Ferromagnetic intermolecular interactions

This chapter will show that only atoms with partially filled shells (i.e. atoms with unpaired electrons) can possess a net magnetic moment in the absence of an external field. Since main group p block) elements have atoms with filled d subshells and tend to form compounds with other p-block elements that result in filled p subshells in accordance with the octet rule, the vast majority of magnetic materials have historically contained transition metal atoms with partially filled d subshells. Nevertheless, some pure organic compounds with free radicals have been found to exhibit ferromagnetic intermolecular interactions, albeit at very low temperamres (several Kelvins). 

Awaga, K., Sugano, T., and Kinoshita, M., Ferromagnetic intermolecular interactions in a series of organic mixed-crystals of galvinoxyl radical and its precursory closed shell compound, J. Chem. Phys., 85, 2211, 1986. 

Hosokoshi, Y, Tamura, M., Kinoshita, M., Sawa, H., Kato, R., Eujiwara, Y., and Ueda, Y., Magnetic properties and crystal structure of the p-fluorophenyl nitronyl nitroxide radical crystal ferromagnetic intermolecular interaction leading to a three-dimensional network of the ground triplet dimer molecules, J. Mater. Chem., 4, 1219, 1994. 

Awaga, K. and Maruyama, Y, Ferromagnetic intermolecular interaction of the organic radical, 2-(4-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-lH-imidazolyl-l-oxy 3-oxide, Chem. Phys. Lett., 158, 556, 1989. 

The temperature dependence of the molar magnetic susceptibility (x) of an assembly of paramagnetic spins without interaction is characterized by the Curie behavior with x = C/T where C = /Vy2( 2.S (.S + l)/3k. It is a very common situation in the organometallic chemistry of radical species when the spin density is essentially localized on the metal atom. Since, in most cases, this atom is surrounded by various innocent ligands, intermolecular interactions are very weak and in most cases are reflected by a small contribution described by a Curie-Weiss behavior, with x = C/(T 0) where 0 is the Curie-Weiss temperature. A positive value for 0 reflects ferromagnetic interactions while a negative value — the most common situation — reflects an antiferromagnetic interaction. 

In Fig. la, the antiferromagnetic (AF) interaction A—A favors the AF interaction between tbe Sg molecular spins. In lb, the AF interaction A---B favors a ferromagnetic (F) interaction between the molecular spins. In Ic, the F interaction A—A favors an F interaction between the molecular spins. In Id, finally, the F interaction A—B favors the AF interaction between the molecular spins. The situation in Fig. lb is by far the most interesting. We have seen that it is much easier to create an antiferro- than a ferromagnetic interaction between two spin carriers. This is true for both intra- and intermolecular interactions. 

The effect of intermolecular interaction becomes important at low temperature when the ground state is magnetically productive. An example is given in Fig. 10.17 where a maximum on the product function (or the effective magnetic moment) replaces a plateau expected for pure ferromagnetic intercentre coupling. 

Takeda, K., Konishi, K., Tamura, M., and Kinoshita, M-, Pressure effects on intermolecular interactions of the organic ferromagnetic crystalline, P-phase p-nitrophenyl nitronyl nitroxide, Phys. Rev. B53, 3374, 1996. 

The family of nitronyl nitroxide (4,4,5,5-tetramethyli-midazoline-l-oxyl 3-oxide) derivatives has been shown to constitute a treasure house of organic magnetism. The derivatives form very stable crystals of various types of symmetry. Table 15.1 compiles the crystallographic data of the nitronyl nitroxide derivative studied in our laboratories. Nitronyl nitroxides often exhibit intermolecular ferromagnetic (FM) interactions as well as antiferromagnetic (AFM) interactions. Furthermore, the interactions are zero-, one-, two- or three-dimensional (OD, ID, 2D or 3D) as a result of various kinds of crystal structures. Thus this group of radicals exhibits a variety of magnetisms. 

Fig. 2 Representation of the ferromagnetic alignment of d localized spins resulting from the interaction with itinerant electrons through short intermolecular contacts...

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