When the field is applied, there is now a partial alignment of the atomic magnetic moments in the direction of the field (see Figure 1.2a) resulting in a net positive magnetization and hence positive susceptibility. Also, the efficiency of the field in aligning the magnetic moments of the paramagnetic materials is reduced by the temperature induced randomizing effect, which results in a temperature dependent susceptibility, known as the Curie-Weiss Law. Typical examples of paramagnetic material are biotite (silicate), siderite (carbonate), pyrite (sulfide), and montmorillonite (clay).

Figure 1.2: Magnetic response of a paramagnetic material with the (a) applied field and (b) temperature.

3. Ferromagnetism (FM):

The atomic moments in FM materials exhibit very strong interactions. These interactions are produced by electronic exchange forces and result in either a parallel or an antiparallel alignment of atomic moments (see Figure 1.3). These exchange forces are considerably large, approximately a 100 million times the strength of the earth's field or equivalent to a field of about 1000 Tesla. Moreover, the exchange force is a quantum mechanical phenomenon due to the relative orientation of the spins of electrons. FM materials exhibit parallel alignment of moments resulting in large net magnetization even after removing the external applied magnetic field.

Figure 1.3: Schematic representation of parallel alignment in FM materials along the applied field direction.