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An ‘air gap’ is non-magnetic material, which is present between a magnet and an attracted object or between two magnets that are attracting each other.
An air gap is best described as a break in the magnetic circuit, which magnetism has to jump through to continue a circuit between north and south poles. The introduction of an air gap weakens the magnetic hold.
An air gap can be air itself or a solid non-ferrous material that does not conduct magnetism such as wood, plastic or aluminium. It could also be a thickness of paint or a surface that is very uneven. Refer to the ‘Pull-gap’ curve entry for a description of how pull strength decreases as the size of an air gap increases.
A magnet is described as anisotropic if all of it’s magnetic domains are aligned in the same direction. This is achieved during the manufacturing process and ensures that the domains are 100% orientated in the same direction to deliver maximum magnetic output. This direction is called the ‘magnetic axis’.
The alignment is achieved by subjecting each magnet to a strong electromagnetic field at a critical point during the manufacturing process, which then ‘locks’ the domains parallel to the applied electromagnetic field.
An anisotropic magnet can only be magnetised in the direction (along its magnetic axis) set during manufacture, attempts to magnetise the magnet in any other direction will result in no magnetism. Anisotropic magnets are much stronger than isotropic magnets, which have randomly orientated magnetic domains producing much less magnetism. However, isotropic magnets have the advantage of being able to be magnetised in any direction.
Sometimes referred to as the ‘magnetisation curve’ or ‘demagnetisation curve’, the B-H curve is a graphical representation showing the relationship between ‘magnetic flux density’ (B) and the ‘magnetic field strength’ (H) required to demagnetise a specific magnet. This graph is the second quadrant of a four quadrant hysteresis curve.
The magnetic flux density increases in proportion to the field strength until it reaches a point of saturation and becomes constant even as the field strength continues to increase.
Magnetic flux density is measured in Gauss (G) or Tesla T, where 10,000 Gauss equals 1 Tesla. Magnetic field strength is measured in Oersteds (Oe). When analysing anisotropic materials if the magnetic field is not applied in parallel or perpendicular to the objects anisotropy axis then the measurements of the B-H curve are not valid.
The properties of all magnetic materials change when they are heated to a particular temperature. The Curie temperature (Tc), or Curie point, is the temperature at which the atomic structure of magnetic material is changed and the object becomes demagnetised.
Once heated to, or passed, the Curie point the magnetic domains of the material are released and become randomised and ‘self keepering’, resulting in permanent magnetic damage. As a result, the magnet will not emit any external magnetic fields.
Demagnetisation occurs when a magnet loses its external magnetic field when in open circuit.
This can be through physical stress or corrosion, through heating the magnet beyond its maximum operating temperature or by exposing the material to a strong demagnetising magnetic field.
Generally, neodymium magnets cannot be re-magnetised once their magnetic properties have been lost.
Density is a measurement of a materials mass per unit of volume. All materials have different densities and a magnet’s density can allow you to calculate its weight. The density values for the different types of magnetic material are as follows:
Magnets are produced in batches and during the machining operations, the tolerance dictates the maximum and minimum permissible size.
Neodymium magnets tend to have a standard tolerance of +/-0.1mm, although +/- 0.05mm can be achieved.
Magnetic materials such as permanent magnets are split into individual microscopic domains. The magnetic domain structure of a material is responsible for its magnetic characteristics such as those displayed by metallic elements and alloys like permanent magnets.
Each domain is a region which has a uniform direction of magnetisation, however, different domains may have different directions of magnetisation. During the process of manufacturing magnetic material, electromagnets align each domain, providing the greatest magnetic energy and giving the finished material anisotropy.
Unlike permanent magnets, the magnetic field exerted by an electromagnet is produced by the flow of electric current. The magnetic field disappears when the current is turned off.
Typically, an electromagnet consists of many turns of copper wire which form a solenoid
When a DC electric current flows around the solenoid coil, a magnetic field is created. If an iron core is inserted into the bore of this solenoid, then magnetism is induced into it and it becomes magnetic, but immediately becomes nonmagnetic when the current stops flowing.
Flux density describes the number of lines of magnetism in each square centimetre of pole area.
The total number of magnetic field lines penetrating each 1cm x 1cm pole area is called the magnetic flux density (also known as magnetic induction). Flux density is measured in Gauss, or Tesla (10,000 Gauss = 1 Tesla).
Named after the famous German mathematician and physicist Carl Friedrich Gauss, the Gauss is a unit of measurement for magnetic flux density.
1,000 Gauss is 1,000 lines of magnetism in each cm2 of pole area.
There are a number of different types of magnet, neodymium, samarium cobalt, ferrite and alnico, for example.
Each type of magnet is manufactured in a number of different grades. The term grade defines the chemical characteristics of the material and its magnetic properties. Each grade of material, depending upon its core elements and how it is manufactured will have different magnetic properties.
You will find a list of magnet grades right here in our Tech Centre.
A four quadrant graph, showing magnetising force relative to resultant magnetisation of a permanent magnet material as it is successively magnetised to its saturation point, then demagnetised, magnetised in the reverse polar direction and then finally re-magnetised.
When the cycles are complete, this four quadrant graph will be a closed loop which illustrates the magnetic characteristics of the magnetic material under test. Magnetically hard materials have a larger area inside the loop which denotes the level of magnetic energy. Magnetically soft materials lose magnetism when the magnetising field is removed and therefore these have very small areas inside the loop. The second quadrant within the four quadrants (+X and -Y) is the most important of the four curves and is known as the demagnetisation curve.
The acronym ‘ID’ refers to the measurement of the inner diameter of a magnet. For example, for a ring magnet, the inner diameter would be the measurement of the diameter of the centre hole.
Magnetic induction, also known as flux density is the number of lines of magnetism in each square centimetre of pole area.
The total number of magnetic field lines penetrating each 1cm x 1cm pole area is called the magnetic flux density (also known as magnetic induction). Flux density is measured in Gauss, or Tesla (10,000 Gauss = 1 Tesla).
If the coercivity of a magnet is the force required to cancel out a saturated magnet’s magnetic field, the intrinsic coercivity is the force required to permanently demagnetise a magnet. Neodymium magnets have large differences between the coercivity and intrinsic coercivity, therefore to permanently demagnetise a neodymium magnet takes much more energy than to just equalise (reduce to zero) a neodymium magnet’s magnetic field. Intrinsic coercivity is measured in kilo-Oersteds (kOe).
Each grade of neodymium magnet has an associated intrinsic coercive force, displayed on the ‘neodymium magnet grades’ page of our Tech Centre.
All magnetism flows from north to south and a magnetic circuit is the journey that it takes to get from north to south.
Magnetism is usually generated by permanent or electromagnets and passes through magnetic paths within the circuit. The circuit may also include one or more ‘air gaps’ filled with non-magnetic material. Magnetic circuits are used in devices such as motors, generators and transformers as an efficient method of channelling magnetic fields.
Magnetic materials such as permanent magnets are split into individual microscopic domains. The magnetic domain structure of a material is responsible for its magnetic characteristics such as those displayed by metallic elements and alloys like permanent magnets.
Each domain is a region which has a uniform direction of magnetisation, however, different domains may have different directions of magnetisation. During the process of manufacturing magnetic material, electromagnets align each domain, providing the greatest magnetic energy and giving the finished material anisotropy.
Magnetic induction, also known as flux density is the number of lines of magnetism in each square centimetre of pole area.
The total number of magnetic field lines penetrating each 1cm x 1cm pole area is called the magnetic flux density (also known as magnetic induction). Flux density is measured in Gauss, or Tesla (10,000 Gauss = 1 Tesla).
The term material refers to the physical composition of a magnet. For example, neodymium magnets are made out of a neodymium alloy (NdFeB) material containing neodymium (Nd), iron (Fe) and boron (B).
There are five main types of magnetic material and they are:
The maximum energy product of a magnet is measured in ‘Mega-Gauss Oersteds’ (MGOe). Known as the maximum energy product value, this is the primary indicator of a magnet’s ‘strength’. In general, the higher the maximum energy product value, the greater the magnetic field the magnet will generate in a particular application. In neodymium grading, the two numbers in a grade name (e.g. N42) represent the maximum energy product for that grade. The higher the value, the greater the magnetic field strength the magnet will exert in a particular application and the smaller the volume of magnet required.
(BH)max is a product of remanence (Br) and coercivity (Hc) and represents the area under the graph of the second quadrant hysteresis loop.
Each grade of neodymium magnet has an associated maximum energy product, displayed on the ‘neodymium magnet grades’ page of the Tech Centre.
The maximum operating temperature is exactly as it sounds, it represents the maximum temperature that a particular grade of magnet will be able to function at, before it becomes permanently demagnetised.
All permanent magnets weaken in relation to their temperature coefficient, but as long as the maximum operating temperature is not exceeded, this is fully recoverable on cooling. If the maximum operating temperature is exceeded, then the losses will not be fully recovered on cooling. Repeatedly heating a magnet above its maximum operating temperature and cooling will significantly demagnetise the magnet.
Neodymium magnets operate best in cold temperatures down to approximately -130oC. Regular neodymium magnets will maintain their magnetism in operating temperatures up to 80oC whereas different variants of neodymium magnets can operate up to temperatures of 230 oC.
The maximum operating temperature for each grade of magnet material is displayed on the ‘How does temperature affect neodymium magnets’ page of the Tech Centre.
Mega Gauss Oersteds is the CGS measure of the maximum energy product of a magnet (BHmax).
The five main types of magnet material have the following typical maximum energy products:
Also known as the hysteresis loop, the M-H loop is a four quadrant graph, showing magnetising force relative to resultant magnetisation of a permanent magnet material as it is successively magnetised to its saturation point, then demagnetised, magnetised in the reverse polar direction and then finally re-magnetised.
When the cycles are complete, this four quadrant graph will be a closed loop which illustrates the magnetic characteristics of the magnetic material under test. Magnetically ‘hard’ materials have a large area inside the loop which denotes the level of magnetic energy. Magnetically ‘soft’ materials lose magnetism when the magnetising field is removed and therefore these have very small areas inside the loop. The second quadrant within the four quadrants (+X and -Y) is the most important of the four curves and is known as the demagnetisation curve.
Some materials, when placed inside a magnetic field, become magnetised themselves. The permeability of a magnetic substance represents the increase or decrease of the magnetic field inside the substance compared to the magnetising field that the substance is located within. Simply put, it is the ability for a material to acquire its own magnetism or for magnetism to flow through it.
Ferromagnetic metals have the greatest permeability of all substances and will become magnetised when exposed to a magnetic field. The rate of magnetic permeability will increase until the substance reaches a point of saturation. ‘Soft’ ferromagnetic materials are easily magnetised, but once the external field is removed they lose most of their magnetism. Conversely, ‘hard’ ferromagnetic materials are difficult to magnetised, but once they are, they will remain magnetised.
Plating is another term for coating. Platings or coatings are applied to raw neodymium magnets to prevent corrosion and demagnetisation. The most common coating is a layer of nickel, followed by a later of copper and then another layer of nickel.
At first4magents.com we can provide many different coatings and platings for bespoke applications, including:
A pull-gap curve plots the ‘pulling power’ of a magnet in direct contact with a thick and flat piece of steel and then though a steadily increasing range of air gaps. Pull follows an inverse square law relationship with distance.
High field gradient magnets have the highest clamping forces in direct contact with ferrous material (zero air gap), but the weakest pull through steadily increasing air gaps.
Low field gradient magnets have the lowest clamping forces in direct contact with ferrous material (zero air gap), but the highest pull through steadily increasing air gaps.
A high field gradient magnet’s pull-gap curve and a low field gradient magnet’s pull-gap curve will cross over if plotted on the same graph.
As a rule of thumb it is five times easier to slide a magnet than to pull it vertically off the surface of a ferrous material.
When a magnet slides on steel, the coefficient of friction is approximately 0.2 and this is how the five times is derived.
Magnets attached to a vertical steel wall will slide down the wall when only 20% of the rated pull is experienced as a load. Rubber coated magnets have a much higher coefficient of friction and therefore will resist sliding at a far higher rate because of the friction caused by the coating.
If the vertical wall is made of thin sheet steel which cannot absorb all the magnetism generated by the magnet, then the holding force will be reduced further.
