Second harmonic generation from 21.5–26 to 43–52 GHz, using about 280 ppm of Fe3+ spins doped into TiO2 crystals has been investigated. The harmonic generator was tunable by use of magnetic fields of a few kilogauss. The most efficient generation is obtained when there are Fe3+ transitions coincident with both frequencies. We report a conversion efficiency of −45 dB, the best so far obtained with paramagnetic crystals. Evidence was found for more efficient conversion occurring inside the crystal. A similar frequency up-conversion experiment yielded an efficiency of −49 dB. With improved materials, frequency down-converters of similar type show promise as a low-noise detector of near-millimeter waves.