Collective magnetism in an artificial 2D XY spin system.

Two-dimensional magnetic systems with continuous spin degrees of freedom exhibit a rich spectrum of thermal behaviour due to the strong competition between fluctuations and correlations. When such systems incorporate coupling via the anisotropic dipolar interaction, a discrete symmetry emerges, which can be spontaneously broken leading to a low-temperature ordered phase. However, the experimental realisation of such two-dimensional spin systems in crystalline materials is difficult since the dipolar coupling is usually much weaker than the exchange interaction. Here we realise two-dimensional magnetostatically coupled XY spin systems with nanoscale thermally active magnetic discs placed on square lattices. Using low-energy muon-spin relaxation and soft X-ray scattering, we observe correlated dynamics at the critical temperature and the emergence of static long-range order at low temperatures, which is compatible with theoretical predictions for dipolar-coupled XY spin systems. Furthermore, by modifying the sample design, we demonstrate the possibility to tune the collective magnetic behaviour in thermally active artificial spin systems with continuous degrees of freedom.

Cardiolipin Models for Molecular Simulations of Bacterial and Mitochondrial Membranes.

Present in bacterial and mitochondrial membranes, cardiolipins have a unique dimeric structure, which carries up to two charges (i.e., one per phosphate group) and, under physiological conditions, can be unprotonated or singly protonated. Exhaustive models and characterization of cardiolipins are to date scarce; therefore we propose an ab initio parametrization of cardiolipin species for molecular simulation consistent with commonly used force fields. Molecular dynamics simulations using these models indicate a protonation dependent lipid packing. A peculiar interaction with solvating mono- and divalent cations is also observed. The proposed models will contribute to the study of the assembly of more realistic bacterial and mitochondrial membranes and the investigation of the role of cardiolipins for the biophysical and biochemical properties of membranes and membrane-embedded proteins.