  The bipartite entanglement across the magnetization process of a highly
frustrated spin-1/2 Heisenberg octahedral chain is examined within the concept
of localized magnons, which enables a simple calculation of the concurrence
measuring a strength of the pairwise entanglement between nearest-neighbor and
next-nearest-neighbor spins from square plaquettes. A full exact
diagonalization of the finite-size Heisenberg octahedral chain with up to 4
unit cells (20 spins) evidences an extraordinary high precision of the
localized-magnon theory in predicting measures of the bipartite entanglement at
sufficiently low temperatures. While the monomer-tetramer phase emergent at low
enough magnetic fields exhibits presence (absence) of the bipartite
entanglement between the nearest-neighbor (next-nearest-neighbor) spins, the
magnon-crystal phase emergent below the saturation field contrarily displays
identical bipartite entanglement between the nearest-neighbor and
next-nearest-neighbor spins. The presented results verify a new paradigm of the
localized-magnon approach concerned with a simple calculation of entanglement
measures.
