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.
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