Paramagnetism describes a form of magnetic behavior where certain materials are weakly attracted to an externally applied magnetic field. This attraction is significantly weaker than the pull felt by common magnets. The defining characteristic of paramagnetic substances is their inability to retain any magnetization once the external magnetic field is completely removed. This temporary magnetic response classifies paramagnetism as a unique category within material science.
The Mechanism of Paramagnetism
In paramagnetic substances, the temporary attraction is due to the presence of unpaired electrons within the atomic or molecular structure. Electrons possess an intrinsic property called spin, which causes each one to act like a tiny magnet, creating a magnetic moment.
When electrons are paired, their opposite spins cause their magnetic moments to cancel out. In paramagnetic atoms, however, unpaired electrons result in a net, permanent magnetic moment for the atom itself. In the absence of an external magnetic field, thermal energy causes these atomic magnets to be randomly oriented, meaning their individual magnetic effects cancel out across the material.
When an external magnetic field is applied, it exerts a torque on the atomic magnetic moments, causing them to partially align in the direction of the field. This alignment produces the overall weak attraction of the material to the magnetic source. The resulting induced magnetization is linear with the strength of the external field, but this alignment is constantly being fought by the material’s thermal energy.
The randomizing effect of thermal motion causes the material to instantly lose its magnetism when the external field is taken away. The slight degree of order imposed by the field is immediately overcome by the agitation of the atoms, returning the magnetic moments to their scattered, non-aligned state. This balance between the aligning force of the magnetic field and the disordering force of temperature governs paramagnetism.
Paramagnetism Versus Other Magnetic Types
Paramagnetism is one of three primary ways materials interact with magnetic fields, distinguished by the strength and permanence of the response. Paramagnetic materials are weakly attracted to a magnetic field, contrasting with diamagnetic materials, which are weakly repelled by it.
Diamagnetic substances have only paired electrons, meaning their atoms have no permanent magnetic moment. The repulsive effect is caused by the external field slightly distorting the electron orbits, inducing a transient magnetic moment that opposes the applied field.
Ferromagnetism, exemplified by iron and nickel, represents the strongest form of magnetic attraction. Ferromagnetic materials possess magnetic domains, which are regions where atomic magnetic moments are permanently aligned with one another. When a field is applied, these domains reorient and remain aligned even after the field is removed, resulting in a permanent magnet.
The strength of attraction is millions of times greater for ferromagnets compared to paramagnets. Paramagnetism is highly dependent on temperature, described by Curie’s Law, which states that magnetic susceptibility decreases as the temperature rises. Ferromagnetic materials lose their permanent magnetic properties entirely when heated above the Curie point, transitioning into a paramagnetic state.
Everyday Examples of Paramagnetic Materials
Many common elements and compounds exhibit paramagnetic properties, though their weak attraction is often not observed in everyday settings. Aluminum is a familiar example of a metal that is paramagnetic, along with other elements like Platinum and Titanium. In their liquid state, some gases also demonstrate this behavior, most notably oxygen.
Liquid oxygen is strongly paramagnetic and can be visibly suspended between the poles of a powerful magnet. A relevant application of paramagnetism is found in medical technology, particularly in Magnetic Resonance Imaging (MRI). Certain paramagnetic substances are used as contrast agents to enhance the clarity of the images produced by the scanner.
Gadolinium, a strongly paramagnetic element, is often incorporated into these contrast agents. When injected into a patient, the gadolinium compound aligns with the MRI machine’s powerful magnetic field, altering the magnetic environment of nearby water molecules. This change in the local magnetic field accelerates the relaxation of protons, which allows for better differentiation between various tissues in the resulting image.