Thermodynamics of magnetically ordered and disordered systems

In this talk I will present the theoretical results regarding thermodynamics of three classes of magnetic systems: insulating and itinerant (especially so-called heavy-fermion compounds) magnetically ordered systems as well as insulating magnets with disorder. The influence of disorder on the properties of magnets will be discussed in details. The theoretical results will be extensively compared with experiment in magnets, diluted magnetic semiconductors and multiferroics. Recent results obtained by team of experimentalists and theorists in PbFe_1/2Sb_1/2O₃ multiferroic will also be discussed.