  Spin-1/2 orthogonal-dimer chain composed of regularly alternating Ising and
Heisenberg dimers is exactly solved in a presence of the magnetic field by the
transfer-matrix method. It is shown that the ground-state phase diagram
involves in total six different phases. Besides the ferromagnetic phase with
fully polarized spins one encounters the singlet antiferromagnetic and
modulated antiferromagnetic phases manifested in zero-temperature magnetization
curves as zero magnetization plateau, the frustrated ferrimagnetic and singlet
ferrimagnetic phases causing existence of an intermediate one-half
magnetization plateau, and finally, the intriguing modulated ferrimagnetic
phase with a translationally broken symmetry leading to an unconventional
one-quarter magnetization plateau. The quantum character of individual ground
states is quantified via the concurrence, which measures a strength of the
bipartite entanglement within the pure and mixed states of the Heisenberg
dimers at zero as well as nonzero temperatures. The parameter region, where the
bipartite entanglement may be in contrast to general expectations reinforced
upon increasing of temperature and/or magnetic field, is elucidated.
