  The spin-1 Ising-Heisenberg diamond chain in a magnetic field is exactly
solved by a rigorous treatment based on the transfer-matrix method. An exact
ground-state phase diagram includes in total three unconventional quantum
ground states with a quantum entanglement of the decorating spin-1 Heisenberg
dimers apart from two ground states with a classical spin arrangement. It is
evidenced that all three values of the magnetization allowed for the spin-1
diamond chain without translationally broken symmetry by the
Oshikawa-Yamanaka-Affleck criterion can become evident in an outstanding
stepwise magnetization curve with three intermediate plateaus at zero,
one-third, and two-thirds of the saturation magnetization.
