Paramagnetism: Weak attraction through magnetic fields

Paramagnetism describes the property of certain materials to be weakly attracted by an external magnetic field. In contrast to ferromagnetic materials, paramagnetic materials do not retain their magnetisation as soon as the magnetic field is removed. This effect is due to the alignment of unpaired electron spins in the atoms or molecules of the material.

How does paramagnetism work?

Paramagnetic materials contain atoms or ions with incompletely filled electron shells. These have unpaired electrons that generate a magnetic moment. In the normal state, these magnetic moments are randomly aligned so that the material shows no macroscopic magnetisation.

However, if an external magnetic field is applied, the magnetic moments partially align along the field. This alignment leads to a weak force of attraction between the material and the magnetic field. As soon as the magnetic field is removed, the magnetic moments return to their random orientation.

Properties of paramagnetism

• Weak magnetisation: Paramagnetic materials show only a slight magnetic reaction compared to ferromagnetic materials.
• Temperature dependence: The strength of paramagnetism decreases with increasing temperature, as the thermal movement of the atoms disturbs the alignment of the magnetic moments.
• No remanence: In contrast to ferromagnetic materials, no magnetisation remains after the magnetic field is removed.

Examples of paramagnetic materials

Many materials exhibit paramagnetic behaviour, including:

• Metals: Aluminium, platinum
• Ions: Transition metal ions such as manganese (Mn²⁺), iron (Fe³⁺) and nickel (Ni²⁺)
• Gases: Oxygen (O₂) also shows weak paramagnetism

Difference between paramagnetism and ferromagnetism

Paramagnetism is fundamentally different from ferromagnetism:

• Magnetic moments: In ferromagnetic materials, the magnetic moments are fully aligned, whereas in paramagnetic materials they are only partially aligned.
• Remainence: Ferromagnetic materials retain their magnetisation after the magnetic field is removed, while paramagnetic materials do not.
• Strength: Paramagnetic effects are significantly weaker than ferromagnetic effects.

Applications of paramagnetism

Paramagnetic materials are used in various scientific and technological fields:

• Magnetic separation: Paramagnetic materials can be isolated from mixtures using strong magnetic fields.
• Medical imaging: Paramagnetic contrast agents such as gadolinium are used in magnetic resonance imaging (MRI) to make tissue structures more visible.
• Sensor technology: Certain gases such as oxygen are measured in gas analysers due to their paramagnetism.

Interesting facts about paramagnetism

Did you know that oxygen can be drawn between the poles of a strong magnet due to its paramagnetism in a liquid state? This fascinating behaviour is often demonstrated in experiments to illustrate the unique magnetic properties of molecules.