Which metals are magnetic?
What is a magnet?
A magnet is an object that generates a magnetic field, these always contain metal, however, not all metals are magnetic. Magnets are almost everywhere you look, in applications as tiny as microchips all the way up to industrial motors, fuel pumps and power generators. In fact, the famous particle accelerator the Large Hadron Collider consists of roughly 10,000 superconducting magnets spanning 27 kilometres!
The most commonly thought of magnetic metal is iron. This also means any alloy containing iron is also magnetic to some degree, including many types of steel. Below id a brief overview of different types of magnetic metals.
Ferromagnetic materials are usually chosen for a combination of strength, corrosion-resistance and coercivity (the ability to resist demagnetisation when exposed to an external magnetic field).
Some rare-earth metals, nickel, cobalt, iron, and alloyed metals that contain these elements are ferromagnetic.
The most common of these are ferrite magnets, made from an alloy of iron oxide and one or more other metal. They are divided into two basic subsets, hard ferrites and soft ferrites.
Hard ferrites, also known as ceramic or permanent magnets, are extremely common and cheap to produce so are found in many household products such as the classic fridge magnet. Alnico magnets (so called as they are primarily composed of aluminium, nickel, and cobalt) are an inexpensive and common example of a strong and stable permanent magnet.
Hard ferrites are also commonly produced by alloying barium or strontium with iron oxide.
Soft ferrites lose their magnetism much quicker and are often found in electronic applications such as transformers and inductors. Common examples included Manganese-Zinc alloys (MnZn) and Nickel-Zinc alloys (NiZn). Soft ferrites are sometimes referred to as temporary or non-permanent magnets.
Simply speaking, an electromagnet is produced by coiling a wire (usually copper) around a metal core made from a ferromagnetic metal such as iron or nickel and sending an electric current down it to generate a magnetic field within the coil. The amount of current running through the wire directly affects the strength of the magnetic field created within it, and magnetism is lost as soon as the current is stopped. Electromagnets can be found in motors, generators, loudspeakers and MRI machines; anywhere that requires an adjustable magnetic field.
Rare Earth – Samarium-Cobalt and Neodymium Magnets
Despite the name, the rare-earth elements are actually found in relative abundance, though they aren’t as evenly distributed or easy to find as other materials. Rare-earth magnets produce a far greater magnetic field than the average ferrite magnet, although they are fragile, brittle and some corrode easily so tend to require additional protection such as plating.
Samarium-Cobalt magnets (unsurprisingly made from the two metallic elements, samarium and cobalt) are extremely strong, corrode less easily and are resistant to demagnetisation and utilised for their ability to perform under extreme temperatures, up to 300 °C. For this reason they can be found in generators, motors, pumps, and many industries that require exposure to the elements. They are also recognisable for their use in a series of well-known Fender guitar pickups.
Neodymium magnets (made from the metals neodymium and iron and the metalloid boron) are also highly coercive and are the strongest kind of permanent magnet available today. Despite their relatively high cost are becoming more and more commonplace; applications include HDDs (computer hard disk drives), loudspeakers, cordless power tools, heavy-duty locks and electric vehicles.
What are non-magnetic metals?
There are other types of magnetic materials, but for the purpose of this article and day-to-day applications of magnets, these would be considered non-magnetic as they are not attracted to a magnet in a way that could be felt or used in normal circumstances.
Paramagnetic materials such as tungsten, aluminium, platinum, and magnesium are extremely weakly magnetic, hundreds of thousands of times weaker than a regular ferrite magnet.
Diamagnetic materials are repellent at both poles of a magnetic field. Examples of diamagnetic metals include gold, mercury and elemental copper.
Ferritic stainless steels, as you would probably expect, are generally magnetic due to their higher iron content, however, some stainless-steel grades are non-magnetic as they are austenitic, (such as 304 and 316) meaning they have been alloyed with a combination of either nickel, manganese, or nitrogen to achieve a specific crystal microstructure that results in them being unaffected by a magnet.
Finally, as precious metals such as silver and gold are non-magnetic, using a magnet is also a good test of the purity of your jewellery; if it is attracted to a magnet, you might want to head back to the jeweller!