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Abstract
The magnetic and thermodynamic properties of the transition metal NiII phosphonatebased cage is reported in detail. The model is known as octanuclear nickel phosphonate cage [Ni8 (μ3 − OH)4 (OMe)2 (O3PR1 )2 (O2C tBu)6 (HO2C tBu)8 ] with the geometry of butterfly-shaped molecular structure in the presence of an external magnetic field. Firstly, using full exact diagonalization (ED) and quantum Monte Carlo simulations, we verify the model without aniostropic terms in order to compare our results with that of the original experimental data expressed in Refs. [18,19]. Then, we consider Heisenberg exchange anisotropy between interacted nickel atoms, besides an uniaxial single-ion anisotropy property for all of them, and examine effects of the both anisotropy parameters on the magnetization process also the specific heat of the model. Interestingly, we see some intermediate magnetization plateaus including 0-plateau, and magnetization jumps accompanying with the magnetic ground-state phase transitions at low temperature 𝑇 = 1K. The magnetization plateaus are strongly dependent on the exchange anisotropy, as well as, single-ion anisotropy parameters. Actually, altering the exchange anisotropy leads to change in width and magnetic position of all intermediate plateaus.
