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 {Gd3}[Mn2](Ge2Ga)O12. [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 1/χ vs T curve is linear for T>35°K with an intercept of −8°K at 1/χ = 0 and a Curie constant of 5.6×10−3 μB deg per oersted per formula unit. The magnetic moment at 0°K probably corresponds to 3nB{Gd3+} −2nB[Mn2+] = 11 μB because the moments observed at temperatures between 4.2 and 1.5°K extrapolate to not less than 9.6 μB at 0°K. The moment decreases for the compositions, {MnGd2}[Mn2](Ge3)O12 and {CaGd2}[Mn2](Ge3)O12, in that order. In these three cases the 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 {MnY2}[Mn2](Ge3)O12 and {CaY2}[Mn2](Ge3)O12 down to 1.3°K. For these compounds the 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.