  We study properties of localized effective spins induced in gapped quantum
spin chains by local inhomogeneities of the lattice. As a prototype, we study
effective spins induced in impunity sites doped AKLT model by constructing the
exact ground state in a matrix product state (MPS) form. We characterize their
responses to external fields by studying an extended Zeeman interaction. We
also study the antiferromagnetic bond-alternating Heisenberg chain with defect
structures. For this model, an MPS representation similar to that for the AKLT
model, "a uniform MPS with windows", is constructed, and it gives a good
approximation of the ground state. We discuss the trade-off relation between
the window length and the precision of the MPS ansatz. The effective exchange
interaction between the induced spins is also investigated by using this
representation.
