Néel temperature inFexZn1−xF2

Wertheim et al. have carefully measured the Néel temperature of the mixed system of iron and zinc fluorides. Over the concentration range $0.25\le x\le 1$, their results show an essentially linear decrease with magnetic concentration of the form $\frac{T_N\mathrm{}\left(x\right)\mathrm{}}{T_N\mathrm{}\left(1\right)\mathrm{}}=mx+c$, where parameters $m$ and $c$ are approximately the following: $m\sim 1$ and $c\sim 0$. Such a result is in direct contrast with the earlier observations of Baker et al. on mixed crystals of manganese and zinc fluorides, where the corresponding choice for the parameters $m$ and $c$ would seem to be $m\sim \frac{5}{4}$ and $c\sim -\frac{1}{4}$. To explain these results we have carried out a coherent-potential-approximation (CPA) analysis which takes into account the relevent exchange interactions (including second-neighbor contributions) and the anisotropy. While for the Mn-Zn system the anisotropy is quite small, for the Fe-Zn system, it is rather large. As such, to treat the latter system we have combined the CPA with the Devlin type of RPA procedure. Our results are in excellent agreement with experiment. Moreover, it appears that the observed differences in the behavior of the two mixed systems are due primarily to differences in their anisotropies.