Paramagnetic Resonance Study at Millimeter Wavelengths of an Axial Fe3+ Spectrum in Strontium Titanate

Paramagnetic resonance measurements have been made on an axially symmetric spectrum of Fe³⁺ (configuration 3d⁵) in SrTiO₃. The SrTiO₃ sample contained a nominal 0.3% Fe impurity. The results were found to be consistent with the assumption that the spin‐Hamiltonian is $${\mathcal{H}}_S{\mathrm{=DO}}_2^0{\mathrm{+G}}_{\parallel }{\mathrm{·S}}_z{\mathrm{+G}}_{\perp }{\mathrm{·S}}_x{\text{+(a/120)\hspace{0.17em}(O}}_4^0{\text{+5O}}_4^4{\text{)+(F/180)\hspace{0.17em}O}}_4^0$$ . From room‐temperature measurements at 35.0 GHz, 65.1 GHz, 69.4 GHz, and 75.0 GHz, values are obtained for the coupling constants in the spin‐Hamiltonian: 2D=(2.707±0.008) cm⁻¹, and (5a/2+5F/3)≅0.01 cm⁻¹. D was determined to be of positive sign in an experiment carried out at 1.2°K and 9.4 GHz. Results of between 25 and 60 G were obtained for the linewidths of some of the transitions.