Real-space perturbation theory for frustrated magnets: application to magnetization plateaus

We present a unified approach to the problem of degeneracy lifting in geometrically frustrated magnets with and without an external field. The method treats fluctuations around a classical spin configuration in terms of a real-space perturbation expansion. We calculate two lowest-order contributions for the Heisenberg spin Hamiltonian and use them to study the magnetization processes of spin-$S$ triangular and kagom\'e antiferromagnets.