Antiferromagnetic coupling magnetic moment

Spin freezing conditions exist for the 2a site (if occupied by magnetic lanthanide or actinide ions) as well. It is believed (Gal et al. 1990) that this behavior of the lanthanide or actinide sublattice is an outcome of conflicting RKKY interactions between the antiferromagnetically coupled (Fe) moments on the 8f site and the ferromagnetic (An) moments on the 2a sites. 

Figure 9.9 Magnetic defects in FeO (a) antiferromagnetic alignment of magnetic moments in nominally stoichiometric FeO with the spins perpendicular to [111] (Z>) the simplest defect cluster in FeO, with the spin on the interstitial Fe lying in (111) and (c) antiferromagnetic coupling of the surrounding Fe ions with all spins lying in (111).

The six high spin Fe sites in the [(Fe6S5X5)(M(CO)3)2]" anions are antiferromagnetically coupled as evidenced by the reduced ambient temperature magnetic moments, and the temperature dependence of the isotropically shifted resonances in the aryloxide derivatives. 

MMCT) transition. A small drop in the effective magnetic moment, observed as the temperature is lowered, is indicative of weak antiferromagnetic coupling. ... 

---