Magnetic Interactions between Rare-Earth Ions in Insulators. III. EPR Measurements ofCe3+Pair-Interaction Constants in LaCl3

In order to investigate the form of interaction between non-$S$-state, Kramers rare-earth ions, the electron paramagnetic resonance spectra from pairs of ${\mathrm{Ce}}^{3+}$ ions in La${\mathrm{Cl}}_3$ have been measured at 4.2°K and at 25 and 36 GHz. The spectra have been fitted to a general effective-spin pair-Hamiltonian in order to obtain values for the g tensors and certain terms in the interaction tensors K. By using additional data from bulk-property measurements on Ce${\mathrm{Cl}}_3$, values for all the terms in K can be deduced. The contributions to the measured parameters from the various interaction mechanisms thought to be present in rare-earth compounds are discussed in detail. Apart from the usual magnetic dipole interaction, it is generally difficult to identify each contribution unambiguously. In the present case, however, because of the simple nature of the ${\mathrm{Ce}}^{3+}$ wave functions, there is clear evidence for the existence of an electric quadrupole-quadrupole interaction between next-nearest-neighbor ions, and its magnitude has been determined. There is also unambiguous evidence for anisotropic superexchange of the type suggested by Levy and by Elliott and Thorpe, and it is found that this has matrix elements of magnitude compariable with the more usual bilinear exchange interaction, which is also present.