Molecular Magnetism Modeling with Applications in Spin Crossover Compounds

Molecular magnetic materials have become flourishing fields for research and technological developments due to their novel behavior compared to classical magnetic materials. Molecular magnetism modeling has reached a certain degree of maturity, although several experimental findings are still open problems. This chapter is aimed at providing a general introduction to physical modeling in molecular materials with a special emphasis placed on spin crossover compounds. This presentation includes Ising-type models and their generalizations, such as Wajnflasz and Pick, Bousseksou et al., Zimmermann and König, Sorai and Seki, and Nasser et al., along with their applications to the characterization of phase transition, hysteresis behavior, and thermal relaxations in spin crossover compounds. Recent experimental findings are explained in this context and the relevance of theoretical results for technological applications is also discussed

Spin state dependence of electrical conductivity of spin crossover materials

We studied the spin state dependence of the electrical conductivity of the spin crossover compound [Fe(Htrz)2(trz)](BF4) (Htrz = 1H-1,2,4-triazole) by means of dc electrical measurements. The low spin state is characterized by higher conductance and lower thermal activation energy of the conductivity, when compared to the high spin state

Synthesis of mild–hard AAO templates for studying magnetic interactions between metal nanowires

The sequential application of mild and hard anodization techniques in the fabrication of porous aluminamembranes allows one to decrease the number of continuous pores in anodized aluminium oxide (AAO) templates. Initially, standard mild anodization techniques were used to create porous templates with 100 nm interpore distances and 70 nm pore diameters. Hard anodization treatment on the same membrane then produced interpore distances of about 265 nm with diameters of 110 nm. At the interface between the two anodization steps, many of the mild-side pores were terminated to create a mild–hard membrane (Mi–Ha AAO) where the functional interpore distances were 200–300 nm. Growth (electrodeposition) of nickel and cobaltnanowires in the various pore structures of the mild and hard sides of the Mi–Ha AAO templates allows one to probe magnetic interactions between nanowires and compares them to wires grown in standard mild templates (Mi-AAO). The magnetic properties of nanowires in Mi–Ha AAO and Mi-AAO showed distinct differences in the squareness of hysteresis loops and coercivity both as a function of pore structure and magnetic component. In general, the squareness of the hysteresis loops increased with aspect ratio and greater interpore distance. Coercivity also showed an increase with aspect ratio, but varied differently with interpore distance for Ni and Co. These various magnetic behaviors are discussed with respect to crystalline structure, morphology, and interactions of adjacent sets of nanowires

Charge Transport and Electrical Properties of Spin Crossover Materials: Towards Nanoelectronic and Spintronic Devices

In this paper, we present a comprehensive review of research on electrical and charge transport properties of spin crossover complexes. This includes both the effect of spin-state switching on the dielectric permittivity and electrical conductivity of the material and vice versa the influence of an applied electrical field (or current) on the spin-state of the system. The survey covers different size scales from bulk materials and thin films to nanoparticles and single molecules and embraces the presentation of several device prototypes and hybrid materials as well

Pressure effect investigations on spin-crossover coordination compounds

The piezochromic properties of spin-crossover complexes have been recognized for a long time, with increasing pressure favouring the low spin state due to its smaller volume and therefore shifting the spin equilibrium towards higher temperatures and accelerating the relaxation at a given temperature. However, the interpretation and quantification of pressure-induced changes have been several times compromised by the relatively poor and incomplete spectral and structural information provided by the detection methods or due to the experimental difficulties related to the need for hydrostatic conditions at low temperatures. The present review is therefore primarily focused on these experimental aspects of high-pressure spin crossover research providing an overview of methods of pressure generation and associated detection methods as well as on selected recent result

Spin Transition Sensors Based on β-Amino-Acid 1,2,4-Triazole Derivative

A β-aminoacid ester was successfully derivatized to yield to 4H-1,2-4-triazol-4-yl-propionate (βAlatrz) which served as a neutral bidentate ligand in the 1D coordination polymer [Fe(βAlatrz)3](CF3SO3)2·0.5H2O (1·0.5H2O). The temperature dependence of the high-spin molar fraction derived from 57Fe Mossbauer spectroscopy recorded on cooling below room temperature reveals an exceptionally abrupt single step transition between high-spin and low-spin states with a hysteresis loop of width 4 K (Tc↑ = 232 K and Tc↓ = 228 K) in agreement with magnetic susceptibility measurements. The material presents striking reversible thermochromism from white, at room temperature, to pink on quench cooling to liquid nitrogen, and acts as an alert towards temperature variations. The phase transition is of first order, as determined by differential scanning calorimetry, with transition temperatures matching the ones determined by SQUID and Mössbauer spectroscopy. The freshly prepared sample of 1·0.5H2O, dried in air, was subjected to annealing at 390 K, and the obtained white compound [Fe(βAlatrz)3](CF3SO3)2 (1) was found to exhibit a similar spin transition curve however much temperature was increased by (Tc↑ = 252 K and Tc↓ = 248 K). The removal of lattice water molecules from 1·0.5H2O is not accompanied by a change of the morphology and of the space group, and the chain character is preserved. However, an internal pressure effect stabilizing the low-spin state is evidenced

Etude théorique et expérimentale de l'effet de la pression et de la taille sur des composés bistables (comportement thermique et étude de la relaxation)

L objectif principal de ce travail est l étude de l effet de la pression et de la taille sur les propriétés hystérétiques des composés à transition de spin (CTS). Notre travail contient quatre parties. La première partie concerne le développement du modèle couplage atome-phonon , afin de pouvoir décrire les différents processus qui ont lieu au sein des solides à transition de spin tel que : l effet LIESST, l Hystérésis Thermique Induite par la Lumière (LITH), le processus de relaxation, l effet de la pression et l effet de taille. La deuxième partie a été dédiée à l étude de l effet d une pression externe sur les propriétés hystérétiques des CTS. En utilisant la méthode du diagramme FORC nous avons étudié l effet de la pression sur les paramètres physiques caractéristiques aux CTS. Cette étude a été complétée soigneusement par l étude de l effet de la pression sur les interactions entre domaines, par l étude des cycles mineurs obtenus entre deux températures fixes, à différentes pressions. Toujours dans cette section, nous avons élucidé la nature de la phase cristallographique induite par la pression dans le composé [Fe(PM-BiA)2(NCS)2]. La troisième partie est consacrée à l étude de l effet des contre-anions dans les composés 1D [Fe(NH2trz)3]Anion nH2O, où une méthode d évaluation de la pression interne induite par les anions de différentes tailles insérés entre les chaînes de molécules, a été proposée. Enfin, dans la quatrième partie nous avons étudié l effet de taille dans les systèmes de nanoparticules du CTS [Fe(NH2-trz)3](Br)2.3H2O.0.03(surfactant) (NH2-trz =4-amino-1,2,4-triazole Surfactant = Lauropal). A ce sujet nous proposons une nouvelle technique qui permet de trouver la dépendance de la largeur du cycle d hystérésis en fonction de la taille du système, en partant d une distribution de taille quelconque.The main aim of this work is the study of the pressure and the size effect on the hysteretic properties of the spin transition compounds (CTS). Our work was split in four main directions. The first part relates to the development of the atom-phonon coupling model, in order to be able to describe the various processes which take place within the spin transition solids, such as: the LIESST effect, the Light Induced Thermal Hysteresis (LITH), the relaxation process, pressure and size effect. The second part was dedicated to the study of the effect of an external pressure on the hysteretic properties of the CTS. By using the FORC diagram method we studied the effect of an applied pressure on the characteristic physical parameters of the CTS. This study was carefully completed by the study of the pressure effect on the inter-domains interactions, by the study of the minor cycles obtained between two fixed temperatures, at different pressures. Always in this section, we elucidated the nature of the pressure-induced crystallographic phase of the [Fe(PM-BiA)2(NCS)2] spin crossover compound. The third part is devoted to the study of the effect of the counter-anions in the [Fe(NH2trz)3]Anion nH2O 1D spin crossover compound, where a method to evaluate the internal pressure induced by the anions of different sizes has been proposed. Finally, in the fourth part, we studied the size effect on the nanoparticle systems of [Fe(NH2-trz)3](Br)2.3H2O.0.03(surfactant) (NH2-trz =4-amino-1,2,4-triazole Surfactant = Lauropal) spin crossover compound. Here we propose a novel method which makes it possible to find the dependence of the hysteresis loop width upon the size of the system.VERSAILLES-BU Sciences et IUT (786462101) / SudocSudocFranceF

Ligand field strength tuning in the model [Fe(H2Bpz2)2(bipy)] spin crossover complex

The original magnetism of a model spin crossover complex [Fe(H2Bpz2)2(bipy)] (H2Bpz2 = dihydrobis(1-pyrazolyl)borate, bipy = 2,2′-bipyridine) has been altered from spin state switching to paramagnetic behavior, through grafting a weak electronic-donating group: bromomethyl (in 1) and Br (in 2) onto the bipy ligand at positions C5 and C5´. The introduction of the electron-donating bromo groups into the bipy ligand reduce the π-acceptor character, leading to a weaker ligand field strength, thus stabilizing the high-spin state

Spin crossover in two 1D Fe(II) polymers with 1,2,4-triazole thiourea building blocks

The 1D chain [Fe(Etutrz)3](ClO4)2∙1.5H2O (2) (Etutrz =1-ethyl-3-(4H-1,2,4-triazol-4-yl) thiourea), displays pronounced thermochromism with a purple color at 77 K while the sample is white at 300 K. Investigation of magnetic properties reveal an abrupt spin transition around 227 K. Differential scanning calorimetry studies on cooling display a first order phase transition at around 200 K with an entropy variation of DeltaS = 61.3 J mol-1 K-1. 57Fe Mössbauer spectroscopy of (2) confirms a complete spin transition with a 100% high-spin population at 300 K (isomer shift deltaHS = 1.04(1) mm/s, quadrupole splitting DeltaEQ = 2.86(2) mm/s). The Fe(II) ions convert to the low-spin state at 78K (deltaLS = 0.53(2)mm/s). The quadrupole splitting, DeltaEQ = 0.29(2) mm/s, confirms the presence of distorted octahedra within the 1D chain. The 1D chain [Fe(Etutrz)3](BF4)2∙2MeOH (1) exhibits a different magnetic behavior with a gradual spin conversion at T1/2 = 221 K, whereas thermochromic properties are maintained