Abstract
The measurement of the temperature dependence of the magnetization, Ms, between 4.2 and 300 K of epitaxial films of Y3+3−xCa2+xFe3+5−y−x′ Ga3+yFe4+x′O2−12, with x<0.4 and 0≤y<1.6, has shown low-temperature deviations from the molecular field theory of a two-sublattice Néel model, generally applicable to yttrium iron garnet with nonmagnetic substitutions. Molecular field calculations of M(x,y,T) have been performed, assuming a temperature-dependent concentration of Fe4+ ions due to the gradual localization of the uncompensated hole at the tetrahedral Fe3+ sites with the lowering of the temperature. The Fe4+ ions are assumed to form a third magnetic sublattice, which is ferromagnetically coupled to the octahedral Fe3+. The hole localization energy is observed to be on the order of 1–4 meV. The calculations are in good agreement with the experimental results.
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