Claude Coulon

Glauber dynamics in a single-chain magnet: From theory to real systems

The Glauber dynamics is studied in a single-chain magnet. As predicted, a single relaxation mode of the magnetization is found. Above 2.7 K, the thermally activated relaxation time is mainly governed by the effect of magnetic correlations and the energy barrier experienced by each magnetic unit. This result is in perfect agreement with independent thermodynamical measurements. Below 2.7 K, a crossover towards a relaxation regime is observed that is interpreted as the manifestation of finite-size effects. The temperature dependences of the relaxation time and of the magnetic susceptibility reveal the importance of the boundary conditions.

Record Ferromagnetic Exchange through Cyanide and Elucidation of the Magnetic Phase Diagram for a CuIIReIV(CN)2 Chain Compound

Reaction of the high-magnetic anisotropy building unit [ReCl(4)(CN)(2)](2-) with [Cu(MeCN)(6)](2+) and hydrotris(pyrazol-1-yl)borate (Tp(-)) affords the zigzag chain compound (Bu(4)N)[TpCuReCl(4)(CN)(2)]. Dc magnetic susceptibility measurements reveal the presence of ferromagnetic exchange coupling between Re(IV) and Cu(II) centers along each chain and a fit to the data gives an exchange constant of J/k(B) = +41 K (+29 cm(-1)), representing the strongest ferromagnetic coupling yet observed through cyanide. Below 11.4 K and at applied fields of less than 3600 Oe, the compound undergoes a phase transition to an antiferromagnetic ground state, stemming from weak π-π interchain interactions of strength J(⊥)/k(B) = -1.7 K (-1.2 cm(-1)). This metamagnetic behavior is fully elucidated using both experimental and theoretical methods. In addition, theoretical modeling provides a detailed determination of the local anisotropy tensors corresponding to the [ReCl(4)(CN)(2)](2-) units and demonstrates that the zigzag arrangement of the Re(IV) centers significantly reduces the effective anisotropy of the chain. These results demonstrate the utility of the Re(IV)-CN-Cu(II) linkage and the importance of anisotropic spin orientation in designing strongly coupled systems, which will aid in both the realization of single-chain magnets with higher relaxation barriers and in the construction of high-dimensional cyano-bridged materials exhibiting higher ordering temperatures.

Synthesis, structure and preliminary results on electrical and magnetic properties of (Perylene)2 [Pt(mnt)2]

Abstract In this paper we report the preparation, crystal structure and some physical properties of the conducting molecular (Perylene)2 [Pt(S4C4(CN)4)]. The crystal structure consists of segregated stacks of perylene and Pt S4C4 (CN)4 units, with a spacing of 3.3 A between the carbon atoms in the perylenes. The temperature dependence of the electrical conductivity is metallic with a maximum at approximately 15 K. At this temperature a sharp transition to an insulating state occurs. Magnetic suceptibility measurements confirm the existence of a transition in this temperature range.

A canted antiferromagnetic ordered phase of cyanido-bridged MnIII2ReIV single-chain magnets

A new cyanido-bridged Re(IV)-Mn(III) heterometallic 1D system, [Mn(III)(5-Me-saltmen)](2)[Re(IV)Cl(4)(CN)(2)]·3CH(3)CN (1), was designed and structurally characterized. Interchain interactions stabilize a canted antiferromagnetic ordered state below 6.2 K that does not prevent slow relaxation of the magnetization reminiscent of the single-chain magnet properties of the individual chains.

Quantum nucleation in a single-chain magnet.

The field sweep rate (upsilon = dH/dt) and temperature (T) dependence of the magnetization reversal of a single-chain magnet is studied at low temperatures. As expected for a thermally activated process, the nucleation field (H(n)) increases with decreasing T and increasing upsilon. The set of H(n)(T,upsilon) data is analyzed with a model of thermally activated nucleation of magnetization reversal. Below 1 K, H(n) becomes temperature independent but remains strongly sweep rate dependent. In this temperature range, the reversal of the magnetization is induced by a quantum nucleation of a domain wall that then propagates due to the applied field.

Spin crossover or intra-molecular electron transfer in a cyanido-bridged Fe/Co dinuclear dumbbell: a matter of state†

The design of molecule-based systems displaying tuneable optical and/or magnetic properties under external stimuli has received a great deal of attention in the past few years. This interest is driven by the potential applications in high-performance molecule-based electronics in the areas of recording media, switches, sensors, and displays. As an example, three-dimensional Fe/Co Prussian blue compounds exhibit a concomitant change in magnetic and optical properties due to a temperature- or light-induced metal-to-metal electron transfer. The foregoing remarkable properties in Prussian blues prompted us to design soluble molecular fragments of these coordination networks through a rational building-block approach in order to mimic their properties on a single molecule. With a judicious choice of the ligands for the iron and cobalt molecular precursors, we prepared a dinuclear cyanido-bridged Fe/Co complex that exhibits an unexpected temperature-dependent spin crossover in the solid state while an intramolecular electron transfer triggered by protonation is observed in solution.

Single-Chain Magnets and Related Systems

In this chapter, the static and dynamic magnetic properties of single-chain magnets and related systems are reviewed. We will particularly focus on the so-called Ising limit for which the magnetic anisotropy energy is much larger than the energy of the intrachain exchange interactions. The simple regular chain of ferromagnetically coupled spins will be first described. Static properties will be summarized to introduce the dominant role of domain walls at low temperature. The slow relaxation of the magnetization will be then discussed using a stochastic description. The deduced dynamic critical behavior will be analyzed in detail to explain the observed magnet behavior. In particular, the effect of applying a magnetic field, often ignored in the literature, will be discussed. Then, more complicated structures including chains of antiferromagnetically coupled magnetic sites will be discussed. Finally, the importance of interchain couplings will be introduced to discriminate between a “real” single-chain magnet and a sample presenting both a magnet-type property and a three-dimensional antiferromagnetic ordered state at low temperature.

Bimetallic cyanido-bridged magnetic materials derived from manganese(III) Schiff-base complexes and pentacyanidonitrosylferrate(II) precursor

Three heterobimetallic MnIIIFeII complexes: [Mn(5-Br-salpn)(H2O)]2[Fe(CN)5NO]·2H2O (1), [{Mn(salen)}2Fe(CN)5NO]·2H2O (2) and [{Mn(saltmen)}4Fe(CN)5NO](ClO4)2·H2O·2CH3OH (3), have been synthesized by the reactions of MnIII/Schiff–base (SB) complexes, [Mn(SB)(H2O)]+, (SB being salen2− = N,N′-ethylenebis(salicylideneiminato) dianion; 5-Br-salpn2− = N,N′-1,3-propylenebis(5-bromosalicylideneiminato) dianion or saltmen2− = N,N′-(1,1,2,2-tetramethylethylene) bis(salicylideneiminato) dianion) with the [Fe(CN)5NO]2− building block. X-Ray diffraction analyses on single crystals reveal that 1 has a trinuclear molecular structure, 2 displays a two-dimensional (2D) network structure, and 3 possesses a new 2D network of MnIII dinuclear motifs. The magnetic measurements show that the interaction between the MnIIIS = 2 spins through the [NC–Fe–CN] bridges is systematically of antiferromagnetic nature: J/kB = −0.95(5) K and J/kB = −1.15(5) K for 1 and 2, respectively, while the direct MnIII–MnIII interaction, in the {Mn2(saltmen)2} dinuclear units present in 3, is ferromagnetic with J/kB = +1.75(5) K. Due to the ST = 4 spin ground state of these dinuclear units and the uniaxial magnetic anisotropy brought by the Mn(III) metal ions, 3 exhibits single-molecule magnet properties. The 400–900 nm optical properties of the three compounds have been also investigated, showing no significant photoactivity unlike in the Na2[Fe(CN)5NO] precursor.

Single crystal synthesis of [(C6H5)4P]2[C70][I] by electrocrystallization and experimental determination of the g-value anisotropy of C70•- and C60•- at 4.2 K

We describe the synthesis by electrocrystallization and the crystal packing of [(C6H5)4P]2[C70][I]. Magnetic susceptibility measurements show Curie-Weiss behaviour. ESR measurements on single crystals of [(C6H5)4P]2[C70][I] and [(C6H5)4P]2[C60][I]x (0<x⪡1) allowed us to determine the g tensor principal values at 4.2 K for both radicals C60•- (gx = gy = 1.9970, gz = 1.9998, gav = 1.9979) and C70•- (gx = gy = 1.9996, gz (long axis) = 2.0150, gav = 2.0047).

One-dimensional coordination polymers of antiferromagnetically-coupled [Mn4] single-molecule magnets

Reactions of the rhombic [MnII2 MnIII2 (hmp)6]4+ complex in acetonitrile with simple carboxylate ligands yield (i) three new isolated [Mn4] complexes, namely [Mn4(hmp)6(CH3COO)2(H2O)2](ClO4)2.4H2O (1), [Mn4(hmp)6(CCl3COO)2(H2O)2](ClO4)2 (2) and [Mn4(hmp)6(C6H5COO)2(H2O)2](ClO4)2.4CH3CN.2H2O (3) in the presence of either bulky carboxylate or of an excess of Mn(II) source; and (ii) two 1D arrangements of [Mn4] complexes connected through double syn-syn carboxylate bridges when using acetate and chloroacetate, namely {[Mn4(hmp)6(CH3COO)2](ClO4)2.H2O}n (4) and {[Mn4(hmp)6(ClCH2COO)2](ClO4)2.2H2O}n (5). The assembly of such building blocks can thus be controlled by an adequate choice of the bridging anion. As expected, the isolated [Mn4] complexes behave as Single-Molecule Magnets as shown by the study of their static and dynamic magnetic properties. Detailed magnetic studies both on polycrystalline samples and single crystals show that the chain compounds are isolated antiferromagnetic chains. The slow relaxation of their staggered magnetization has been studied thanks to finite-size effects induced by the intrinsic defects of the material