Spectral origins of high Faraday rotation at 1.5-m wavelength from Fe and Co in SrTiO₃ films

Measurements of Faraday rotation (FR) in [001]-oriented epitaxial films of SrTi_1-xFe_xO₃ and SrTi_1-xCo _xO₃ with x = 0.4 and 0.3, respectively, indicate that FR constants (∼10³deg/cm) of these perovskite compounds can approach those of (Bi,Y)₃Fe₅O₁₂ garnet used in fiber-optical isolators at a wavelength of 1.5 m. Because the magnetic site in a stoichiometric host is occupied by Ti⁴⁺, substituted Fe or Co must also be 4. A reduction to Fe³⁺, Fe²⁺, and Co³⁺, Co²⁺ is also expected from O^2- loss during the deposition process. The orbital energy-level structures in an octahedral site are examined for the existence of transitions that could account for the similar high FR effects. It was determined that only Fe⁴⁺ and Co⁴⁺ in low-spin states can satisfy the selection rules. The observation that the respective coercive fields of the FR(H) hysteresis loops exceed those of the magnetic moment M(H) lends credence to the argument that the unquenched orbital angular momentum acting with spin-orbit coupling can become the dominating influence on collective magnetic alignment.