Define the following :
ferromagnetic and ferrimagnetic crystals.

Explanation

Ferrimagnetic Substances and Ferrimagnetism
The property by virtue of which certain substances which are expected to be ferromagnetic but are in fact weakly attracted by magnetic fields is called ferrimagnetism and such substances are called ferrimagnetic substances. Atoms, molecules or ions of such substances not only have one or more unapired electrons, but these are also grouped into small rgions called domains as in ferromagnetic substances. However, in such substances these domains are aligned in parallel and antiparallel directions in unequal numbers and therefore such substances are weakly attracted by magnetic field as compared to ferromagnetic substances.
Examples : Magnetite `(Fe_(3)O_(4))` Ferrites like `MgFe_(2)O_(4),ZnFe_(2)O_(4)` etc.