Magnetocaloric effect in the Ising model with RKKY interaction on the Shastry-Sutherland lattice

The classical Monte Carlo method is used to examine effects of electron and hole doping on the magnetocaloric effect in the generalized 2D Ising model with the long-range RKKY interaction on the Shastry-Sutherland lattice. The electron and hole doping in this model is controlled by deviations of the Fermi wave vector kF from the initial value kF = 2x/1.24 which models the most realistically situation in the undoped metallic Shastry- Sutherland rare-earth tetraboride TmB4. For the undoped case we have found a heating hill (the entropy change AS > 0) occurring in the vicinity of metamagnetic transitions and a cooling depression in the paramagnetic phase which is in a very good qualitative agreement with experimental measurements in TmB4. In the case of small electron doping (kF = 2x/1.26), a significant decrease of the maximum of -AS is observed, while in the strongly hole (electron) doped case kF = 2x/1.17 (kF = 2x/1.43) the maximum of -AS is considerably enhanced against the undoped system, which points to the fact that some of doped systems could have better cooling properties than the undoped one. This is confirmed independently by calculations of relative cooling power for a wide range of parameter kF, where we have found two intervals kF & ISIN; (2x/1.15,2x/1.21) and kF & ISIN; (2x/1.39,2x/1.5) with much larger relative cooling power in comparison to the undoped case.