Paramagnetism
Introduction
Paramagnetism is a form of magnetism whereby certain materials are attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, diamagnetic materials are repelled by magnetic fields and form induced magnetic fields in the direction opposite to that of the applied magnetic field. Paramagnetic materials include most chemical elements and some compounds; they have a relative magnetic permeability slightly greater than 1 (i.e., a small positive susceptibility to magnetic fields) and hence are attracted to magnetic fields. Paramagnetic materials have a small, positive susceptibility to magnetic fields. The magnetic moment induced by the applied field is linear in the field strength and rather weak.
Theory
The attraction experienced by a paramagnetic material to an external magnetic field, although weak, is distinct from the attraction exhibited by ferromagnetic materials. This attraction is due to the alignment of individual atomic magnetic moments with the external field. Unlike ferromagnets, paramagnets do not retain any magnetization in the absence of an externally applied magnetic field because thermal motion randomizes the spin orientations. Thus, the total magnetization drops to zero when the applied field is removed.
Paramagnetism requires that the atoms individually have both a permanent magnetic moment and a magnetic moment that can be induced in them. This can occur if the atom is ionized such that it has an incomplete inner electron shell, or if the spin of one or more of its electrons is unpaired. In these cases, the electron orbitals are not completely filled, and the electrons can move freely.
Paramagnetic Materials
Paramagnetic materials, those with unpaired electrons, include both atoms and molecules. They can be metal ions such as iron and copper. The higher the number of unpaired electrons, the stronger the paramagnetic behavior. A simple model for the behavior is a molecule with one electron in an orbital of the molecule. The electron would move in the magnetic field and create a magnetic moment opposing the applied field.
Curie's Law
Curie's Law describes the paramagnetic properties of materials at different temperatures. The law states that the magnetization of a paramagnetic material is directly proportional to an applied magnetic field. However, it is inversely proportional to temperature. Therefore, the magnetic susceptibility of paramagnetic materials is inversely proportional to the absolute temperature.
Applications
Paramagnetism finds its applications in a diverse range of fields. It is used in the field of Magnetic Resonance Imaging (MRI) where paramagnetic contrast agents are used to enhance the contrast between different tissues, making it easier to differentiate between them.
Paramagnetic materials are also used in the design of magnetic sensors, which are devices used to measure magnetic fields. These sensors can be used in a variety of applications, such as detecting the presence of ferromagnetic materials in non-ferrous materials.