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.Comment: Submitted to PRL 10 May 2003. Submitted to PRB 12 December 2003; published 15 April 200

Quantum nucleation in a single-chain magnet

The field sweep rate (v=dH/dt) and temperature (T) dependence of the magnetization reversal of a single-chain magnet (SCM) is studied at low temperatures. As expected for a thermally activated process, the nucleation field (H_n) increases with decreasing T and increasing v. The set of H_n(T,v) 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.Comment: 5 pages, 4 figure

Cyanomethylene-bis(phosphonate) as ditopical ligand: stepwise formation of a 2-D heterometallic Fe(III)-Ag(I) coordination network

A new heteroditopic ligand, cyanomethylene-bis(5,5-dimethyl-2-oxo-1,3,2 lambda(5)-dioxa-phosphorinane) 1 (bphosCN), has been reacted with Fe(ClO4)(3) to afford the mononuclear complex Fe(bphosCN)(3) 2 which crystallized in the cubic system, space group Pa (3) over bar. The iron center, chelated by the oxygen atoms of the ligand, shows an almost perfect octahedral geometry, with the CN groups disposed at 120 degrees each other. Further reaction with AgClO4 provided the heterometallic coordination polymer (infinity)(2) {[(Fe (bphosCN)(3))(3)Ag-3(H2O)(2)]center dot(ClO4)(3)} 3 as the unique crystalline polymorph, in the monoclinic space group P21/a. The targeted 2D honeycomb type structure has been achieved through an interplay between coordinative CN-Ag bonds and CN center dot center dot center dot H2O bonds. The magnetic measurements demonstrate the existence of isolated paramagnetic Fe(III) centers in both complexes

Radical cation salts of TTF donors with XF6 (X = Re,Ta) anions

Electrocrystallization of the dianionic Re(IV)F6 species with different organic π electron donors was carried out. Depending on the first oxidation potential of the TTF derivatives this crystallization technique gave rise to various radical cations salts involving Re(IV) or Re(V) anions. With tetramethyltetrathiafulvalene (TMTTF) the (TMTTF)2Re(IV)F6  salt was obtained. However, with tetramethyltetraselenafulvalene (TMTSF) we obtained very small needles of the (TMTSeF)2Re(V)F6 system, analogue to the so called “Bechgaard salts” [1]. We present some characteristics of this new metallic phase. We compare to the diamagnetic (TMTSeF)2Ta(V)F6 obtained by the same strategy using TBATaF6 prepared according to the Browstein’s method [2] and explore electrocrystallization of the tantalum based anions [3], [4]. References: [1] K. Bechgaard, C.S. Jacobsen, K. Mortensen, H.J. Pedersen, N. Thorup, Solid State Commun. 1980, 33, 1119-1125. [2] S. Brownstein, Inorg. Chem. 1973, 12, N°3, 584-589. [3] C. Lenoir, K. Boubekeur, P. Batail, E. Canadell, P. Auban, O. Traetteberg, D. Jérome, Synth. Met. 1991, 42, 1939-1942. [4] F. Iwase, K. Sugiura, K. Furukawa, T. Nakamura, J. Phys. Soc. Jpn. 2009, 78, N°10, 104717:1-104717:7

Assisted Self-Assembly to Target Heterometallic Mn-Nd and Mn-Sm SMMs: Synthesis and Magnetic Characterisation of [Mn7_{7}Ln3_{3}(O)4_{4}(OH)4_{4}(mdea)3_{3}(piv)9_{9}(NO3_{3})3_{3}] (Ln=Nd, Sm, Eu, Gd)**

Assisted self-assembly by using a preformed [Mn$_{6}$O$_{2}$(piv)$_{10}$(4-Me-py)$_{2}$(pivH)$_{2}$] (1) (pivH=pivalic acid) cluster leads to 3 different outcomes of the reaction with Ln(NO$_{3}$)$_{3}$ ⋅ xH$_{2}$O (Ln=Pr−Ho and Y): Mn$_{7}$Ln$_{3}$ and in some cases the inverse butterfly Mn$_{2}$Ln$_{2}$ for Ln=Pr−Eu, Mn$_{5}$Ln$_{4}$ for Ln=Tb-Ho and in the special case of Gd all three possibilities can form. Whilst the magnetic properties for the Mn$_{5}$Ln$_{4}$ systems show the expected known SMM properties, for Mn$_{7}$Ln$_{3}$ two new examples of Nd- and Sm-containing SMMs have been discovered. in an assisted self-assembly approach starting from the [Mn$_{6}$O$_{2}$(piv)$_{10}$(4-Me-py)$_{2}$(pivH)$_{2}$] cluster a family of Mn−Ln compounds (Ln=Pr−Yb) was synthesised. The reaction of [Mn$_{6}$O$_{2}$(piv)$_{10}$(4-Me-py)$_{2}$(pivH)$_{2}$] (1) with N-methyldiethanolamine (mdeaH$_{2}$) and Ln(NO$_{3}$)$_{3}$ ⋅ 6H$_{2}$O in MeCN generally yields two main structure types: for Ln=Tb−Yb a previously reported Mn$_{5}$Ln$_{4}$ motif is obtained, whereas for Ln=Pr−Eu a series of Mn$_{7}$Ln$_{3}$ clusters is obtained. Within this series the Gd$^{III}$ analogue represents a special case because it shows both structural types as well as a third Mn$_{2}$Ln$_{2}$ inverse butterfly motif. Variation in reaction conditions allows access to different structure types across the whole series. This prompts further studies into the reaction mechanism of this cluster assisted self-assembly approach. For the Mn$_{7}$Ln$_{3}$ analogues reported here variable-temperature magnetic susceptibility measurements suggest that antiferromagnetic interactions between the spin carriers are dominant. Compounds incorporating Ln=Nd$^{III}$(2), Sm$^{III}$(3) and Gd$^{III}$ (5) display SMM behaviour. The slow relaxation of the magnetisation for these compounds was confirmed by ac measurements above 1.8 K

Cyanomethylene-bis(phosphonate)-Based Lanthanide Complexes: Structural, Photophysical, and Magnetic Investigations

10 pagesInternational audienceThe syntheses, structural investigations, magnetic and photophysical properties of a series of 10 lanthanide mononuclear complexes, containing the heteroditopic ligand cyanomethylene-bis(5,5-dimethyl-2-oxo-1,3,2λ5-dioxa-phosphorinane) (L), are described. The crystallographic analyses indicate two structural types: in the first one, [LnIII(L)3(H2O)2]*H2O (Ln = La, Pr, Nd), the metal ions are eight-coordinated within a square antiprism geometry, while the second one, [LnIII(L)3(H2O)]*8H2O (Ln = Sm, Eu, Gd, Tb, Dy, Ho, Er), contains seven-coordinated LnIII ions within distorted monocapped trigonal prisms...