Nuclear-magnetic-resonance study and simulation of the spin configuration in In- and Ga-substitutedCdCr2S4

The hyperfine fields at the nuclei of ${\mathrm{Cd}}^{2+}$, ${\mathrm{Ga}}^{3+}$, and ${\mathrm{In}}^{3+}$ ions in the substituted ferromagnetic spinel ${\mathrm{CdCr}}_2$ ${\mathrm{S}}_4$ have been measured by the spin-echo method at liquid-helium temperature. The impurity, when located at the B site, produces a large shift in the resonance frequency of the ${\mathrm{Cd}}^{2+}$ ions nearest to it, and shows an anomalous field dependence of its own resonance frequency. A well-defined canted spin structure was then deduced from these NMR data. Such a structure was reproduced by simulation on a system of N=1185 spins, taking into account all positive nearest-neighbor and negative third-neighbor pair interactions plus local interactions around the impurity. The simulation results showed that these local interactions must be introduced in order to create the canted spin structure, and are much greater than the original ones. This strong effect was ascribed to the superexchange interaction via the impurity s valence electron whose spin density, as deduced from the hyperfine interaction, is unusually high.