A molecule that can be magnetized in an external magnetic field and remains magnetized when the field is removed. It usually consists of atoms of transition metals, such as manganese or iron, separated by non-magnetic atoms or groups of atoms; here the magnetism is due not to interactions between molecules but to superexchange interactions between the transition metal atoms in each molecule. It is also possible to have molecular magnets even when there are no transition metal atoms present, particularly if one of the magnetic units has a stable triplet ground state. Molecular magnets have proved very useful for studying many aspects of magnetism.