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Rare-earth iron garnets with large magnetic gyrotropy, made with reduced thermal budgets, are ideal magneto-optical materials for integrated isolators. However, reduced thermal budgets impact Faraday rotation by limiting crystallization, and characterization of crystallinity is limited by resolution or scannable area. Here, electron backscatter diffraction (EBSD) is used to measure crystallinity in cerium substituted yttrium- and terbium-iron garnets (CeYIG and CeTbIG) grown on planar Si, crystallized using one-step rapid thermal processes, leading to large Faraday rotations > −3500 °/cm at 1550 nm. Varying degrees of crystallinity are observed in planar Si and patterned Si waveguides, and specific dependences of crystallite size are attributed to the nucleation/growth processes of the garnets and the lateral dimensions of the waveguide. On the other hand, a low thermal budget alternative–exfoliated CeTbIG nanosheets–are fully crystalline and maintain high Faraday rotations of −3200 °/cm on par with monolithically integrated thin film garnets.