Faraday rotation in the 10 μm region in InSb at liquid‐helium temperature

Faraday rotation has been measured at 10.59 and 9.66 μm in n‐type InSb at ∼6 K having a carrier concentration of ∼2×1014 cm−3, using magnetic fields up to 50 kG. In addition to the interband and free‐carrier plasma contributions to the Faraday rotation, an appreciable contribution from the conduction electron spins has been observed. The latter contribution is proportional to the spin alignment that saturates at magnetic fields exceeding ∼5 kG. Its saturation value of −17±1°/cm at 10.59 μm is in good agreement with the value calculated from published theoretical results. A value of −1.8±0.1°/kG cm has been deduced for the interband contribution at 10.59 μm, in disagreement with the previously published value of −3.8°/kG cm. Design for a 10 μm Faraday isolator is proposed for operation at ∼5 kG, a field well within the range of permanent magnets.