Paramagnetism

Paramagnetism is a specific type of magnetism found in certain materials. These materials are weakly attracted to an externally applied magnetic field. When a field is present, the material forms an internal, induced magnetic field. This internal field points in the same direction as the external one. This behavior is the opposite of diamagnetism. Diamagnetic materials are actually repelled by magnetic fields. In diamagnetism, the induced field points in the opposite direction of the applied field.

The mechanism behind paramagnetism is found at the level of electrons. Every electron possesses a property called spin. Because of this spin, an unpaired electron acts like a tiny magnetic dipole. In most atoms, electrons exist in pairs within their atomic orbitals. When electrons are paired, their magnetic moments cancel each other out. However, atoms with incompletely filled orbitals often have unpaired electrons. These unpaired electrons create a permanent magnetic moment. When an external magnetic field is applied, it exerts a torque on these dipoles. This force causes the electron spins to align parallel to the field. This alignment creates a net attraction to the magnet.

There are different ways this magnetic response can manifest in matter. Most paramagnetic materials consist of localized electrons. In these cases, the magnetic moments come from individual atoms or molecules. A clear example is molecular oxygen. Even as a frozen solid, oxygen contains di-radical molecules. These molecules have unpaired spins in their p orbitals. Another type is known as Pauli paramagnetism. This occurs in conductive metals where electrons are delocalized. In these solids, electrons travel freely through the material. When a field is applied, the conduction band splits into spin-up and spin-down bands. This creates a small surplus of one spin type, resulting in a weak magnetic response.

Scientists use specific rules to describe how these materials behave. One important principle is Curie's law. This law states that magnetic susceptibility is inversely proportional to absolute temperature. In simpler terms, paramagnetic materials become more magnetic as they get colder. This happens because thermal motion causes random orientations of the spins. At high temperatures, this jiggling overcomes the magnetic alignment. As temperature decreases, the thermal agitation weakens. This allows the external field to align the spins more effectively. However, if the field is extremely strong or the temperature is extremely low, the material reaches saturation. At saturation, all dipoles are already aligned, so the magnetization cannot increase further.

Paramagnetism is much weaker than ferromagnetism. Ferromagnetic materials, like the iron in a refrigerator magnet, have a very strong and permanent attraction. They can retain magnetization even after the external field is removed. In contrast, paramagnets do not retain magnetization. Once the external field is gone, thermal motion randomizes the spin orientations again. The total magnetization drops to zero. The magnetic susceptibility for most paramagnets is quite small, typically between 10⁻⁳ and 10⁻⁵. Some special synthetic materials, called ferrofluids, can reach a much higher susceptibility of 10⁻¹.

Many different elements and compounds exhibit paramagnetic properties. Common examples include aluminum, titanium, and iron oxide (FeO). In the periodic table, elements with d or f electrons often show stronger magnetic effects. This is because these electrons are frequently more localized. For instance, lanthanide atoms can have very large magnetic moments. Gadolinium(III) can carry up to seven unpaired electrons. Because of these high moments, lanthanides are used in MRI technology. Other elements like platinum, tungsten, and magnesium also show measurable paramagnetic susceptibility. Platinum has a susceptibility of 26 × 10⁻⁵, while magnesium is much lower at 1.2 × 10⁻⁵.

Understanding paramagnetism helps scientists study the fundamental nature of matter. It connects the study of chemistry to quantum mechanics. The true origins of spin alignment can only be explained through quantum properties like angular momentum. Researchers use highly sensitive tools to detect these weak forces. One such tool is the SQUID magnetometer. This device allows for the precise measurement of the tiny magnetic moments produced by paramagnetic substances. By studying these effects, scientists gain insight into how electrons occupy energy bands and how temperature influences atomic structure.