At Magnet Expert, we believe in sharing our knowledge of and passion for magnets. Incredible benefits and efficiencies can be realised using the power of magnets but finding easy-to-understand, reliable information can be difficult. The theory of magnetism is complex, but its application of it doesn’t have to be.
Here, you will find the information directly from our team of experts covering the questions we are frequently asked. Our aim is to explain everything there is to know about using permanent magnets in an easy-to-apply way, however, if you have a technical question not answered here please do not hesitate to contact our team.
There are five main types of permanent magnet material; these are, in order of strength from strongest to weakest, neodymium, samarium cobalt, alnico, ferrite, and flexible rubber. When designing a magnetic solution, physical strength is not always the most important factor and each material has its own unique characteristics.
When designing an application for permanent magnets it is essential to consider the temperature range that the magnet or magnets will be expected to work in. Yet, with so much information available (some of it incorrect) it is possible to associate the wrong magnetic property with the type of magnetic material you are considering, particularly where thermal stability is concerned; the result being disappointing magnetic performance.
By their very nature, every permanent magnet contains a form of iron. Iron displays the most dramatic ferromagnetic properties of all elements, which is why it is found in the most powerful magnets. However, iron is highly reactive to water and hence makes magnets with high iron content very susceptible to corrosion.
Other ferromagnetic elements used in different types of permanent magnets include nickel, cobalt, gadolinium and dysprosium all of which have an impact on both magnetic strength and resistance to corrosion.
Most modern magnets are manufactured with a preferred direction of magnetism, which means they are anisotropic. A magnet is described as anisotropic if all of its individual atomic magnetic domains are aligned in the same direction. This is achieved during the manufacturing process 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.
A magnet made of magnetically isotropic material has no preferred direction of magnetism and has the same properties along either axis. During manufacture, isotropic material can be manipulated so that the magnetic field is applied in any direction.
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.
How a permanent magnet works is all to do with its atomic structure. All ferromagnetic materials produce a naturally occurring, albeit weak, magnetic field created by the electrons that surround the nuclei of their atoms.
These groups of atoms can orient themselves in the same direction and each of these groups is known as a single magnetic domain. Like all permanent magnets, each domain has its own north pole and south pole. When a ferromagnetic material is not magnetised its domains point in random directions and their magnetic fields cancel each other out.
To make a permanent magnet, ferromagnetic material is heated at incredibly high temperatures, while exposed to a strong, external magnetic field. This causes the individual magnetic domains within the material to line up with the direction of the external magnetic field to the point when all the domains are aligned and the material reaches its magnetic saturation point. The material is then cooled and the aligned domains are locked in position. This alignment of domains makes the magnet anisotropic.
After the external magnetic field is removed hard magnetic materials will keep most of their domains aligned, creating a strong permanent magnet.
Gauss is a measure of magnetic induction and a value of density. Simply put, a magnet’s Gauss measurement represents the number of magnetic field lines per square centimetre, emitted by a magnet. The higher the value, the more lines of magnetism emitted by a magnet however, alone, it isn’t necessarily a representation of a magnet’s strength.
As well as the material, geometry also has an effect on a magnet’s Gauss value, for example, if you have two different sized magnets made from the same material with the same surface Gauss, the larger magnet will always be stronger. Sometimes, a small magnet may have a high surface Gauss but will be able to support less weight than a larger magnet with a lower surface, Gauss.
The Br or remanence value is the theoretical maximum density of a magnetic field within a magnet, used in closed circuit conditions. Magnets in open circuit conditions rarely exceed a value of 7,000 Gauss. The open circuit (not attached to any other ferrous object) surface Gauss value is the density of the magnetic field at any point on the surface of the magnet.
For example, a 25mm diameter by 20mm thick N52 neodymium magnet, made from one of the strongest magnetic materials commercially available will measure a maximum of 6,250 Gauss on the magnet’s surface and considerably less as you move away from the surface.
Some of our disc, rod and ring magnets are described as diametrically magnetised, which means rather than having their north and south pole on opposite flat faces the north pole is on one curved side and the south pole is on the other. Diametrically magnetised magnets are not often designed to hold the maximum possible weight for the size of the magnet but instead are used to provide rotational movement.
Magnetic fields will pass through plastic, wood, aluminium and even lead as if it was not there. There is no material that will block magnetism. Ferrous materials such as iron, steel or nickel can conduct magnetic fields and redirect magnetism.
All magnetic fields seek the shortest path from north to south and a piece of steel can provide a short cut making the journey from north to south much easier than flowing through the air. To remove magnetism from where you do not want it to be, you can use steel to provide the magnet with a shortcut to redirect the magnetism flow via an alternative route.
The simplest example is putting a steel keeper across the poles of a horseshoe magnet, all the magnetism flows through the steel and there is no external magnetic field. When we send highly magnetised materials overseas, the airlines stipulate that there should be no magnetism on the outside of the box.
To achieve this, we put the magnets in the centre of the box and then line all 6 sides of the inside of the box with sheet steel. Stray magnetism which would normally pass through the walls of the box is suddenly diverted as they conduct through the steel on their journey from north to south.
Using two magnets together would be the same as having one magnet of their combined size. For example, if you stacked two 10mm diameter x 2mm thick magnets on top of each other you would have effectively created a 10mm diameter x 4mm thick magnet, essentially doubling the magnets strength and pull.
Once the length of the magnet exceeds the diameter of the magnet, the magnet is working at an optimum level and further additions to magnetic length will provide only small increases in performance.
A permanent magnet, if kept and used in optimum working conditions, will keep its magnetism for years and years. For example, it is estimated that a neodymium magnet loses approximately 5% of its magnetism every 100 years.
When a magnet is not in direct, flush contact with a steel surface or another magnet, their ability to attract/repel does decline significantly. How much, is roughly exponential however every shape and size of magnet is different. We test the holding strength of all of our magnets in direct contact with a steel plate and through a series of ‘air gaps’ ranging from 0.1mm to 20mm.
All magnets have a 'pull' rating measured in kilograms and this relates to how much force acting perpendicular to the magnet is required to pull the magnet from a steel plate or equal thickness when indirect, flush contact.
The 'pull' rating is obtained under the following ideal conditions:
In actual applications, perfect conditions are unlikely and the following factors will reduce the given pull:
If a magnet needs the contact steel to be 10mm thick to absorb all the magnetism and deliver maximum pull, then fixing the magnet to a 1mm thick sheet steel surface will result in 90% of the magnetism being wasted and the actual pull delivering only 10% of its capability.
To test if the contact steel is thick enough to absorb all the magnetism from a given magnet, simply fix the magnet in place and then offer a small steel plate behind the contact steel, directly behind the magnet and if it sticks, then it is being held in place by stray magnetism which is breaking out from insufficiently thick steel. If it falls away, then the contact steel is absorbing and conducting all the magnetism and increasing the thickness of the steel will not increase the 'pull' from the magnet.
If the contact steel is rusty, painted or uneven, then the resulting gap between the magnet and the contact steel will lead to a reduced 'pull' from the magnet. As this gap increases, the pull decreases using an inverse square law relationship.
All pull tests use mild steel as contact steel. Alloy steels and cast irons have a reduced ability to conduct magnetism and the pull of a magnet will be less. In the case of cast iron, the pull will reduce by as much as 40% because cast iron is much less permeable than mild steel.
Subjecting a magnet to temperatures above its maximum operating temperature will cause it to lose performance that won’t be recovered on cooling. Repeatedly heating beyond the maximum operating temperature will result in a significant decrease in performance.
It is five times easier to slide a magnet than to pull it vertically away from the surface it is attracting to. This is entirely down to the coefficient of friction which is typically 0.2 for steel on steel faces. Magnets with a rated pull of 10kg will only support 2kg if they are being used on a vertical steel wall and the load is causing the magnets to slide down the wall.