Granular and molecular magnetic materials present a wide range of magnetic relaxation phenomena. The chemists bottom-up approach to build molecular nanostructures provided this field with some unprecedented model systems, as the quasi isolated nanomagnets Mn12 and Fe8, ferrimagnetic chains and planar structure, disordered systems, presenting very slow relaxation, and glassy behaviour. In this presentation we will review some aspects of magnetic relaxation and its temperature dependence, as the crossover from quantum to thermally activated relaxation in nanomagnets, the influence of weak interactions and some signatures of thermally activated quantum tunnelling. The temperature dependence of magnetic relaxation in granular systems and magnetic molecular systems will be discussed.
Instituto de Fisica Universidade Federal do Rio de Janeiro