Measurements made on the EPR spectrum of a pair of interacting nearest-neighbor neodymium ions in lanthanum ethyl sulfate show that the interaction is predominantly dipolar, but that there is an additional term $A_{\mathrm{ij}}{S}_{\mathrm{iz}}{S}_{\mathrm{jz}}+\frac{1}{2}{B}_{\mathrm{ij}}(S_{i+}{S}_{j-}+S_{i-}{S}_{j+})$, with $A_{\mathrm{ij}}=(+32\mathrm{}\pm 6)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and $B_{\mathrm{ij}}=(-3\mathrm{}\pm 3)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. A semiquantitative discussion is given of the possible origins of this nondipolar contribution including super-exchange, electric quadrupole-quadrupole, and virtual-phonon exchange interactions.

• Received 2 July 1964

DOI:https://doi.org/10.1103/PhysRev.136.A1341