Bipartite quantum and thermal entanglement is quantified within pure and mixed states of a mixed spin-($1/2,1$) Heisenberg dimer with the help of negativity. It is shown that the negativity, which may serve as a measure of the bipartite entanglement at zero as well as nonzero temperatures, strongly depends on intrinsic parameters—for instance, exchange and uniaxial single-ion anisotropy—in addition to extrinsic parameters such as temperature and magnetic field. It turns out that a rising magnetic field unexpectedly reinforces the bipartite entanglement due to the Zeeman splitting of energy levels, which lifts the twofold degeneracy of the quantum ferrimagnetic ground state. The maximal bipartite entanglement is thus reached within a quantum ferrimagnetic phase at sufficiently low but nonzero magnetic fields under the assumption that the gyromagnetic $g$ factors of the spin-1/2 and spin-1 magnetic ions are equal and the uniaxial single-ion anisotropy is half of the exchange constant. It is suggested that the heterodinuclear complex $\left[\mathrm{Ni}\left(\mathrm{dpt}\right)\left({\mathrm{H}}_2\mathrm{O}\right)\mathrm{Cu}\left(\mathrm{pba}\right)\right]·2{\mathrm{H}}_2\mathrm{O}$ [pba = 1,3-propylenebis(oxamato) and dpt = bis-(3-aminopropyl)amine], which affords an experimental realization of the mixed spin-($1/2,1$) Heisenberg dimer, remains strongly entangled up to relatively high temperatures (about 115 K) and magnetic fields (about 140 T) that are comparable with the relevant exchange constant.

• Received 26 August 2020
• Revised 19 October 2020
• Accepted 4 November 2020

DOI:https://doi.org/10.1103/PhysRevB.102.184419