Ferrofluid composites containing dispersed metallic particles of tin or copper have a magnetically induced dichroism, which is particle concentration dependent, in the 3 mm microwave region. The system relaxes when the magnetic field ( approximately equals 50 Oe) is removed with an initial relaxation time tau approximately equals 1 s. The subsequent relaxation through Brownian motion is significantly longer with tau approximately equals 10**3 s.