The specific heat and magnetic properties of two species of spin-1/2 ladders with butterfly-shaped unit blocks

Studies of the structural effects on the thermodynamic properties of two types of Ising–Heisenberg ladders are comprehensively reported. Each structure comprises spin-1/2 particles partitioned into adjoined blocks or cages formed from butterfly-shaped plaquettes and interacting in an Ising manner. We use the transfer-matrix approach to determine the partition functions of these models and numerically investigate their magnetization and specific-heat properties. Both models illustrate all characteristic features of low-temperature magnetization processes and thermodynamics properties such as abrupt variations of the magnetization curves, Schottky-type peak and a double-peak structure of the specific heat. Comparisons between the two ladders reveal some differences in their magnetic and thermodynamic behaviors. For instance, difference in the number of intermediate magnetization plateaus, also difference in the height and temperature-position of the specific heat peaks. We also present that the fluctuations of the specific heat with respect to the magnetic field are in highly accordance with the magnetization plateaus and magnetization jumps. Moreover, we find quite interesting threshold reentrance points within the matrix-plot of the specific heat shown in the (T/J  −  B/J) plane, at which the temperature-position of the Schottky peak alternatively change. Finally, we prove that the reentrance points can be satisfactorily considered as magnetization plateau witnesses even in the high temperatures. (paper)