Magnetic properties of nanocomposite Fe–Pt films with Fe concentration higher than 50 at % have been investigated in this study. Fe/Pt multilayers were produced by sputtering and magnetic hardening was observed after heat treatment including rapid annealing. The final nanocomposite films consisted of the hard face-centered tetragonal FePt phase and a soft face-centered-cubic phase. The maximum energy products of the optimally processed samples exceeded 40 MGOe. Evidence for exchange coupling of the hard and soft phases was found.
Polymer chains form lamellar structures during crystallization which display a memory of thermal history. Using molecular dynamics simulations and primitive path analysis, we show a direct dependence of both density and crystalline stem length on the local entanglement length. The slow relaxation of the entanglement state after a change of external conditions can directly explain the role of thermal history for polymer crystallization, in particular memory effects. The analysis of the local entanglement state can be used to predict the occurrence of nucleation events. Our results present a fresh insight of the nonequilibrium properties of polymer crystals which might be identified as "frozen topology" of polymer melts.
Using
molecular dynamics simulations and primitive path analysis,
we show that hot entangled polymer melts can crystallize faster with
higher crystallinities and larger crystalline stem lengths, as compared
to cold melts under rapid quenching conditions or during cold-crystallization.
This counterintuitive phenomenon similar to the so-called Mpemba effect
observed for water can be explained by the temperature dependence
of entanglements. Our results demonstrate the key role of the entanglement
state for crystallization properties and provide a new approach to
understand the role of thermal history and to the open question of
thickness selection in polymer crystallization.
Large scale and long time molecular dynamics simulations
and primitive
path analysis are used to investigate the disentanglement of long
linear polymer chains during their crystallization from the melt state.
In general, two competitive processes, a slow decrease of average
entanglement length during cooling caused by stiffening of chains
and a strong increase during crystallization, can be observed. In
both homogeneous and heterogeneous nucleation, disentanglement occurs
via forming folds from locally unentangled segments and continues
in postcrystallization processes (slow reorganization), in particular,
during annealing. Re-entanglement processes after melting are slow
and can lead to memory effects in heating–recooling protocols
such as self-seeding.
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