Electron Spin Resonance of ${\mathrm{Nd}}^{3+}$ Pairs in La${\mathrm{Cl}}_3$ and La${\mathrm{Br}}_3$

We have examined the electron-spin-resonance spectra due to ${\mathrm{Nd}}^{3+}$ pairs in La${\mathrm{Cl}}_3$ and La${\mathrm{Br}}_3$. These measurements were made at 4.2°K and 9 kMc/sec using 0.1-2.0 at.% doping of enriched even-even isotopes of neodymium. A general anisotropic exchange interaction between two real spins is transformed into an effective-spin Hamiltonian formalism containing a Zeeman term and a traceless coupling tensor $A_{\mathrm{ij}}$ between two effective spins each of magnitude ½. This latter term represents an anisotropic effective spin interaction within the triplet levels of the system. About 5000 resonances due to allowed and forbidden transitions within these triplet levels were observed and fitted to this Hamiltonian for various angles $\theta$, between the magnetic field H and the crystal axis, and $\phi$, between H and the plane containing a second-neighbor pair. Some resonances due to first-neighbor pairs were shifted by 2500 Oe from the unpaired-ion resonance and require analysis techniques other than perturbation theory. The observed symmetry of the second-neighbor interaction tensors in these two salts suggests that the halide ions play an important role. By removing the dipolar contributions to $A_{\mathrm{ij}}$, quantitative results regarding the nature of the nondipolar spin-spin interaction are obtained. No resonances attributable to pairs were detected in La${\mathrm{Cl}}_3$ doped with either ${\mathrm{Ce}}^{3+}$ or ${\mathrm{Er}}^{3+}$.