Quantum Entanglement Generation in the Heterometallic Ni$^\text{2+}_4$Gd$_4^\text{3+}$ Complexes

Abstract: We investigate tetrapartite and bipartite quantum entanglement in octanuclear heterometallic $3d/4f$ complexes denoted as Ni$^\text{2+}_4$Gd$_4^\text{3+}$ under an external magnetic field using exact diagonalization. These molecular magnets that can be effectively characterized by Heisenberg spin models, consist of two Ni$^\text{2+}_2$Gd$_2^\text{3+}$ cubane subunits bridged by acetate and hydroxide ligands. We detect that their magnetization exhibits intermediate plateaus at low temperatures, reflecting distinct ground states characteristic of Gd-containing compounds. Using negativity as a quantum entanglement measure, we analyze the effects of single-ion anisotropy and magnetic field on tetrapartite and bipartite entanglements in two families of Ni$^\text{2+}_4$Gd$_4^\text{3+}$ complexes: $\boldsymbol{(1)}$ without and $\boldsymbol{(2)}$ with anisotropy. Complex $\boldsymbol{(1)}$ exhibits strong tetrapartite entanglement, persisting up to $T \approx 2.5$ K and $B \approx 4.0$ T, but considerably weak bipartite entanglement between Ni$\cdots$Ni and Ni$\cdots$Gd links. Conversely, complex $\boldsymbol{(2)}$ shows strong bipartite entanglements but negligible tetrapartite entanglement negativity. These findings highlight the critical role of single-ion anisotropy in generating and shaping entanglement properties in heterometallic Ni$^\text{2+}_4$Gd$_4^\text{3+}$ complexes.