Title : Nanoscale thermodynamics of liquid-like molecular spins in on-surface synthesized nanoclusters
Abstract:
With an average diameter of about 2 nm, on-surface synthesized amino-ferrocene nanoclusters are chemisorbed onto a graphene oxide (GO) nanosheet [1], where their Fe ions change from the zero-spin state (S = 0) to the high-spin state (S = 5/2) by charge transfer between the molecules and the GO nanosheet [2,3]. The structure of the molecular nanocluster is characterized using a transmission electron microscope and an atomic force microscope. In this two-dimensional (2D) nanomaterial, the molecular spins in a given nanocluster are weakly magnetic dipole interacting under the conditions of short intermolecular distance and large magnetic moment. It generates spin correlations and slow dynamics accessible by magnetic susceptibility and Mössbauer spectroscopy at a temperature where the magnetic anisotropy is normally negligible. Stochastic simulations with the model in the same configuration as the experimental one show that minimizing its magnetic dipole energies produces spatially entangled structures of the spin orientations under thermal perturbations substantially at T ? 15 K and that the structures behave like a spin liquid [3]. The term “entangled structure” introduced here refers to the “entangled polymers” below the glass transition temperature. The competition between the formation of the entangled structures and their thermal destruction produces the slow dynamics and spin correlations at low temperatures. The origin of the liquid-like behavior and the interpretation of another experimental results are analyzed microscopically by using the simulated distribution of the dipole energies and energy fluctuations at the sites in the nanocluster model. The present results demonstrate the validity of nanoscale thermodynamics.
