Spin gap in low-dimensional magnets

The main mechanisms of formation of a spin gap in low-dimensional metaloxide compounds are reviewed. Among the objects in which a singlet ground state is formed are dimers, alternating chains with half-integer spin, uniform chains with integer spin, spin ladders, and two-dimensional ensembles of exchange-coupled spins. In some of these systems the spin gap in the spectrum of magnetic excitations is present from the start, while in others it is formed as a result of magnetoelastic interaction, charge ordering, or orbital ordering.