  The spin-1/2 Ising-Heisenberg branched chain, inspired by the magnetic
structure of three isostructural polymeric coordination compounds
[(Tp)2Fe2(CN)6X(bdmap)Cu2(H2O)].H2O to be further denoted as Fe2Cu2 (Tp =
tris(pyrazolyl)hydroborate, bdmapH = 1,3-bis(dimethylamino)-2-propanol, HX =
acetic acid, propionic acid or trifluoroacetic acid), is rigorously studied
using the transfer-matrix method. The overall ground-state phase diagram
reveals three distinct phases: a quantum antiferromagnetic phase, a quantum
ferrimagnetic phase and a classical ferromagnetic phase. In the
zero-temperature magnetization curve, two quantum ground states are manifested
as intermediate plateaus at zero and half of the saturation magnetization,
while the magnetization reaches its saturated value within the classical
ferromagnetic phase. The bipartite entanglement between nearest-neighbor
Heisenberg spins is more pronounced in the quantum ferrimagnetic phase compared
to the quantum antiferromagnetic phase due to a fully polarized nature of the
Ising spins. A reasonable agreement between theoretical predictions for the
spin-1/2 Ising-Heisenberg branched chain and experimental data measured for a
temperature dependence of the magnetic susceptibility and a low-temperature
magnetization curve suggests strong antiferromagnetic coupling between
nearest-neighbor CuII-CuII magnetic ions and moderately strong ferromagnetic
coupling between nearest-neighbor CuII-FeIII magnetic ions in the polymeric
compounds Fe2Cu2. A thermal entanglement between nearest-neighbor CuII-CuII
magnetic ions persists up to a relatively high threshold temperature T = 224 K
and undergoes a transient magnetic-field-driven strengthening.
