Interactions spin ladders

Concerning molecule-based magnets, the first spin-ladder was synthesized (/ -EPYNN)[Ni (dmit)2] (/ -EPYNN = / -7V-ethylpyridinium o-nitronyl nitroxide). Within the crystal lattice, the radical cation /7-EPYNN units are arranged in ID chains with ferromagnetic interactions. The chains of [Ni(dmit)2] moieties in the ladder formation exhibit coexistent antiferromagnetic interactions.1031,1032... 

A deeper analysis [80] on this Cu - Dy chain showed that NN and NNN interactions are efficient above Tc and that inter spin-ladder ferromagnetic interaction is observed in this family of compounds [79-82]. Attempts were made to use the 3d tectons with other lanthanide precursors, such as hexanuclear lanthanide clusters. Regular chains were obtained with Dy-Cu ferromagnetic interaction but their close packing prevents observation of SCM behaviour [83]. 

Very often, however, these dyads are not isolated in the solid state and interact with neighboring ones at least along one preferential direction. In this case, we can distinguish two important situations the alternated spin chain and the spin ladder. As shown in Scheme 5, the alternated spin chain is characterized with two different magnetic interactions, noted / and aJ with 0 < a < 1. Note that if a = 0, one recovers the singlet-triplet behavior while if a = 1, we are in the presence of a uniform spin chain. The spin ladder is also characterized by two J values, noted J// and J in the following. 

One of these models is the spin- ladder with competing interactions of the ferro- and antiferromagnetic types at the F-AF transition line. The exact singlet ground-state wave function on this line is found in the special form expressed in terms of auxiliary Bose-operators. The spin correlators in the singlet state show double-spiral ordering with the period of spirals equal to the system size. 

This section is devoted to four main topics spin-Peierls systems, spin-ladder systems, systems exhibiting ferromagnetic interaction, and bulk ferromagnets. To our knowledge, no dithiolene-based films (including LB films) have been studied for their ferromagnetic properties and no patent has claimed the use of these materials in this area. 

The related (p-EPYNN)[Au(dmit)2] complex has also been synthesized. However, its crystal structure does not correspond to a true spin-ladder system but rather to ID chains of [Au(dmit)2 alternating with ID chains of (p-EPYNN)+ still presenting ferromagnetic interactions at low temperature (304). 

Although isostructural to the Au complex, the Ni and Pt salts do not form spin-ladder systems. This may be due to slight differences in the transfer integrals within the DT-TTF stacks and also to the paramagnetism of [M(mnt)2] (M = Ni, Pt) that may interact with the DT-TTF system. Related Co and Fe compounds have also been reported (311), but they are just simple ionic salts as shown by their 1 1 stoichiometry (instead of the 2 1 stoichiometry for the Au, Ni and Pt compounds). Very recently, Ribera et al. (307) published an excellent and detailed report on this family of compounds. 

The low-temperature magnetization (insert of previous figure) of BPCB is essentially zero until 5 tesla when the gap is closed by the Zeeman interaction. There is a rapid increase until 17 tesla when the field overcomes both the rung and rail exchanges. The ladder and alternating chain models make different predictions for M(H) leading to the conclusion that BPCB is a spin ladder with IJrwJk = -13.3 K and Trail/ = -3.8 K. 

From the magnetic point of view, the magnetic interactions define the topology of a 2-leg spin-ladder. Figure 5 shows five of such spin-ladders a central one surrounded... 

Figure 7. Magnetic topology of the TOLKEK crystal. Each radical is represented by a point site and the connection lines indicate pairs of radicals where the Jab (4i) interaction is non-negligible. Note that in the Figure Jab (4i) = J. There is a central spin-ladder (their J, J, and Js, pairs are specified) and four surrounding spin-ladders, numbered 1 to 4 as in Figure 5.

Besides the electrical conductivity, a number of other applications of [M(dmit)2] have been investigated. For example, molecular-based magnets, such as the spin ladder compound p-EPYNN][Ni(dmit)2] [p-FFYNN]" " = p-N-ethylpyridinium a-nitronyl nitroxide) was reported in 1996. In the crystal, the radical cationic molecules, p-EPYNN, form one-dimensional chains with ferromagnetic interactions, while the chains of [Ni(dmit)2] monovalent anions, in a ladder formation, exhibit antiferromagnetic interactions. The magnetisation is explained by the sum of them. 

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