Magnet

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Magnets are a commonly-encountered form of material that can be useful to the amateur chemist.

General magnetic properties

Curie temperature

The Curie temperature represents the temperature at which all magnetic crystal domains will realign randomly, effectively causing the magnet to lose all traces of magnetic force. This temperature is useful for removing the dangerous magnetic fields of stronger magnets.

Magnetic field strength

The magnetic field strength is how much force the magnet can exert at a given size and magnetization. For example, a 1-pound 2"x2"x1" block of neodymium-iron-boron alloy, when fully magnetized, can lift over 500 pounds of weight.

Types of magnets

Neodymium-iron-boron magnets

Neodymium magnets are composed of an alloy of neodymium, iron and boron surrounded by a protective, oxygen-excluding metallic shell. See Neodymium iron boron magnet for more information. These magnets are approximately 27% Nd by weight. The typical range of Curie temperature for this alloy is 310 to 340 degrees C. These alloys may contain significant (>10%) amounts of cerium, gadolinium, praseodymium, and other rare earths, as substituting these for Nd does not significantly impact the magnetic properties.

Samarium-cobalt magnets

Samarium-cobalt magnets are composed of an alloy of samarium and cobalt, typically surrounded by a protective metallic or Teflon shell. Some laboratory stir bars may use the SmCo alloy as the magnetic material owing to its higher temperature resistance than neodymium magnets. Two types of SmCo alloys are commonly made: SmCo5 and Sm2Co17. The first is approximately 34% Sm by weight, and the second is approximately 23% Sm by weight. The typical range of Curie temperature for this alloy is 700 to 800 degrees C.

Alnico magnets

Alnico magnets are composed of an alloy of aluminium, nickel and cobalt, hence the name. These magnets' composition is not as tightly controlled as are rare-earth magnets', and as such the magnet composition can consist of a range of 8-12% Al, 15-26% Ni, 5-24% Co, 0-6% Cu, 0-1% Ti, and the remainder is Fe.

Ferrite magnets

Ferrite magnets are chiefly composed of a ceramic magnetic crystal form of iron oxide, with trace additions of nickel, manganese, zinc, strontium, and/or barium to create a spinel crystal structure consisting of the compound AB2O4, where A is Fe, B is some 2+ cation such as zinc or barium, and O is oxygen.

Availability

Magnets can be cheaply purchased online or from the electronic stores. You can also salvage them from old machines, toys, electronics, though old ferrite and alnico magnets tends to become weaker with age, and they aren't very useful.

Projects

Handling

Safety

Rare earths can cause heavy metal poisoning if ingested, but are otherwise safe to handle with basic PPE. Magnetized materials, however, can exert strong enough forces to injure the human body, and should thus be either demagnetized at their Curie temperature or kept away from other magnetic/ferrous materials.

Storage

Stronger magnets such as neodymium/samarium alloy magnets should be stored in soft foam padding, with spacers between individual magnets to avoid injury. Weaker varieties such as Alnico and ferrites can be stored anywhere.

Disposal

Stronger magnets should always be demagnetized by heating before disposal to prevent injury.

References

Relevant Sciencemadness threads