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Elastic Frustration Causing Two-Step and Multistep Transitions in Spin-Crossover Solids: Emergence of Complex Antiferroelastic Structures
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posted on 2016-02-09, 00:00 authored by Miguel Paez-Espejo, Mouhamadou Sy, Kamel BoukheddadenTwo-step
and multistep spin transitions are frequently observed
in switchable cooperative molecular solids. They present the advantage
to open the way for three- or several-bit electronics. Despite extensive
experimental studies, their theoretical description was to date only
phenomenological, based on Ising models including competing ferro-
and antiferro-magnetic interactions, even though it is recognized
that the elastic interactions are at the heart of the spin transition
phenomenon, due to the volume change between the low- and high-temperature
phases. To remedy this shortcoming, we designed the first consistent
elastic model, taking into account both volume change upon spin transition
and elastic frustration. This ingredient was revealed to be powerful,
since it was able to obtain all observed experimental configurations
in a consistent way. Thus, according to the strength of the elastic
frustration, the system may undergo first-order transition with hysteresis,
gradual, hysteretic two-step or multistep transitions, and incomplete
transitions. Furthermore, the analysis of the spatial organization
of the HS and LS species in the plateau regions revealed the emergence
of complex antiferro-elastic patterns going from simple antiferro-magnetic-like
order to long-range spatial modulations of the high-spin fraction.
These results enabled us to identify the elastic frustration as the
fundamental mechanism at the origin of the very recent experimental
observations showing the existence of organized spatial modulations
of the high-spin fraction inside the plateau of two-step spin transitions.