Antiferromagnetism and Ferrimagnetism

The only type of magnetic order which has been considered thus for ferromagnetism, in which, in the fully magnetized state, all the dipoles are aligned in exactly the same direction. There are, however, substances which show different types of magnetic order. In antiferromagnetic materials such as Cr and Mno, the dipoles have equal moments, but adjacent dipoles point in opposite directions. Thus the moments balance each other, resulting in a zero net magnetization. The gross magnetic effects of such materials are very small. When heated above critical temperature, called the Neel temperature, the exchange interaction ceases to exist and these materials become paramagnetic.

In ferromagnetic materials (also called ferrites) such as Mn Fe2 O4, the magnetic moments of adjacent ions are anti parallel and of unequal strength. So there is a finite net magnetization. This results in a net magnetism, which lies in between ferromagnetism and antiferromagnetism. Here again, when the temperature is above a critical value, the material becomes paramagnetic.

By suitable choice of rare-earth ions in the ferrite lattices it is possible to design ferromagnetic substances with specific magnetizations for use in electronic components. Ferrites are good insulators and have very small eddy current losses when subjected to very high field. For this reasons, ferrites are often used in microwave systems.