Magnetism is often understood through the strong pull exerted on materials like iron or steel. This intense attraction is characteristic of only a few substances; the vast majority of materials do not exhibit this response. These substances, colloquially called “non-magnetic,” include glass, wood, many common metals, and even air. While they do not visibly stick to a refrigerator magnet, they still interact with magnetic fields in scientifically quantifiable ways.
Defining Magnetic Responses
Materials are broadly classified into three categories based on how they react to an external magnetic field. The most familiar is ferromagnetism, describing the powerful attraction seen in materials such as iron, nickel, and cobalt. Ferromagnetic substances can retain their magnetism after the external field is removed, leading to permanent magnets. The two other classifications, which define “non-magnetic” materials, are paramagnetism and diamagnetism. Paramagnetic materials show a very weak attraction, while diamagnetic materials exhibit a very weak repulsion. Both responses are temporary, meaning the material immediately loses the induced magnetic property once the external field is taken away.
Paramagnetic Materials and Examples
Paramagnetic materials are slightly drawn toward an external magnetic field, but this attraction is extremely faint. The small positive magnetic susceptibility means the material’s internal magnetic alignment is momentarily strengthened in the direction of the applied field. This induced magnetism is temporary; the slight alignment of the atoms is quickly randomized by thermal energy once the external field is withdrawn. Common examples include the metal aluminum, used in foil and beverage cans, and platinum. Surprisingly, liquid oxygen also exhibits paramagnetism, and in a strong magnetic field, a stream of liquid oxygen can be visibly suspended between the poles of a magnet.
Diamagnetic Materials and Examples
Diamagnetic materials represent the second category of non-magnetic substances, characterized by weak repulsion to a magnetic field. When an external magnetic field is applied, these materials generate a faint opposing magnetic field, causing them to be slightly pushed away. This effect is present in all matter, but it is only the dominant response in materials lacking stronger paramagnetic or ferromagnetic effects. Water is one of the most common examples of a diamagnetic substance, meaning a powerful magnet will slightly repel it. Other examples include non-metallic materials like wood and most plastics, as well as metals such as copper, gold, and silver.
The Electron Structure Underlying Non-Magnetism
The difference between paramagnetic and diamagnetic materials depends on the configuration of electrons within their atoms. Electrons possess spin, which makes each one act like a tiny magnet. In many atoms, electrons exist in pairs with opposite spins, causing their individual magnetic moments to cancel each other out. Diamagnetic materials have all electrons paired, resulting in no net intrinsic magnetic moment. When an external field is applied, it slightly alters the orbital motion of these paired electrons, generating a weak magnetic field that opposes the external one. Conversely, paramagnetic materials contain one or more unpaired electrons, providing the atom with a small, permanent magnetic moment that can weakly align with an external magnetic field.