Disorder-induced transition good ground-state seeker to glassy behavior in two-dimensional ensembles magnetic nanoparticles

The collective static and dynamic behavior of two-dimensional ensembles of interacting magnetic nanoparticles (MNPs) has been investigated. These systems are most important for a number of applications, including biosensors, cancer therapies, and magnetic storage media. In addition, their complex many-particle behavior makes them very challenging from a theoretical physics perspective. Using the methodology of statistical physics, energy landscapes and networks, we have shown that disorder leads to a profound transformation of the static and dynamic behavior of these nanostructures. Weakly disordered systems are so-called good structure seekers, which means that they have relatively few metastable states and a very stable ground state. Consequently, their dynamics is fast, proceeding unhindered towards the ground state and thermal equilibrium. In contrast, strongly-disordered systems have an extremely large number of metastable states with comparable energies, separated by high energy barriers. This leads to an extremely complex and slow dynamics, which is characterized by the presence of different time scales as in structural or spin glasses. An example of the complexity of the energy landscape of disordered MNP ensembles is shown in the figure below.

Figure: Disconnectivity graph of a strongly disordered ensemble of MNPs. In (a) all metastable states are shown, while (b) is restricted to the lowest 400 states. Notice that the numerous low metastable states are separated by large energy barriers, often much larger than the actual energy differences. The insets illustrate the magnetic configurations of the ground state (E₀), the second excited state (E₂), and the third excited state (E₃). The first excited state (not shown) is very similar to the ground state.

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