  In this contribution, we will present a review of our works on the time
dependence of magnetization in nanoparticle systems starting from
non-interacting systems, presenting a general theoretical framework for the
analysis of relaxation curves which is based on the so-called $\svar$ scaling
method. We will detail the basics and explain its range of validity, showing
also its application in experimental measurements of magnetic relaxation. We
will also discuss how it can be applied to determine the energy barrier
distributions responsible for the relaxation. Next, we will show how the
proposed methodology can be extended to include dipolar interactions between
the nanoparticles. A thorough presentation of the method will be presented as
exemplified for a 1D chain of interacting spins, with emphasis put on showing
the microscopic origin of the observed macroscopic time dependence of the
magnetization. Experimental application examples will be given showing that the
validity of the method is not limited to 1D case.
