Paramagnetism refers to a property of certain materials that are weakly attracted to magnetic fields. When exposed to an external magnetic field, internal induced magnetic fields form in these materials that are ordered in the same direction as the applied field. Once the applied field is removed, the materials lose their magnetism as thermal motion randomizes the electron spin orientations.
Materials that display paramagnetism are called paramagnetic. Some compounds and most chemical elements are paramagnetic under certain circumstances. However, true paramagnets display magnetic susceptibility according to the Curie or Curie-Weiss laws and exhibit paramagnetism over a wide temperature range. Examples of paramagnets include the coordination complex myoglobin, transition metal complexes, iron oxide (FeO), and oxygen (O2). Titanium and aluminum are metallic elements that are paramagnetic.
Superparamagnets are materials that show a net paramagnetic response, yet display ferromagnetic or ferrimagnetic ordering at the microscopic level. These materials adhere to the Curie law, yet have very large Curie constants. Ferrofluids are an example of superparamagnets. Solid superparamagnets are also known as mictomagnets. The alloy AuFe (gold-iron) is an example of a mictomagnet. The ferromagnetically coupled clusters in the alloy freeze below a certain temperature.
How Paramagnetism Works
Paramagnetism results from the presence of least one unpaired electron spin in a material's atoms or molecules. In other words, any material that possesses atoms with incompletely filled atomic orbitals is paramagnetic. The spin of the unpaired electrons gives them a magnetic dipole moment. Basically, each unpaired electron acts as a tiny magnet within the material. When an external magnetic field is applied, the spin of the electrons aligns with the field. Because all the unpaired electrons align the same way, the material is attracted to the field. When the external field is removed, the spins return to their randomized orientations.
The magnetization approximately follows Curie's law, which states that the magnetic susceptibility χ is inversely proportional to temperature:
M = χH = CH/T
where M is magnetization, χ is magnetic susceptibility, H is the auxiliary magnetic field, T is the absolute (Kelvin) temperature, and C is the material-specific Curie constant.
Types of Magnetism
Magnetic materials may be identified as belonging to one of four categories: ferromagnetism, paramagnetism, diamagnetism, and antiferromagnetism. The strongest form of magnetism is ferromagnetism.
Ferromagnetic materials exhibit a magnetic attraction that is strong enough to be felt. Ferromagnetic and ferrimagnetic materials may remain magnetized over time. Common iron-based magnets and rare earth magnets display ferromagnetism.
In contrast to ferromagnetism, the forces of paramagnetism, diamagnetism, and antiferromagnetism are weak. In antiferromagnetism, the magnetic moments of molecules or atoms align in a pattern in which neighbor electron spins point in opposite directions, but the magnetic ordering vanishes above a certain temperature.
Paramagnetic materials are weakly attracted to a magnetic field. Antiferromagnetic materials become paramagnetic above a certain temperature.
Diamagnetic materials are weakly repelled by magnetic fields. All materials are diamagnetic, but a substance isn't usually labeled diamagnetic unless the other forms of magnetism are absent. Bismuth and antimony are examples of diamagnets.