Here's an element factoid for you: Not all iron is magnetic. The a allotrope is magnetic, yet when the temperature increases so that the a form changes to the b form, the magnetism disappears even though the lattice doesn't change.
Key Takeaways: Not All Iron Is Magnetic
- Most people think of iron as a magnetic material. Iron is ferromagnetic (attracted to magnets), but only within a certain temperature range and other specific conditions.
- Iron is magnetic in its α form. The α form occurs below a special temperature called the Curie point, which is 770 °C. Iron is paramagnetic above this temperature and only weakly attracted to a magnetic field.
- Magnetic materials consist of atoms with partially-filled electron shells. So, most magnetic materials are metals. Other magnetic elements include nickel and cobalt.
- Nonmagnetic (diamagnetic) metals include copper, gold, and silver.
Why Iron Is Magnetic (Sometimes)
Ferromagnetism is the mechanism by which materials are attracted to magnets and form permanent magnets. The word actually means iron-magnetism because that is the most familiar example of the phenomenon and the one scientists first studied. Ferromagnetism is a quantum mechanical property of a material. It depends on its microstructure and crystalline state, which can be affected by temperature and composition.
The quantum mechanical property is determined by the behavior of electrons. Specifically, a substance needs a magnetic dipole moment in order to be a magnet, which comes from atoms with partially-filled electron shells. Atoms will filled electron shells are not magnetic because they have a net dipole moment of zero. Iron and other transition metals have partially-filled electrons shells, so some of these elements and their compounds are magnetic. In atoms of magnetic elements nearly all of the dipoles align below a special temperature called the Curie point. For iron, the Curie point occurs at 770 °C. Below this temperature, iron is ferromagnetic (strongly attracted to a magnet), but above it the iron changes its crystalline structure and become paramagnetic (only weakly attacted to a magnet).
Other Magnetic Elements
Iron isn't the only element that displays magnetism. Nickel, cobalt, gadolinium, terbium, and dysprosium are also ferromagnetic. As with iron, the magnetic properties of these elements depends on their crystal structure and whether the metal is below its Curie point. α-iron, cobalt, and nickel are ferromagnetic, while γ-iron, manganese, and chromium are antiferromagnetic. Lithium gas is magnetic when cooled below 1 kelvin. Under certain condition, manganese, the actinides (e.g., plutonium and neptunium), and ruthenium are ferromagnetic.
While magnetism most often occurs in metals, it also occurs rarely in nonmetals. Liquid oxygen, for example, may be trapped between the poles of a magnet! Oxygen has unpaired electrons, allowing it to react to a magnet. Boron is another nonmetal that displays paramagnetic attraction greater than its diamagnetic repulsion.
Magnetic and Nonmagnetic Steel
Steel is an iron-based alloy. Most forms of steel, including stainless steel, are magnetic. There are two broad types of stainless steels which display different crystal lattice structures from one another. Ferritic stainless steels are iron-chromium alloys that are ferromagnetic at room temperature. While normally unmagnetized, ferritic steel become magnetized in the presence of a magnetic field and remain magnetized for some time after the magnet is removed. The metal atoms in ferritic stainless steel are arranged in a body-centered (bcc) lattic. Austenitic stainless steels tend to be nonmagnetic. These steels contain atoms arranged in a face-centered cubic (fcc) lattice.
The most popular type of stainless steel, Type 304, contains iron, chromium, and nickel (each magnetic on its own). Yet, atoms in this alloy usually have the fcc lattice structure, resulting in a nonmagnetic alloy. Type 304 does become partly ferromagnetic if the steel is bent at room temperature.
Metals That Aren't Magnetic
While some metals are magnetic, most are not. Key examples include copper, gold, silver, lead, aluminum, tin, titanium, zinc, and bismuth. These elements and their alloys are diamagnetic. Nonmagnetic alloys include brass and bronze. These metals weakly repel magnets, but not usually enough that the effect is noticeable.
Carbon is a strongly diamagnetic nonmetal. In fact, some types of graphite repel magnets strongly enough to levitate a strong magnet.
- Devine, Thomas. "Why don't magnets work on some stainless steels?" Scientific American.