Ferromagnetism, antiferromagnetism and ferrimagnetism

When a bridging ligand facilitates the coupling of electron spins on adjacent metal centres, the mechanism is one of superexchange. This is shown schematically in diagram 21.11, in which the unpaired metal electrons are represented in red. 

In a superexchange pathway, the unpaired electron on the first metal centre, Mj, interacts with a spin-paired pair of electrons on the bridging ligand with the result that the unpaired electron on M2 is aligned in an antiparallel manner with respect to that on Mj. 

Whenever we have mentioned magnetic properties so far, we have assumed that metal centres have no interaction with each other (Fig. 20.32a). This is true for substances where the paramagnetic centres are well separated from each other by diamagnetic species. Such systems are said to be magnetically dilute (see Section 4.9). For a paramagnetic material, the magnetic susceptibility, x. is inversely proportional to temperature. This is expressed by the Curie law 

Antiferromagnetism occurs below the Neel temperature, TW. As the temperature decreases, less thermal energy is available and the paramagnetic susceptibility falls 

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There are several types of magnetic behaviour that affect the specific heat of a material paramagnetism, ferromagnetism, antiferromagnetism and ferrimagnetism. Diamagnetism, being independent of temperature, causes no specific heat contribution and is not considered. 

FIGURE 33.11 Schematic comparing dipole alignments in ferromagnetic, antiferromagnetic, and ferrimagnetic materials. 

Long-range order ferromagnetism, antiferromagnetism and ferrimagnetism... 

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