paramagneto

Paramagnetism is a form of magnetism observed in materials that are weakly attracted to external magnetic fields due to the presence of unpaired electron spins. In the absence of a field, the random orientations of these spins average to zero, so there is no spontaneous magnetization. When a magnetic field is applied, the spins tend to align with the field, producing a small net magnetization.

The magnetic susceptibility of paramagnets is positive but small, meaning they are only weakly attracted. For many substances, the temperature dependence follows Curie’s law, χ = C/T, where χ is the magnetic susceptibility and C is a material-specific constant. In the presence of weak interactions between moments, the Curie-Weiss law χ = C/(T − θ) better describes the behavior, with θ (the Weiss temperature) typically small and positive or near zero. Paramagnetism arises in atoms or ions with unpaired electrons, such as transition metal ions (for example Mn2+, Fe3+), rare-earth ions, and molecules with unpaired spins, including the oxygen molecule O2. By contrast, materials with itinerant electrons can show Pauli paramagnetism, a temperature-independent form of weak paramagnetism.

Paramagnetism is distinguished from diamagnetism, where all electrons are paired and the material is repelled by magnetic fields, and from ferromagnetism, which features spontaneous magnetization and strong interactions between moments. Commonly studied paramagnets include aluminum, platinum, and certain oxide or mineral materials; oxygen is also paramagnetic. Techniques such as magnetometry and electron paramagnetic resonance (EPR) are used to study paramagnetic behavior, while nuclear paramagnetism underpins methods like nuclear magnetic resonance and magnetic resonance imaging.