Molecule-based magnetocalorics

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Magnetic frustration as well as the design of quantum criticalpoints can be the source of huge magnetocaloric effects atKelvin and sub-Kelvin temperatures. This proposal aims at asystematic investigation of frustrated molecular couplingschemes with respect to their magnetocaloric properties and the suggestion of promising structures of few spin systemsfor magnetic coolingapplications in certain regions of the T-B plane. Thefocus will be put on the generation of massive degeneracieseither in the ground state or in a field-induced groundstate.The resulting isentropes shall be investigated and single-shot as well as cyclic cooling processes shall bediscussed and suggested. One additional interesting aspect is given by thefact that some points in the T-B plane can be connected byadiabatic as well as isothermal processes at the same time.Key figures of merit are the isothermal entropy change and theadiabatic temperature change of which the cooling rate is thederivative with respect to the field. Also important is theamount of heat that can be absorbed during the lower temperatureisothermal process step. The second part of this proposal is devoted to the investigationof the influence of anisotropy (single-ion as well as exchange) and in particular of theanisotropic dipolar interaction on magnetocaloricproperties. Anisotropy tends to split degeneracies of levelswhich decreases the isothermal entropy change with field andtends to decrease the cooling rate. We want to investigatestructures that are more robust than others against sucheffects. On the other hand anisotropy allows to constructmagnetocaloric processes that work by reorientation. Thisconstitutes a completely new direction in the field of molecularmagneto-refrigerants. Processes of this kind shall beinvestigated and discussed.As a last part we would like to start investigations ona realistic dynamics of cooling processes using the Lindbladformalism in order to describe the coupling to heat baths in thecourse of a magnetocaloric cycle. Especially the observation,that sometimes two different processes connect the same state,could be studied further. It would be interesting to find out whether realisticprocesses run differently on those paths.In order to not confuse our proposal with other efforts we wouldlike to state that we do not design or investigate real chemicalmaterials, but schemes of exchange interactions that can bepossibly realized as magnetic molecules in the future. We wouldalso like to state that we do not aim at room temperatureapplications orapplications in the few-ten Kelvin range for gas liquification,since molecules are not appropriate for this purpose due to their small (Kelvin-) energy scale.