Towards the Ultimate Size Limit of the Memory Effect in Spin‐Crossover Solids

Abstract

The phenomenon of spin crossover (SCO) between high-spin (HS) and low-spin (LS) states of 3d 4 -3d 7 transition-metal complexes has attracted much interest.[1] Although the origin of the spin-crossover phenomenon is purely molecular, the macroscopic behavior of these systems in the solid state is strongly influenced by short-and long-range interactions (of mainly elastic origin) between the transition-metal ions, giving rise to remarkable cooperative phenomena, such as first-order phase transitions.[2] One of the most interesting open questions in this research field concerns the effect of size reduction on the cooperativity and on the relevant physical properties. Notably, the hysteresis, which in certain cases accompanies the thermal spin transition, is considered to be an important property, as it confers a memory effect on these systems. As was suggested by Kahn and Martinez, [3] this phenomenon might be used for information storage. The same authors have estimated from statistical considerations that approximately 10 3 strongly interacting metal ions (i.e. comprising a particle with a diameter of a few nanometers) should be the approximate lower size limit for which an SCO solid might still exhibit hysteresis. The size dependence of the hysteresis width has also been investigated by Kawamoto and Abe on a model system taking into account only short-range interactions, [4] but the complexity of the elastic interactions in real systems and the poor knowledge of the relevant lattice properties (and their size dependence) makes it very difficult to reliably predict the interaction energies and hence the hysteresis width.To our knowledge, the first results concerning the sizereduction effect in SCO systems was reported by LØtard et al. [5,6] [7] At the same time, some of us reported an alternative method for the fabrication of nanoobjects (down to 30 nm) exhibiting SCO properties based on the lithographic patterning of thin films. [8,9] Such ordered arrays of SCO micro-and nanostructures are particularly interesting for potential applications of this phenomenon, but in the sub-micrometer range it is rather challenging to characterize their structure and physical properties, owing to the small signals. Moreover, it is difficult to obtain nanoobjects with sizes below 10 nm by means of lithographic techniques. This range is, however, accessible by chemical methods, which have the further advantage of allowing the synthesis of a large number of particles, and thus their physical characterization becomes feasible by standard averaging techniques, such as magnetometry.Herein, we report the synthesis and study of ultra-small monodisperse nanoparticles of the 3D spin crossover coordination polymer [Fe(pyrazine){Ni(CN) 4 }], [10] obtained using the biopolymer chitosan as matrix. It should be noted that investigations on the synthesis of cyano-bridged coordination polymers at the nanoscale level have attracted a great deal of attention in recent years, and numerous methods have been reported, including the use of reverse m...

Light‐Induced Stored Information in Nanoparticles

“…[10] To precisely control the particle size in this study, the reactant concentration was kept constant (0.05 ) and only the mass of the nonionic polyoxyethylenic surfactant Ifralan D0205 was varied (25, 78 and 85 %). In other reports of nanoparticle preparation, different sized particles were obtained through varying the solution metal concentration, [8,9] and we found that the strategy of keeping the concentration constant and varying the oil/water ratio allowed the size to be varied more precisely.…”

Mechanochemical synthesis of cooperative spin crossover materials

“…This is attributed to amorphisation and a reduction in the coherent domain length of individual particles, through increased defects and a more significant surface contribution. 11 Cooperativity can be assessed through evaluation of the abruptness of the SCO curve and hysteresis widths via magnetometry. 12 Previous studies on the effects of grinding SCO materials involved grinding solutionsynthesized molecular SCO materials for several hours.…”

Matrix and size effects on the appearance of the thermal hysteresis in 2D spin crossover nanoparticles

“…In all these studies, the research of the control of the thermal hysteresis loop, and particularly, the identification of the relevant physical parameters allowing monitoring the hysteresis width is a key point for the practical applications of these systems. The current work aims to address this problem on SC nanoparticles on which an extensive work was devoted this last decade, both from the chemical view point by synthesizing new core-shell systems [28][29][30] and from the theoretical side, in which some attempts to explain the interplay between the matrix and the SCO properties [31][32][33][34][35][36] have been developed. Mikolasek et al [37] has studied the influence of surface/interface on the lattice dynamic of SCO nanoparticules using a spring-ball model.…”