Strong axial electron paramagnetic resonance spectrum of Fe3+ in SrTiO3 due to nearest-neighbor charge compensation

An axial electron paramagnetic resonance (EPR) spectrum in iron-doped (cubic) strontium titanate has been observed at 3.3 and 1.85 cm wavelength with effective g values ge=5.993±0.001 and ge=5.961±0.001, respectively, and gIIe=g=2.0054±0.0007. The same spectrum was found after charge displacement due to heat treatment in the dark or by reduction in crystals which contained, in addition to iron, other transition metal ions. The spectrum is attributed to ΔM=1 transitions of the Sz=±12 level of Fe3+(3d5) in a strongly tetragonal electric crystalline field produced by local charge compensation at a nearest-neighbor oxygen site. The theory is developed for ions of half-integral spin in an axial field much greater than the Zeeman splitting. By applying the theory to the special case of S=52 and using the measured effective ge values and resonance magnetic fields g=2.0101±0. 0008 and a zero-field splitting parameter |2D| of 2.85±0.15 cm-1 is obtained. This is the largest splitting which has so far been observed for Fe3+ in any inorganic crystal and could be useful for submillimeter maser applications.