Temperature Variation of ${\mathrm{Gd}}^{3+}$ Electron-Spin-Resonance Spectra in La${\mathrm{Cl}}_3$ and La${\mathrm{Br}}_3$ Single Crystals

The electron spin resonance of gadolinium was observed and the temperature variation of the splitting was studied in single crystals of La${\mathrm{Cl}}_3$ and La${\mathrm{Br}}_3$ where the symmetry of the local crystalline field was axial. Absolute signs of spin-Hamiltonian parameters have been determined from the relative intensity of the resonance lines at low temperatures. For La${\mathrm{Br}}_3$, the temperature variation of the second-order term in the axial spin Hamiltonian in the presence of an appreciable fourth-order term is sufficient to change the relative positions of the resonance lines between 77°K and room temperature for $H$ parallel to the crystal axis.