Paramagnetic Resonance of Fe 3 + and Gd 3 + in Yttrium Orthoaluminate

The paramagnetic-resonance spectra of ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Gd}}^{3+}$ in yttrium orthoaluminate were measured as part of an effort directed at understanding the isomorphic orthoferrites. The low symmetry of the ion sites and the complicated nature of the spectra necessitated the development of a special method of analysis for obtaining the relevant spin Hamiltonians. The method consisted of constructing directly from the angular dependence of the data "anisotropy surfaces," classical counterparts of the spin Hamiltonian, from which the spin Hamiltonian itself could be extracted by the method of operator equivalents. The predominant term in both ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Gd}}^{3+}$ Hamiltonians was quadratic in the spin components. The fourth-order term of the ${\mathrm{Fe}}^{3+}$ Hamiltonian manifested very nearly cubic symmetry with the biquadratic coefficient $a$ negative, contrary to general expectations. The ${\mathrm{Gd}}^{3+}$ spin Hamiltonian displayed no visible cubicity in the fourth-order terms. The "preferred" spin directions as determined in the orthoaluminate were compared with the observed spin configurations in the orthoferrites, and with the predictions of crystal-field calculations based on the ion position parameters of GdFe${\mathrm{O}}_{3}$. Correlation was very poor in both instances. No conclusive explanation of the discrepancies is at present available; the most likely explanation involved differences in the local crystal distortions between the orthoaluminate and the orthoferrite.