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>Disorder-Induced Transformation of the Energy Landscapes and Magnetization Dynamics in Two-Dimensional Ensembles of Dipole-Coupled Magnetic Nanoparticles
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Disorder-Induced Transformation of the Energy Landscapes and Magnetization Dynamics in Two-Dimensional Ensembles of Dipole-Coupled Magnetic Nanoparticles
The interaction-energy landscapes (ELs) and magnetization dynamics of two-dimensional ensembles of dipole-coupled magnetic nanoparticles are theoretically investigated. Extended nanostructures are modeled by considering nonoverlapping nanoparticles (NPs) in a square unit cell with periodic boundary conditions. The local minima and connecting transition states of the EL are determined systematically for representative NP arrangements having different degrees of disorder. The topology of the ergodic networks of stationary points is analyzed from both local and energy perspectives by using kinetic networks and disconnectivity graphs. We show that increasing the degree of disorder not only increases, most significantly, the number of local minima and transition states but also changes the shape of the EL in a very profound way. While slightly disordered ensembles correspond to good structure seekers, which are funneled towards the global minima, strongly disordered systems show very rough landscapes with multiple low-energy local minima separated by relatively large energy barriers. The consequences of this transition on the long-time Markovian dynamics of the nanostructures are quantified by calculating the field-free magnetic relaxation after saturation and after quenching. The simulations indicate that the relaxation of weakly disordered systems follows a slightly stretched exponential law, with a single characteristic timescale for a wide range of temperatures. In contrast, strongly disordered systems show a much more complicated relaxation dynamics involving multiple timescales, slowing down and trapping, which is reminiscent of spin glasses.
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