K Boukheddaden et al 2007 J. Phys.: Condens. Matter 19 106201 doi:10.1088/0953-8984/19/10/106201
Theoretical investigations on an axial next nearest neighbour Ising-like model for spin crossover solids: one- and two-step spin transitions
K Boukheddaden1, J Linares1, R Tanasa1,2 and C Chong1
Show affiliationsWe investigate the thermodynamical properties of an adapted ANNNI-like (axial next nearest neighbour Ising-like) model for 1D spin crossover solids. We performed an exact treatment within the framework of the well known transfer matrix method and derived the thermal evolution of the high spin fraction, the correlation function and the heat capacity. We show that a model of competition of the interactions between similar spin state ions reproduces qualitatively the main features of experimental data: gradual, abrupt and two-step spin crossover transitions. In addition, we found that the intermediate region of the two-step transition exhibits two types of spatial organization, according to the sign of the interactions. Indeed, antiferro-like and 'antiphase' (called the phase 
in the ANNNI model) structures are obtained. We also found that these two configurations can be distinguished through the thermal evolution of their corresponding correlation functions.
The existence of the phase , predicted by the present model, is in good agreement with recent observations on a spin crossover sample of [{Fe(CNBH3)(4phpy)}2(μ-bpypz)2] for which this phase was evidenced for the first time, in these materials.
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Issue 10 (14 March 2007)
Received 23 October 2006, in final form 19 January 2007
Published 15 February 2007
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Theoretical investigations on an axial next nearest neighbour Ising-like model for spin crossover solids: one- and two-step spin transitions
K Boukheddaden et al 2007 J. Phys.: Condens. Matter 19 106201
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