where A1/(1+A2H 2 ) describes the quantum tunnelling of magnetization (QTM), B1H describes direct relaxation and D stands for processes that are magnetic field independent. The temperature dependence of the relaxation time was analyzed using Eq. 2 and is presented in Figure for 1 (black) 1UV (blue) and 1vis (red) and in Figure for 2 (black) and 2UV (blue):