八核膦酸镍笼的磁性和热力学性质
Magnetic and thermodynamic properties of the octanuclear nickel phosphonate-based cage
摘要：
我们报告了一个详细的理论研究各向异性的影响的磁性和热力学性质的八核镍膦酸笼与蝴蝶形的分子几何形状，即$\mathrm{Ni}_8(μ_3-\mathrm{OH})_4(\mathrm{OMe})_2(\mathrm{O}_3\mathrm{PR}_1)_2 (\mathrm{O}_2\mathrm{C}^t\mathrm{Bu})_6 (\mathrm{HO}_2\mathrm{C}^t\mathrm{Bu})_8$。为了验证我们的精确对角化方法，我们首先比较结果与模拟和实验中的各向同性的情况下。建立了共存，我们然后介绍单轴单离子各向异性和海森堡交换相互作用的镍原子之间的各向异性。然后，我们检查两个各向异性参数的磁化过程中，以及对模型的比热的影响。我们预测了中间磁化平台，包括零平台，和磁化跳磁基态相变在低温$T=1$ K。磁化平台强烈依赖于交换各向异性和单离子各向异性的水平。改变前者导致所有中间高原的宽度和磁位置的变化，而它们在增加后者时变得更宽。当系统为各向同性时，在弱磁场存在下，该模型的比热在中等温度下表现出Schottky型最大值。各向异性的引入导致比热的热行为的显著变化。实际上，通过调整各向异性参数，肖特基峰转换为双峰温度依赖性，这与磁化跳变一致。我们呼吁这些理论预测在低温下进行实验验证。
英文摘要：
We report a detailed theoretical investigation into the influence of anisotropy on the magnetic and thermodynamic properties of an octanuclear nickel phosphonate cage with butterfly-shaped molecular geometry, namely $\mathrm{Ni}_8(μ_3-\mathrm{OH})_4(\mathrm{OMe})_2(\mathrm{O}_3\mathrm{PR}_1)_2 (\mathrm{O}_2\mathrm{C}^t\mathrm{Bu})_6 (\mathrm{HO}_2\mathrm{C}^t\mathrm{Bu})_8$. To validate our exact diagonalization approach, we firstly compare results with simulations and experiment in the isotropic case. Having established concurrence, we then introduce uniaxial single-ion anisotropy and Heisenberg exchange anisotropy between interacted nickel atoms. We then examine effects of both anisotropy parameters on the magnetization process, as well as on the specific heat of the model. We predict intermediate magnetization plateaus, including zero plateau, and magnetization jumps with magnetic ground-state phase transitions at low temperature $T=1$K. The magnetization plateaus are strongly dependent on both the levels of exchange anisotropy and single-ion anisotropy. Varying the former leads to change in width and magnetic position of all intermediate plateaus while they become wider upon increasing the latter. The specific heat of the model manifests a Schottky-type maximum at moderate temperature in the presence of weak magnetic fields, when the system is isotropic. The introducion of aniostropy results in substantial variations in the thermal behavior of the specific heat. Indeed, by tuning anisotropy parameters the Schottky peak convert to a double-peak temperature dependence that coincided with the magnetization jumps. We call for these theoretical predictions to be verified experimentally at low temperature.
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