About magnetism

 


Magnetism and ferromagnetism

Brief summary: (see also the original Wikipedia article on which this is based)

Back in ancient times, people discovered that magnetite crystals attract or repel each other depending on their orientation. This physical phenomenon is referred to as magnetism. The words magnetite and magnesium are both derived from Magnesia, the name of an area in the Thessaly region of Greece where magnetic stone can be found in abundance.

It is the iron in the rock that is responsible for the magnetic properties of magnetite. Many iron alloys possess magnetic properties. In addition to iron, nickel, cobalt and gadolinium have magnetic properties as well.

Although ferromagnetic (and ferrimagnetic) materials are the only kinds with strong enough magnetic properties to be drawn to a magnet (and therefore generally considered to be magnetic), all other substances also respond weakly to a magnetic field, via one or more other types of magnetism.

Ferromagnetic materials can be divided into magnetically 'soft' materials, such as annealed iron, which can be magnetized but usually do not retain the magnetization indefinitely, and magnetically 'hard' materials that do remain magnetized. Permanent magnets are made of 'hard' ferromagnetic materials such as Alnico and ferrite, which undergo special processing in a powerful magnetic field during production to 'align' their internal microcrystalline structure, making them very resistant to demagnetization.

Objects which strongly exhibit this behaviour are called magnets. There are natural and man-made magnets (e.g. Alnico, Fernico, ferrites). All magnets have two poles, which are referred to as the north pole and the south pole. Magnetic monopoles (a north or south pole occurring independently, without its opposite) are also theoretically possible, but their existence has never yet been demonstrated experimentally. The north pole of a magnet repels the north pole of other magnets and attracts the south pole of other magnets. Two south poles also repel.

Because the earth has a magnetic field as well, with its magnetic south pole close to the geographic north pole and its magnetic north pole close to the geographic south pole, a free-spinning magnet will always take on a north-south orientation. The names of the poles of a magnet are derived from this. For the sake of convenience, but nevertheless slightly confusing, the south pole of “the 'earth magnet' is called the magnetic north pole and the north pole of the 'earth magnet' is called the magnetic south pole.

A related phenomenon is electromagnetism, or magnetism that is generated by an electric current. In essence, all magnetism is caused by either rotating or revolving electrical charges in eddy currents.

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Magnetic behaviour of materials

When a material is exposed to a magnetic field it can respond in various ways.

We distinguish between:

In colloquial language when we say a material is magnetic we usually mean it exhibits ferromagnetic (or sometimes ferrimagnetic) behaviour. The forces that occur in diamagnetic and paramagnetic behaviour are much weaker, and materials exhibiting such behaviour do not spontaneously produce their own magnetic field. They can more or less be considered to be non-magnetic. Diamagnetic materials have the tendency to repel lines of flux from their core, while ferromagnetic, ferrimagnetic and paramagnetic materials more or less concentrate them.

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Remanence or residual magnetism

Residual magnetism is often caused (either intentionally or unintentionally) by magnetic fields from the immediate vicinity, such as clamping tables, magnetic conveyors or induction heating. Other causes include arc welding, machining processes, cold forming and even mechanical vibrations.

The consequences of residual magnetism may be desirable, problematic or even very costly. A nut that clings to the end of a screwdriver is handy, but two products that stick together in a mould disrupt production, resulting in financial losses. Other possible consequences of undesired magnetism: a coarse surface after galvanization, welds that only penetrate on one side, rapid wear of bearings, or metal chips that stick to the parts.

These consequences can be avoided by demagnetizing the material.

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Measurement of quantity of magnetism in materials

The simplest manner to determine whether magnetism is present is with a paperclip. By attaching one to a thread and dangling it above the surface you can locate the magnetic areas. If the product actually draws the paperclip towards it and the paperclip sticks to it, the magnetic value is at least 20 Gauss. Below 20 Gauss the paperclip will fall off, and above 40 Gauss it will be firmly held in place.

Iron filings will be held in place at levels above just 10 Gauss. This is very little, as the Earth's magnetism (depending on the location on Earth) is around 0.5 Gauss.

Using a magnetic field strength meter it is possible to measure the exact field strength or direction of the field.

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Demagnetization

        

Depending on the type, shape, dimensions, speed and quantity, there are several options available for eliminating undesired magnetism. A few examples:

More info about demagnetization


Nice to know: magnetization/demagnetization of CRT TVs and computer monitors:

The Earth's magnetic field can cause colour distortions on old CRT (cathode ray tube) televisions and computer monitors. Try holding a magnet (e.g. from a loudspeaker) near a TV screen (not too close!). You will see considerable shifting of the colours. This can also occur when you place the TV in a different part of the room or turn it to face a different direction.

These CRT colour TVs and computer monitors always have a built-in demagnetization system. This consists of a coil of copper wire wrapped in a particular way around the metal shielding of the CRT. In the case of a television, the demagnetization is often only performed when the mains power is switched on. Sometimes there is a button on the device and/or on the remote control. Computer monitors have a button or a control function ('degauss') with which you can demagnetize the components in and around the CRT.

If you have a problem with colour distortion on your old CRT TV, you should try switching it off for a half hour (not just stand-by; remove the plug from the socket) and then switch it on again. You will then hear a loud 'boing' sound, which is caused by the demagnetization process. If you do not hear the 'boing' and you are having problems with colour shifting, you should call a repair service, because something is broken.

Modern flat screen monitors, video projectors (beamers) and flat TVs are not affected by magnetic fields, so they do not have a demagnetization system.

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The magnetization of ferromagnetic materials

Ferromagnetic (or magnetically conductive) materials such as iron and steel and their alloys can very easily become magnetic. Depending on the type of material or alloy, the product may remain magnetic; this is referred to as remanent magnetism. Even non-ferrite stainless steel can become magnetized as a result of deformation or welding!

In that case, the induced magnetism often originates from other magnetic sources, such as lifting magnets, clamping tables, loudspeakers or magnetic transport systems. Magnetic fields near transformers, welding cables and welding processes can also induce magnetism. Even certain processes, such as drilling, grinding, sawing and polishing of the material, can result in remanent magnetism. As mentioned above, this can even occur in stainless steel.

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Curie temperature



The Curie temperature is named after Pierre Curie (1859-1906).

It is the temperature above which ferromagnetic materials lose their permanent magnetic field. Above this temperature the material behaves paramagnetically. As the temperature rises, the molecular excitement gradually disrupts the spin alignment. When the Curie temperature is reached the alignment collapses because the thermal energy has exceeded the energy of the magnetic interaction.

It is difficult to measure the Curie temperature exactly. For one thing, the permanent magnetic field around the material only partly disappears. Secondly, the Curie temperature varies greatly based on even small quantities of contaminants in the material.

For example, if an AlNiCo magnet is heated above its Curie temperature of 850 °C, it will no longer be ferromagnetic. It then becomes paramagnetic. Once the magnet has cooled off again, the permanent magnetic field does not return. There will, however, be new magnetic fields present in small areas within the material, the so-called Weiss areas (Weiss 1865-1904), but these fields are aligned in random directions so their vector sum does not result in an external magnetic field. Nevertheless, it is possible to remagnetize the magnet.

The ferromagnetic elements and alloys with their Curie temperatures:

Material

 Curie temp.

 Fe 770°C 
 Co 1115°C 
 Ni 354°C 
 Gd 19°C 
 Alnico 850°C 
 Ferrite 450°C 
 Sm Cobalt  750-825°C
 Nd-Fe-B  310-340°C

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Electromagnets

     

In general, electromagnets consist of a core of magnetic or ferromagnetic material, such as soft iron, around which a coil has been wound. As long as an electric current flows through the coil, the core remains magnetic.

Working principle: A magnetic field is generated around a conductive wire through which an electric current flows. The generated magnetic flux can be expressed as follows:

Φ = L * I
where:
Φ is the magnetic flux expressed in Weber
L is the self-induction in Henry
I is the current in Ampere

A strong magnetic field is obtained with high current or large self-induction. High currents are not always feasible or desirable (danger, heat production), which is why a high self-induction is usually utilized, obtained by winding a wire into a coil shape, referred to as a 'solenoid'. The fields generated in each winding act collectively, resulting in a strong magnetic field.

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Permanent magnets

A permanent magnet is a ferromagnetic material that possesses permanent magnetic properties, even when it is not located within a magnetic field.

One end of the magnet is called the north pole, the other the south pole. North and south poles attract, and this attraction is inversely proportional to the square of the distance between the two poles. Like poles (north-north and south-south) repel.

Lines of flux are imaginary lines that indicate the direction of the magnetic field at a certain point. For magnets they can be made visible by placing a sheet of paper on the magnet and sprinkling some iron filings on the paper. The iron filings will cluster along the lines of flux, allowing you to see them.
In certain circumstances a permanent magnet can lose its magnetic properties. This can be caused by high temperature (see Curie temperature), a physical shock (impact) or exposure to external magnetic fields. Magnets supplied by Goudsmit are of such high quality that the loss of magnetic properties can be considered negligible, provided they are used within the specified operating parameters, which pertain to aspects such as temperature range and vicinity to external magnetic fields.

Permanent magnets were once made of steel, but we now have all sorts of alloys available that are better suited for this purpose.

Goudsmit supplies magnets and magnet systems based on the following four main categories of magnetic materials:

AlNiCo magnets
Ferrite magnets
Samarium-Cobalt magnets
Neodymium-Iron-Boron magnets (also sold by Goudsmit under the brand name Neoflux®)

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Direction of magnetization

Most permanent magnets available on the market are anisotropic, i.e. the magnet has a preferred direction of magnetic orientation and can only be magnetized along one axis. It is possible, however, to reverse the polarity of the magnet, which exchanges its north and south pole. Goudsmit has very powerful magnetization equipment with which permanent magnets can be magnetized to their maximum saturation.

Ferrite magnets are also available in isotropic versions, which can be magnetized in any direction. Segment-shaped ferrite and Neoflux® magnets are also available; these are radially anisotropic and can therefore only be magnetized in the radial direction.

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Safety

Neodymium-iron-borion (Nd-Fe-B) magnets, sold by Goudsmit under the brand name Neoflux®, should always be handled carefully. Neodymium magnets smaller than a penny yet powerful enough to lift over 10 kilograms! These magnets are dangerous, as they can pinch the skin or fingers when suddenly attracted to iron or steel. Neodymium magnets are made with special powders and coatings and are therefore brittle and easily broken, especially at temperatures above 150 °C or when they slam together. When they break, this occurs so suddenly and violently that flying pieces may cause eye or other injuries.

Neodymium magnets should also be kept far from electrical appliances, magnetic cards, CRT monitors, pacemakers, watches and similar items as such items may be permanently damaged by the strong magnetic field.

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