A magnet is an object made of certain materials which create a magnetic field. Every magnet has at least one north pole and one south pole. By convention, we say that the magnetic field lines leave the North end of a magnet and enter the South end of a magnet. This is an example of a magnetic dipole ("di" means two, thus two poles). If you take a bar magnet and break it into two pieces, each piece will again have a North pole and a South pole. If you take one of those pieces and break it into two, each of the smaller pieces will have a North pole and a South pole. No matter how small the pieces of the magnet become, each piece will have a North pole and a South pole. It has not been shown to be possible to end up with a single North pole or a single South pole which is a monopole ("mono" means one or single, thus one pole).
Ferromagnetism When a ferromagnetic material is placed near a magnet, it will be attracted toward the region of greater magnetic field. This is what we are most familiar with when our magnet picks up a bunch of paperclips. Iron, cobalt, nickel, gadolinium, dysprosium and alloys containing these elements exhibit ferromagnetism because of the way the electron spins within one atom interact with those of nearby atoms. They will align themselves, creating magnetic domains forming a temporary magnet. If a piece of iron is placed within a strong magnetic field, the domains in line with the field will grow in size as the domains perpendicular to the field will shrink in size.
Diamagnetism When a diamagnetic material is placed near a magnet, it will be repelled from the region of greater magnetic field, just opposite to a ferromagnetic material. It is exhibited by all common materials, but is very weak. Metals such as bismuth, copper, gold, silver and lead, as well as many nonmetals such as graphite, water and most organic compounds are diamagnetic.
Paramagnetism When a paramagnetic material is placed near a magnet, it will be attracted to the region of greater magnetic field, like a ferromagnetic material. The difference is that the attraction is weak. It is exhibited by materials containing transition elements, rare earth elements and actinide elements. Liquid oxygen and aluminum are examples of paramagnetic materials.
The types of magnets There are three main types of magnets:
Permanent magnets
Temporary magnets
Electromagnets
Permanent Magnets Permanent magnets are those we are most familiar with, such as the magnets hanging onto our refrigerator doors. They are permanent in the sense that once they are magnetized, they retain a level of magnetism. As we will see, different types of permanent magnets have different characteristics or properties concerning how easily they can be demagnetized, how strong they can be, how their strength varies with temperature, and so on.
Temporary Magnets Temporary magnets are those which act like a permanent magnet when they are within a strong magnetic field, but lose their magnetism when the magnetic field disappears. Examples would be paperclips and nails and other soft iron items.
Electromagnets An electromagnet is a tightly wound helical coil of wire, usually with an iron core, which acts like a permanent magnet when current is flowing in the wire. The strength and polarity of the magnetic field created by the electromagnet are adjustable by changing the magnitude of the current flowing through the wire and by changing the direction of the current flow.
Materials used for permanent magnets There are four classes of permanent magnets:
Neodymium Iron Boron (NdFeB) magnets
Samarium Cobalt (SmCo) magnets
Aluminium nickel cobolt (AlNiCo) magnets
Ceramic (Ferrite) magnets
The table below gives us some of the special characteristics of the four classes of magnets. Br is the measure of its residual magnetic flux density in Gauss, which is the maximum flux the magnet is able to produce. ( 1Gauss is like 6.45 lines/sq in) Hc is the measure of the coercive magnetic field strength in Oersted, or the point at which the magnet becomes demagnetized by an external field. ( 1Oersted is like 2.02 ampere-turns/inch) BHmax is a term of overall energy density. The higher the number, the more powerful the magnet. Tcoef of Br is the temperature coefficient of Br in terms of % per degree Centigrade. This tells you how the magnetic flux changes with respect to temperature. -0.20 means that if the temperature increases by 100 degrees Centigrade, its magnetic flux will decrease by 20%! Tmax is the maximum temperature the magnet should be operated at. After the temperature drops below this value, it will still behave as it did before it reached that temperature (it is recoverable). (degrees Centigrade) Tcurie is the Curie temperature at which the magnet will become demagnetized. After the temperature drops below this value, it will not behave as it did before it reached that temperature. If the magnet is heated between Tmax and Tcurie, it will recover somewhat, but not fully (it is not recoverable).
Material
Br
Hc
BHmax
Tcoef of Br
Tmax
Tcurie
NdFeB
12,800
12,300
40
-0.12
150
310
SmCo
10,500
9,200
26
-0.04
300
750
AlNiCo
12,500
640
5.5
-0.02
540
860
Ferrite
3,900
3,200
3.5
-0.20
300
460
Both the NdFeB and the SmCo magnets are generally known as rare earth magnets since their compounds come from the rare earth or Lanthanide series of theperiodic table of the elements. They were developed in the 1970's and 1980's. As can be seen in the table, these are the strongest of the permanent magnets, and are difficult to demagnetize. However, the Tmax for NdFeB is the lowest. AlNiCo magnet is made of a compound of aluminum, nickel and cobalt. AlNiCo magnets were first developed in the 1940's. As can be seen in the table, this magnet is least affected by temperature, but is easily demagnetized. This is the reason why bar magnets and horseshoe magnets made of AlNiCo will easily become demagnetized by other magnets, by dropping it, and by not storing it with a keeper. Its Tmax, though, is the highest.
Ferrite magnets are the most popular types of magnets available today. The flexible magnets we use are a type of ferrite magnet, with the magnetic powders fixed in a flexible binder. These were first developed in the 1960's. This is a fairly strong magnet, not as easy to demagnetize as AlNiCo, but its magnetic strength will vary the most as its temperature changes.
Shapes Permanent magnets can be made in most any shape imaginable. They can be made into round bars, rectangular bars, horseshoes, rings or donuts, disks, rectangles, multi-fingered rings, and other custom shapes. Some are cast into a mold and require grinding to achieve final dimensions. Others start as a powder which is pressed into a mold or pressure bonded or sintered.
How are magnets made? There are 6 basic steps to making a magnet, such as a Neodymium Iron Boron magnet = Nd2Fe14B or Nd15Fe77B8.
1. Make an alloy of iron, boron and neodymium. You will need about 14 grams of boron and 369 grams of neodymium for every 1000 grams of iron to make an alloy of Nd2Fe14B. This will have to be heated above 1538 degrees Centigrade to make it melt. The mixing of the materials with the iron is very important, just like thoroughly mixing the ingredients for a cake.
2. Grind the alloy into a powder. After the alloy has cooled, you will need to grind it or mill it into a very fine powder.
3. Compress the powder into a shape. Since the magnet will have a specific shape when you are done, you use a mold of that shape to make the magnet. For example, you may want a disk. Pour the powder into a mold that has a disk shape, but is also deeper than the thickness of the final part. Next, you will compress the powder with hundreds of pounds of pressure to compact the powder into a solid disk. Heat is often used to help fuse the particles together, and is called a sintered magnet. Sometimes a glue is used to help keep it all together, and is considered to be a bonded magnet. To achieve precise final dimensions, you may need to grind the part.
4. Coat the magnet. In order to improve the corrosion resistance of the magnet, the disk needs to be plated with a thin film of nickel. Sometimes a film of gold is used, or zinc, or an epoxy coating.. Nickel does not oxidize like iron, so it works great for magnets you will be touching.
5. Magnetize the magnet. All this time, the powder and the disk is not magnetized. It would be attracted to and stick to a magnet, but it would not be able to pick up a paper clip all by itself. So, it would be placed into a magnetizing fixture that has a coil of wire through which a very large pulse of current is passed for a very short period of time. The magnet has to be held in place so it doesn't shoot out and hit something or someone. It takes about a thousandth of a second to actually magnetize the magnet.
6. Pack and ship it. You now have a magnet for whatever you need. Engineers often require special shapes or specific magnetization configurations to make the product they are designing work properly. They talk with the magnet manufacturer and they determine how to best make the magnet that is needed. That's why there are so many different shapes and sizes of magnets in the catalogs.
Magnetization configurations How the magnet is magnetized is as important as its shape. For example, a segment magnet can be magnetized where N is on the inside and S on the outside, or N is on one edge and S on the opposite edge, or N is on the top side and S on the bottom side, or multiple N and S poles all around the outside edge, etc. A big help in visualizing how a magnet may be magnetized is by using a magnetic viewing film. Obtain one of these viewing cards, and look at the magnets you have around your house. The white line marks the boundary between the N and S poles. Make a sketch of what each magnet looks like under the viewing film. You will be surprised by some.
