Ferrimagnetic coupling

In yttrium iron garnet Y3Fe5012 ( YIG ) a ferrimagnetic coupling (superexchange) is active between the octahedral and the tetrahedral sites. Since the tetrahedral sites are in excess, the magnetic moments do not compensate each other. The magnetic properties can be varied by substitution of yttrium by lanthanoids. 

In the ferrimagnetic coupling scheme, polyradical possesses aCUs and the spin sites connected in an alternating mode. Furthermore, the spin sites with unequal values of S associated with each spin site alternate, i.e., large, small, large, small, etc. This leads to uncompensated spin canceling for most connectivities between the spin sites, e.g., linear, branched, macrocyclic, etc. 

Annelated macrocyclic polyradicals consist of calix[4]arene-based macrocycles annelated via bis(3,4 -biphenylene)methyl linkers. This connectivity is consistent with the ferromagnetic-ferrimagnetic coupling scheme, for which scaling of S with the number of radical sites (or molecular weights) was attained.8,112... 

SQUID magnetic studies of 57 in THF-d8 matrix revealed a relatively high average value of S 18. Values of Msat 0.4-0.6 pB indicate 40-60% of unpaired electrons in the limit of very low temperature and high field for both polyradicals, which is consistent with the ferromagnetic-ferrimagnetic coupling scheme.8,113 These results may be compared with the value of Sx 40-50 expected for defect-free... 

Values of Msat 0.5 fiB, indicating presence of approximately 50% of unpaired electrons at low temperature, did not show any systematic dependence on degree of polymerization of polyether. Overall, this results were consistent with the ferromagnetic-ferrimagnetic coupling scheme and presence of chemical defects.8... 

Fig. 38 Polymers 57 and 58 with the ferromagnetic-ferrimagnetic coupling scheme with the macrocyclic (S — 3 and 5 = 2) and cross-linking (5=1 /2) modules.

Compared with the compounds with no B-moment the R-3d compounds are simpler with respect to spin structures, only ferrimagnetic coupling being present in the vast majority of cases, whereas the compounds with no B-moment exhibit an elaborate variety of spin structures as seen in section 2. On the other hand, however, the R-3d intermetallics possess three types of magnetic coupling (R-R, R-3d, 3d-3d) which are more difficult to describe theoretically than the R-R interactions in compounds where 3d-moments are absent. In addition magnetocrystalline anisotropy of the 3d-moments is extremely difficult to treat... 

As in the other series of R-3d compounds where R is a heavy lanthanide the RFe2 compounds show ferrimagnetic coupling between the R and Fe moments. The magnetization versus temperature curves can be well described by molecular field calculations (Hilscher et al., 1973 Burzo, 1974). Compensation points are found only for R = Tm and Er. The absence of compensation points for the other lanthanides is a result of the considerable negative R-Fe interaction (Buschow and van Stapele, 1971). The Fe-Fe interaction is however still the strongest (Burzo, 1974). 

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