Permanent magnets are vital for many modern everyday appliances. You are never far from a permanent magnet,
you are likely to have one in front of you right now, or if you are reading this article on a smartphone, you might even have one in your hand!
What is Permanent magnets?
Permanent magnets are materials where the magnetic field is generated by the internal structure of the material itself. Inside atoms and crystals you have both electrons and the nucleus of the atom. Both the nucleus and the electrons themselves act like little magnets, like little spinning chunks of electric charge, and they have magnetic fields inherent in the particles themselves. There’s also a magnetic field that’s generated by the orbits of the electrons as they move about the nucleus. So the magnetic fields of permanent magnets are the sums of the nuclear spins, the electron spins and the orbits of the electrons themselves. In many materials, the magnetic fields are pointing in all sorts of random directions and cancel each other out and there’s no permanent magnetism. But in certain materials, called ferromagnets, all the spins and the orbits of the electrons will line up, causing the materials to become magnetic. This would be your normal iron, cobalt, nickel. Permanent magnets are limited by the structure of the material. And the strongest magnetic field of a permanent magnet is about 8,000 gauss. The strongest magnets here at the Magnet Lab are 450,000 gauss, which would be almost 50 times stronger than that.
There are two main different types of magnet, permanent magnets and electromagnets. A permanent magnet is called a permanent magnet because its magnetism is ‘always on’, it generates its own persistent magnetic field unlike an electromagnet which is made from a coil of wire wrapped around a ferrous core and requires an electric current to generate a magnetic field. An electromagnet’s magnetism can be controlled and turned off and on at the flick of a switch as the magnetism depends on a constant flow of electricity.
In addition to permanent magnets and electromagnets there are temporary magnets. Some metals are defined as ferromagnetic, this means that they exhibit their own magnetic properties and are defined as magnetically ‘soft’ materials. Permanent (hard) magnets and temporary (soft) magnets are both ferromagnetic but temporary magnets only display noticeable magnetic properties when influenced by a permanent magnet and tend to not stay magnetised. Magnetically soft materials such as steel conduct magnetism when attached to a magnet but this ceases when the magnet is removed.
Types of Permanent Magnet
You might ask, what makes a good permanent magnet? The answer isn’t always straightforward as it depends on what the permanent magnet is going to be used for. There are several types of permanent magnets, each manufactured differently from different materials with different properties. The five types of permanent magnets are alnico, samarium cobalt, ferrite, flexible rubber and the strongest permanent magnets, neodymium magnets.
You can follow the links to right for more information about each type of permanent magnet:
Super holding power / strongest magnetic material in the world.
|Samarium Cobalt Magnets
Extremely powerful, small size, rare earth magnets.
Powerful, economically priced and differing strengths.
Horseshoe, rod and bar shapes, feature high heat resistance.
How Does a Permanent Magnet Work?
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.
Magnetic Strength of Permanent Magnet
There are several measurements that all contribute to a permanent magnet’s strength, which can often seem confusing. Magnetic field strength (remanence), resistance to demagnetisation (coercivity) and pulling force are all often summarised as strength and all are desired attributes of a permanent magnet. The single primary indicator of a permanent magnet’s strength is its maximum energy product value measured in Mega Gauss Oersteds (MGOe). The higher the maximum energy product value, the greater the magnetic field the magnet will generate in a particular application.
Maximum energy product, also referred to as BHmax is calculated by multiplying a magnet’s remanence (Br) and coercivity (Hc). The strongest magnets in the world are neodymium magnets, which are manufactured in different grades, however, each grade is given a handy name that allows you to instantly judge which magnet is stronger. Commercially available neodymium magnets range from N35 to N52 grade; the number after the letter ‘N’ represents the magnet’s maximum energy product.
A magnet’s individual performance is altered by temperature and some types of magnets perform better at high temperatures than others, such as alnico and samarium cobalt magnets. All magnets lose a percentage of their magnetism per ℃ rise in temperature and the five types of magnetic material lose magnetism at different rates and have different maximum operating temperatures. If the temperature exceeds the maximum operating temperature of the individual grade it will be permanently demagnetised.
Most permanent magnets are inherently brittle and should not be utilised as structural components. Dimensions and tolerances vary from manufacturer to manufacturer but most will produce magnets to the tolerance of +/- 0.1mm for all dimensions quoted. Permanent magnets are produced in many shapes such as standard rings, bars and discs plus custom shapes such as trapezoid, arc, mitre and even the ‘top hat’.
Permanent magnets are often coated to improve their performance. Take neodymium magnets for example, they are the strongest permanent magnets available but they are the most prone to corrosion due to their high iron content, therefore they are virtually always supplied with a coating. Typical coatings include, nickel, stainless steel, PTFE (Teflon), epoxy, rubber, gold, titanium, chrome and many more.
Permanent magnets have several advantages that electromagnets do not. For example, permanent magnets do not require any power source and usually produce a powerful magnetic field compared to their size. Contrastingly, electromagnets must be continuously connected to a power source that may be quite large depending on the magnetic field needed.
Although permanent magnets are advantageous, they do have several disadvantages. For example, permanent magnets constantly produce a magnetic field and cannot be turned off like electromagnets. Likewise, it is not easy to control the intensity of a permanent magnet’s magnetic field. Furthermore, permanent magnets do not usually have a very large magnetic field, a property that makes it difficult to use them over long distances.
Permanent magnets can be produced with various magnetic directions. The term ‘magnetic length’ refers to the dimension of a magnet which follows the direction of a magnet’s magnetic axis. A magnet’s magnetic axis is always listed last when referring to a magnet’s physical dimensions.
The following diagrams show the types of magnetic direction of permanent magnets.
Oriented Through Thickness, Oriented Through Length, Multi-Pole Segments On Single Face, Axially Oriented In Segments, Axially Oriented Through Thickness, Multi-Pole on Outside Diameter, Multi-pole, Diametrically magnetised, Radially magnetised, Oriented Through Diameter, Oriented Through Diameter …
As permanent magnets are often out of sight it is easy overlook the important role they play in modern life.
Without permanent magnets, there would be no electric motors, loudspeakers, computers, hard disc drives, smartphones and many other commercial appliances. They also have a vital role in engineering, power generation, processing and manufacturing.