Modelling of isothermal remanence magnetisation curves for an assembly of macrospins

•A framework to compute IRM curves for nanoparticle assemblies is presented.•A simple analytic expression (for a single size) is compared to more evolved models.•The simple expression can reliably simulate IRM curves for realistic samples.•Irreversible susceptibility and the influence of several parameters is discussed.

We present a robust and efficient framework to compute isothermal remanent magnetisation (IRM) curves for magnetic nanoparticle assemblies. The assembly is modelled by independent, randomly oriented, uniaxial macrospins and we use a Néel model to take into account the thermal relaxation. A simple analytic expression is established for a single size, in a sudden switching approximation, and is compared to more evolved models. We show that for realistic samples (necessarily presenting a size dispersion) the simple model is very satisfactory. With this framework, it is then possible to reliably simulate IRM curves, which can be compared to experimental measurements and used in a best fit procedure. We also examine the influence of several parameters on the IRM curves and we discuss the link between the irreversible susceptibility and the switching field distribution.