Highly Isotropic Magnetoresistances of High Sheet-Resistance Copper-Particle Films

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We have measured the magnetoresistances in ≈30-Å-thick Cu-particle films between 1.2 and 4.2 K and in magnetic fields H up to 6 T, with sheet resistances R_□(4.2K) of the films varying from ≈8 kΩ to 2 MΩ. We find that the magnetoresistances are positive, exhibiting broad maxima, at low H, while they change sign and become negative at high H (typically, around 4T). The magnitudes of the normalized magnetoresistances, ΔR_□(H)/R_□(0)=[R_□(H)-R_□(0)]/R_□(0), are of the order of a tenth of a percent or smaller, depending on the values of R_□. Particularly, the magnetoresistances are highly isotropic, regardless of the orientation of H being applied parallel or perpendicular to the film plane. These observations are closely connected with the fine-particle structure of our films, and can be qualitatively explained in terms of Zeeman effects in producing magnetoresistances in strongly localized, interacting electrons.
1994-12-15