Distant-neighbor exchange constants from magnetization steps inZn1−xCoxTe

Direct measurements of several exchange constants ${\mathit{J}}_{\mathit{i}}$, for several neighbors located at different distances ${\mathit{r}}_{\mathit{i}}$, are possible via the magnetization-steps method. Predictions for the distance dependence of the exchange constant can then be tested. The theory of the magnetization steps (MS’s) caused by distant neighbors is presented. This theory is based on a sequence of cluster models. The problem of identifying the particular exchange constant ${\mathit{J}}_{\mathit{i}}$ that is responsible for an observed series of MS’s is also discussed. The magnetization of ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Co}}_{\mathit{x}}$Te, with x=0.007, was measured at 0.6 K in magnetic fields up to 27 T, and at 0.08 K in fields up to 20 T. Two series of MS’s were observed. These series were identified as being due to pairs of next-nearest neighbors (${\mathit{J}}_2$ pairs), and pairs of third neighbors (${\mathit{J}}_3$ pairs). The exchange constants deduced from the data are ${\mathit{J}}_2$/${\mathit{k}}_{\mathit{B}}$=-(5.7±0.6) K and ${\mathit{J}}_3$/${\mathit{k}}_{\mathit{B}}$=-(2.7±0.3) K. These values, and the earlier result ${\mathit{J}}_1$/${\mathit{k}}_{\mathit{B}}$=-38 K for the nearest-neighbor exchange constant, are compared with several proposed dependences of ${\mathit{J}}_{\mathit{i}}$ on distance.