Figure 1.5: Schematic drawing of unequal magnitude of antiparallel alignment in FiM materials.

5. Antiferromagnetism:

If the sublattice moments M_↑ and M_↓ are exactly equal but opposite (see Figure 1.6), then the net moment in a given material is zero. This type of magnetic ordering is called antiferromagnetism (AFM). The clue to AFM is the behavior of susceptibility above a critical temperature, called the Néel temperature (T_N). Above T_N, the susceptibility obeys the Curie-Weiss law for paramagnets as shown in the Figure 1.7 but with a negative intercept indicating negative exchange interactions.

Figure 1.6: Schematic representation of equal magnitude of antiparallel alignment in AFM materials.

Figure 1.7: Variation of susceptibility and inverse susceptibility with temperature in an AFM material.

6. Superparamagnetism:

Superparamagnetism is another form of magnetism, which appears in ferromagnetic or ferrimagnetic nanoparticles. In small enough nanoparticles (diameter < 100 Å), magnetization direction is flipped randomly under the influence of thermal energy.