The magnetic relaxation is temperature-independent at low temperatures (below 5, 6, and 8 K for 1 -3, respectively), describing the significant dominance of quantum tunnelling of the magnetization (QTM) effect in that region. Above this QTM regime, magnetic relaxation is temperature-dependent and is influenced by the thermal effects. Finally, it follows a thermally activated process at high temperatures. Origin of the paramagnetic relaxation behaviour in SMM, can be defined in terms of magnetization relaxation time () by four main mechanisms: Raman, direct, thermally activated (Arrhenius or Orbach-like), and QTM processes as described in the following equations ( ) and ( )