  Ordered double perovskite oxides of the general formula, A2BB'O6, have been
known for several decades to have interesting electronic and magnetic
properties. However, a recent report of a spectacular negative
magnetoresistance effect in a specific member of this family, namely Sr2FeMoO6,
has brought this class of compounds under intense scrutiny. It is now believed
that the origin of magnetism in this class of compounds is based on a novel
kinetically-driven mechanism. This new mechanism is also likely to be
responsible for the unusually high temperature ferromagnetism in several other
systems, such as dilute magnetic semiconductors, as well as in various
half-metallic ferromagnetic systems, such as Heussler alloys.
