Based on the magnetic response, materials can be classified into the following five major groups: (1) Diamagnetism; (2) Paramagnetism; (3) Ferromagnetism; (4) Ferrimagnetism; and (5) Antiferromagnetism. While the materials in the first two groups do not exhibit any collective magnetic exchange interactions and are not magnetically ordered, the next three groups exhibit long-range magnetic order below a certain temperature, called critical (or Curie/Neel) temperature. Ferromagnetic and ferrimagnetic materials are generally considered as magnetic and the remaining three are assumed to be non-magnetic. In addition to these five groups, there exists a new type of materials called superparamagnetism. Although it is NM in its demagnetized state, it exhibits magnetic saturation behavior similar to FM materials under an applied field. Here, we shall briefly discuss all types of magnetism.

1. Diamagnetism:

Diamagnetism, a very weak fundamental property of all matter, arises due to the non-cooperative behavior of orbiting electrons when exposed to an externally applied magnetic field. Diamagnetic substances are composed of atoms which have no net magnetic moments, i.e., all the orbital shells are filled and there are no unpaired electrons. However, a negative magnetization is generated when exposed to external magnetic field, and hence the susceptibility, a ratio between the induced magnetization and the applied magnetic field, is negative (see Figure 1.1a). Note that the magnetization is zero when the external applied field is zero. The other characteristic behavior of diamagnetic materials is that the susceptibility is generally temperature independent (see Figure 1.1b). Well-known diamagnetic substances are quartz, calcite, and water. A superconductor in its superconducting states is an example of an ideal diamagnet.

Figure 1.1: (a) Magnetic response of a diamagnetic material to an applied field and (b) the variation of diamagnetic susceptibility with temperature.

2. Paramagnetism:

This class of the materials has a net magnetic moment due to the existence of unpaired electrons in partially filled orbitals. However, the individual magnetic moments do not interact magnetically and hence the magnetization is zero when there is no external applied field or when the externally applied field is removed.