Abnormal field-dependence of magnetocaloric effect in ErB4 and TmB4

The coupled system of multi-degrees of freedom, such as charge, spin, orbital and lattice, has recently received much attention due to its potential to improve the magnetocaloric effect (MCE). The exotic inverse MCE was observed in rare-earth tetraborides of Ho1-xDyxB4 (x = 0.0, 0.5, and 1.0), associated with a strong coupling between magnetic dipoles and orbital quadrupoles in the strong spin-orbit coupling and geometric frustration. Here, the magnetism and magnetocaloric effects of ErB4 and TmB4 are investigated. It shows the maximum entropy changes of 11.4 J/kgK, and 12.6 J/kgK with the field of Delta H approximate to 40 kOe (H parallel to c) in ErB4 and TmB4, respectively. The field- and temperature-dependence of the entropy change is found to be quite different from those of the conventional MCE. And the entropy change is also found to have strong correlation with the field induced meta-magnetic transition. Because the field induced transition is due to magnetic moment reorientation, which is strongly coupled with quadrupole moment, the abnormal MCE of ErB4 and TmB4 is attributed to the dipole-quadrupole interaction and magnetic frustration. Thus, it supports the fact that the strong coupling between quadrupole and magnetic dipole moments plays important role in the exotic inverse MCE in rare-earth tetraboride system.