Magnets, molecular Ferromagnetism

The field of the heteropolymetallic magnetic systems is still rather new. As a matter of fact, the first EPR spectrum of an heteropair appeared in 1967 and the first preliminary magnetic data dealing with compounds of this type in 1968. This article is, to our knowledge, the first review on the subject. It largely concerns heterobinuclear complexes and covers the field up to the first attempts to obtain molecular ferromagnets. One may anticipate that, in a few years, another review will be necessary and that its main emphasis will be the systems of higher nuclearity and the extended systems with subtle spin orders. 

As early as 1967, the chloride complex [FeCl(S2CNEt2)2] was shown to be a molecular ferromagnet (1253). Over the past 20 years a number of further studies have been carried out on the magnetism of this system (and the related bromide), most notably by De Fotis et al. (1254—1258). It has a ferromagnetic transition temperature of 2.457 K, while mixtures of [FeCl(S2CNEt2)2] and [FeBr(S2CNEt2)2] are characterized by transition temperatures of between 2.213 and 1.613 K (1255). 

The little atomic magnets are of course quantum mechanical, but Weiss s original theory of paramagnetism and ferromagnetism (1907) [7] predated even the Boln atom. He assumed that in addition to the external magnetic field Bq, there was an additional internal molecular field B. proportional to the overall magnetization M of the sample. 

The dielectric permittivity as a function of frequency may show resonance behavior in the case of gas molecules as studied in microwave spectroscopy (25) or more likely relaxation phenomena in soUds associated with the dissipative processes of polarization of molecules, be they nonpolar, dipolar, etc. There are exceptional circumstances of ferromagnetic resonance, electron magnetic resonance, or nmr. In most microwave treatments, the power dissipation or absorption process is described phenomenologically by equation 5, whatever the detailed molecular processes. 

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