A magnetic study of the ferro-magnesian olivines (Fe xMg 1− x)SiO 4, 0 < x < 1

Abstract

The magnetic properties of the ferromagnesian olivines, (Fe x Mg 1− x ) 2SiO 4, have been studied between 4 and 300°K. P eff for Fe 2+ ions in the olivines is equal to 5·2 and is constant throughout the series. The asymptotic Curie temperature varies roughly as θ A = −87 x, the observed deviation from linearity being consistent with an increasing exchange energy due to the contracting lattice. θ N falls with decreasing x in a manner predicted by calculations based on the series expansion of the susceptibility in terms of reciprocal temperature, with an initial slope [1/θ N (1)] dθ N ( x)/d x = 1·5 ± 0·2, after correcting for lattice contraction. The θ N— x curve also exhibits a ‘nose’ of the type predicted theoretically. The critical concentration below which long range magnetic order is not observed is x c ∼ 0·3. θ t, the spin structure transition temperature, falls more slowly than θ N and below x = 0·5, θ t cannot be distinguished from θ N . It may be that in the composition range 0·3 < x < 0·5, the transformation at θ N is from paramagnetism directly to a canted spin system. The observed ratio for θ N /θ A (= 0·72 ± 0·07) for x = 1 is in good aggreement with the value calculated from molecular field theory. Despite the complexity of the magnetic structure the behaviour of the series is in surprisingly good aggreement with theoretical predictions based on very much simpler systems.