Magnetic chain, thermodynamic

Exchange constants for low-dimensional magnets are most commonly obtained via comparison of experimental data to the predicted behavior of a thermodynamic property for a given model, usually the magnetic susceptibility. Johnston et al. showed that the molar susceptibility xm of the uniform chain can be expressed as a ratio of polynomials in powers ofthe reduced temperature t t = hgT/ 2J ). The coefficients N and D are listed in Table 1. 

The low-temperature (10 mK < T < 20 K) thermodynamics of quinolinium (TCNQ)2 are now rather well represented by random-exchange Heisenberg anti-ferromagnetic chains (REHACs). While MP-TCNQ has similar power laws , its magnetic specific heat is quite different possibly due to facile methyl rotation about the 6-fold MP barrier . Disorder is surely important, but MP-TCNQ is complicated in this respect also. 

Thermodynamic properties of non-regular chains have been described in reference [27]. In that case, several correlation functions T) should be introduced. Taking the simple example of a chain of spin S with an alternation of 7i and J2 magnetic exchanges (i.e. a chain of dimers with Ji and J2 being the intra-dimer and inter-dimer interactions, respectively), the corresponding Ising Hamiltonian reads ... 

The above expressions show that the key analysis of a single-chain magnet system is the comparison between susceptibility and relaxation data. In fact, the observation of an activated relaxation time is not characteristic of SCM behavior (a similar behavior is obtained for other systems like SMMs) and a discussion based only on the dynamic data remains highly ambiguous. The situation is different if the thermodynamic and dynamic properties are compared as A can be deduced from susceptibility data and then considered to... 

An nuclear magnetic resonance (NMR) study of the same complex indicates that the observed hexacoordination in the crystal structure is combined with a fast exchange of the donor thioethers in solution as all carbon atoms of the side chains have equivalent signals. Even at temperatures of 183 K, the signals remain equivalent. This could lead to an extra stability, but despite the high thermodynamic stability the kinetic stability is still too low and slow exchange with serum proteins has been observed. 

---