Test of Spin Hamiltonian forIron3+in Strontium Titanate

Abstract

The applicability of a conventional spin Hamiltonian to the paramagnetic spectrum of ${\mathrm{Fe}}^{3+}$ in strontium titanate is investigated. The work was inspired by a paper by Müller who finds deviations from a conventional spin Hamiltonian which he attributed to covalent bonding. The spectrum is remeasured and compared with the more general theory of Koster and Statz. It is found that the conventional Hamiltonian describes the spectrum about as well, in this case, as the improved theory. The remaining discrepancies vary from crystal to crystal and are due to random distortions of the ${\mathrm{Fe}}^{3+}$ site. A rather good agreement with theory was obtained for one crystal which apparently was more perfect than the other measured samples. From perturbation theory, it is concluded that the deviations from a conventional Hamiltonian should be about 0.1 Mc/sec if covalency and exchange effects can be neglected. The experimental errors in the present experiments are about 1 to 2 Mc/sec. Even though for the present example it is unnecessary to resort to the improved theory, it is shown that, even in the absence of covalency, measurable deviations from a conventional spin Hamiltonian are expected in substances where the zero-field splittings and the applied magnetic field are large.