and the material is no longer able to hold magnetisation.

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Demagnetisation Curve

The second quadrant of the hysteresis loop, generally describing the behavior of magnetic characteristics in actual use. Also known as the B-H Curve.

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Eddy Currents

Circulating electrical currents that are induced in electrically conductive elements when exposed to changing magnetic fields, creating an opposing force to the magnetic flux. Eddy currents can be harnessed to perform useful work (such as damping of movement), or may be unwanted consequences of certain designs, which should be accounted for or minimized.

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Electromagnet

A magnet, consisting of a solenoid with an iron core, which has a magnetic field existing only during the time of current flow through the coil.

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Energy Product

Indicates the energy that a magnetic material can supply to an external magnetic circuit when operating at any point on its demagnetisation curve. Calculated as Bd x Hd, and measured in Mega Gauss Oersteds, MGOe.

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Ferromagnetic Material

A material whose permeability is very much larger than 1 (from 60 to several thousand times 1), and which exhibits hysteresis phenomena.

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Flux

The condition existing in a medium subjected to a magnetising force. This quantity is characterized by the fact that an electromotive force is induced in a conductor surrounding the flux at any time the flux changes in magnitude. The cgs unit of flux is the Maxwell.

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Flux Density, B

The magnetic flux per unit area of a section normal to the direction of flux. Also known as magnetic induction. Measured in Gauss, in the cgs system of units.

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Fluxmeter

An instrument that measures the change of flux linkage with a search coil.

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Gauss

Lines of magnetic flux per square centimetre, cgs unit of flux density, equivalent to lines per square inch in the English system, and Webers per square meter or Tesla in the SI system.

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Gaussmeter

An instrument that measures the instantaneous value of magnetic induction, B. Its principle of operation is usually based on one of the following: the Hall effect, nuclear magnetic resonance (NMR), or the rotating coil principle.

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Hysteresis Loop

A closed curve obtained for a material by plotting corresponding values of magnetic induction, B, (on the abscissa) against magnetising force, H, (on the ordinate).

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Induction, B

The magnetic flux per unit area of a section normal to the direction of flux. Measured in Gauss, in the cgs system of units.

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Intrinsic Coercive Force, Hci

Measured in Oersteds in the cgs system, this is a measure of the materials inherent ability to resist demagnetisation. It is the demagnetisation force corresponding to zero intrinsic induction in the magnetic material after saturation. Practical consequences of high Hci values are seen in greater temperature stability for a given class of material, and greater stability in dynamic operating conditions.

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Intrinsic Induction, Bi

The contribution of the magnetic material to the total magnetic induction, B. It is the vector difference between the magnetic induction in the material and the magnetic induction that would exist in a vacuum under the same field strength, H. This relationship is expressed as: Bi = B-H.

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Irreversible Loss

Defined as the partial demagnetisation of a magnet caused by external fields or other factors. These losses are only recoverable by re-magnetisation. Magnets can be stabilized to prevent the variation of performance caused by irreversible losses.

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Isotropic Magnet

A magnet material whose magnetic properties are the same in any direction, and which can therefore be magnetised in any direction without loss of magnetic characteristics.

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Keeper

A piece of soft iron that is placed on or between the poles of a magnet, decreasing the reluctance of the air gap and thereby reducing the flux leakage from the magnet.

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Knee of the Demagnetisation Curve

The point at which the B-H curve ceases to be linear. All magnet materials, even if their second quadrant curves are straight line at room temperature, develop a knee at some temperature. Alnico 5 exhibits a knee at room temperature. If the operating point of a magnet falls below the knee, small changes in H produce large changes in B, and the magnet will not be able to recover its original flux output without re-magnetisation.

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Leakage Flux

That portion of the magnetic flux that is lost through leakage in the magnetic circuit due to saturation or air-gaps, and is therefore unable to be used.

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Length of air-gap, Lg

The length of the path of the central flux line in the air-gap.

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Load Line

A line drawn from the origin of the Demagnetisation Curve with a slope of -B/H, the intersection of which with the B-H curve represents the operating point of the magnet. Also see Permeance Coefficient.

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Magnetic Circuit

An assembly consisting of some or all of the following: permanent magnets, ferromagnetic conduction elements, air gaps, electrical currents.

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Magnetic Flux

The total magnetic induction over a given area. When the magnetic induction, B, is uniformly distributed over an area A, Magnetic Flux = BA.

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Magnetic Flux Density, B

The magnetic flux per unit area of a section normal to the direction of flux. Also known as magnetic induction. Measured in Gauss, in the cgs system of units.

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Magnetic Induction, B

The magnetic flux per unit area of a section normal to the direction of flux. Measured in Gauss, in the cgs system of units.

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Magnetising Force, H

The magnetomotive force per unit length at any point in a magnetic circuit. Measured in Oersteds in the cgs system.

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Magnetomotive Force, F

Analogous to voltage in electrical circuits, this is the magnetic potential difference between any two points.

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Maximum Energy Product, BHmax

The point on the Demagnetisation Curve where the product of B and H is a maximum and the required volume of magnet material required to project a given energy into its surroundings is a minimum. Measured in Mega Gauss Oersteds, MGOe.

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North Pole

That pole of a magnet which, when freely suspended, would point to the north magnetic pole of the earth. The definition of polarity can be a confusing issue, and it is often best to clarify by using "north seeking pole" instead of " north pole" in specifications.

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Oersted, Oe

A cgs unit of measure used to describe magnetising force. The English system equivalent is Ampere Turns per Inch, and the SI systems is Ampere Turns per Meter.

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Orientation Direction

The direction in which an anisotropic magnet should be magnetised in order to achieve optimum magnetic properties. Also known as the "axis", "easy axis", or "angle of inclination".

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Paramagnetic Material

A material having a permeability slightly greater than 1.

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Permeance

The inverse of reluctance, analogous to conductance in electrical circuits.

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Permeance Coefficient, Pc

Ratio of the magnetic induction, Bd, to its self demagnetising force, Hd. Pc = Bd / Hd. This is also known as the " load line", "slope of the operating line", or operating point of the magnet, and is useful in estimating the flux output of the magnet in various conditions. As a first order approximation, Bd / Hd = Lm / Lg, where Lm is the length of the magnet, and Lg is the length of an air gap that the magnet is subjected to. Pc is therefore a function of the geometry of the magnetic circuit.

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Pole Pieces

Ferromagnetic materials placed on magnetic poles used to shape and alter the effect of lines of flux.

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Relative Permeability

The ratio of permeability of a medium to that of a vacuum. In the cgs system, the permeability is equal to 1 in a vacuum by definition. The permeability of air is also for all practical purposes equal to 1 in the cgs system.

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Reluctance, R

Analogous to resistance in an electrical circuit, reluctance is related to the magnetomotive force, F, and the magnetic flux by the equation R = F/( Magnetic Flux), paralleling Ohm's Law where F is the magnetomotive force (in cgs units).

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Remanence, Bd

The magnetic induction that remains in a magnetic circuit after the removal of an applied magnetising force. If there is an air gap in the circuit, the remanence will be less than the residual induction, Br.

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Residual Induction, Br

This is the point at which the hysteresis loop crosses the B axis at zero magnetising force, and represents the maximum flux output from the given magnet material. By definition, this point occurs at zero air gap, and therefore cannot be seen in practical use of magnet materials.

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Return Path

Conduction elements in a magnetic circuit which provide a low reluctance path for the magnetic flux.

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Reversible Temperature Coefficient

A measure of the reversible changes in flux caused by temperature variations.

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Saturation

The condition under which all elementary magnetic moments have become oriented in one direction. A ferromagnetic material is saturated when an increase in the applied magnetising force produces no increase in induction. Saturationflux densities for steels are in the range of 16,000 to 20,000 Gauss.

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Search Coil

A coil conductor, usually of known area and number of turns that is used with a fluxmeter to measure the change of flux linkage with the coil.

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South Pole

That pole of a magnet which, when freely suspended, would point opposite to the north magnetic pole of the earth. See North Pole definition.

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Stabilization

Exposure of a magnet to demagnetising influences expected to be encountered in use in order to prevent irreversible losses during actual operation. Demagnetising influences can be caused by high or low temperatures, or by external magnetic fields.

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Temperature Coefficient

A factor, which describes the change in a magnetic property with change in temperature. Expressed as percent change per unit of temperature.

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Weber

The practical unit of magnetic flux. It is the amount of magnetic flux which, when linked at a uniform rate with a single-turn electric circuit during an interval of 1 second, will induce in this circuit an electromotive force of 1 volt. Wb = V s = m 2 kg/s 2 A.

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Magnets In The Home

This picture demonstrates the many uses for magnets in our homes and in our every day life. One can only imagine the countless other applications for magnets in our work places, in our schools, in heavy industry, in shopping centers and in the wider community.

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Directions of Magnetism

  Axially Magnetised (Through the thickness)
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  Magnetised through the thickness
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  Axially Magnetised with two poles (Isotropic only)
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  Diametrically Magnetised (Isotropic only)
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  Magnetised - Multipoles on one face only (Isotropic only)
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  Axially Magnetised - Multipoles on one face only (Isotropic only)
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  Magnetised Multipoles on one face only (Isotropic only)
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  Radially Magnetised with multipoles (Isotropic only)
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  Magnetised with multipoles on the inside (Isotropic only)
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  Magnetised with multipoles on the circumference (Isotropic only)
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  Diameterically Magnetised
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  Radially Magnetised
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