Exchange Field Induced Magnetoresistance in Colossal Magnetoresistance Manganites. It has been discovered that the exchange field of magnetic impurities in a superconductor behaves as if it were a macroscopic applied magnetic field. This phenomena is known as the Jaccarino-Peter effect. Work by MRSEC supported students Krivorotov (Dahlberg), Nikolaev (Goldman) and Dobin (Wentzcovitch) have found a second example of this equivalence. They find the exchange field from either an antiferromagnet or a ferromagnet alters the resistance of a colossal magnetoresistance (CMR) material. An important difference from the earlier Jaccarino-Peter effect is the explict discovery that the CMR is determined by the magnitude of the vector sum of the exchange field and an applied field (in the earlier work, the exchange field was either parallel or antiparallel to the applied field). In addition to this discovery, the data support the interfacial origin of the low temperature magnetoresistance observed in thin films of these materials. The MBE-grown heterostructure illustrated at the top of the accompanying figure exhibits a resistance which is the vector sum of an applied magnetic field and the exchange field from the two insulating AF layers whereas the unbiased structure at the bottom exhibits the usual CMR in an applied magnetic field. [I.N.Krivorotov, K.R.Nikolaev, A.Yu.Dobin, A.M.Goldman, and E.Dan Dahlberg, Phys. Rev. Lett. 86, 5779-5782 (2001)]