Determination of the Sign of the Interactions between Pairs ofNd3+Ions in LaCl3by Electron Paramagnetic Resonance

This paper describes measurements of the interactions between nearest-neighbor (nn) and next-nearest-neighbor (nnn) ${\mathrm{Nd}}^{3+}$ ions in La${\mathrm{Cl}}_3$ from the electron-paramagnetic-resonance (EPR) pair spectra, which are very similar to those recently reported by Brower et al. Our numerical values are in substantial agreement with those of these authors, but by working at much higher frequency we have been able to determine the signs of the interactions. The signs are opposite to those assumed by Brower et al., and this considerably alters the interpretation of the nondipolar part of the interaction. In contrast to the suggestion made by Eisenstein et al., which was apparently confirmed by Brower et al., this part of the interaction cannot be described for nnn by an isotropic exchange between real spins, or for nn by a ferromagnetic isotropic exchange between real spins.