Magnetic-Ion Interaction in Gd3Mn2Ge2GaO12 and Related Garnets

The existence of appreciable (c)‐(a) ferrimagnetic exchange interaction in the garnet structure has been established by the observation of ferrimagnetism in $\{{\mathrm{Gd}}_3\mathrm{\}[}{\mathrm{Mn}}_2\mathrm{](}{\mathrm{Ge}}_2\mathrm{Ga}){\mathrm{O}}_{12}$ . [The dodecahedral, or 24(c), ions are enclosed by {}, the octahedral, or 16(a), ions by [] and the tetrahedral, or 24(d), ions by ().] The Néel temperature is about 8°K; the $\text{1/χ}$ vs $T$ curve is linear for $\text{T>35°}\mathrm{K}$ with an intercept of −8°K at $\text{1/χ\hspace{0.17em}=\hspace{0.17em}0}$ and a Curie constant of ${\text{5.6×10}}^{\text{−3}}{\mathrm{\hspace{0.17em}\mu }}_B$ deg per oersted per formula unit. The magnetic moment at 0°K probably corresponds to ${\text{3n}}_B\{{\mathrm{Gd}}^{\text{3+}}{\text{}\hspace{0.17em}−2n}}_B[{\mathrm{Mn}}^{\text{2+}}{\text{]\hspace{0.17em}=\hspace{0.17em}11\hspace{0.17em}μ}}_B$ because the moments observed at temperatures between 4.2 and 1.5°K extrapolate to not less than ${\text{9.6\hspace{0.17em}μ}}_B$ at 0°K. The moment decreases for the compositions, $\{{\mathrm{MnGd}}_2\mathrm{\}[}{\mathrm{Mn}}_2\mathrm{](}{\mathrm{Ge}}_3){\mathrm{O}}_{12}$ and $\{{\mathrm{CaGd}}_2\mathrm{\}[}{\mathrm{Mn}}_2\mathrm{](}{\mathrm{Ge}}_3){\mathrm{O}}_{12}$ , in that order. In these three cases the $\text{1/χ}$ vs $T$ curves are concave downward below 20°K, as would be expected in the case of ferrimagnetism. No spontaneous magnetization is observed in $\{{\mathrm{MnY}}_2\mathrm{\}[}{\mathrm{Mn}}_2\mathrm{](}{\mathrm{Ge}}_3){\mathrm{O}}_{12}$ and $\{{\mathrm{CaY}}_2\mathrm{\}[}{\mathrm{Mn}}_2\mathrm{](}{\mathrm{Ge}}_3){\mathrm{O}}_{12}$ down to 1.3°K. For these compounds the $\text{1/χ}$ vs $T$ curve is concave upward below 20°K as would be expected for a weak (a)‐(a) antiferromagnetic interaction. Only a very weak (c)‐(a) interaction is present in the first case as a consequence of one‐third occupancy of dodecahedral sites by Mn²⁺ ions.