HIGHLY ISOTROPIC MAGNETORESISTANCES OF HIGH SHEET-RESISTANCE COPPER-PARTICLE FILMS

We have measured the magnetoresistances in approximate to 30-Angstrom-thick Cu-particle films between 1.2 and 4.2 K and in magnetic fields H up to 6 T, with sheet resistances R(square) (4.2 K) of the films varying from approximate to 8 k Omega to 2 M Omega. 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 4 T). The magnitudes of the normalized magnetoresistances, Delta R(square) (H)/R(square) (0)= [R(square) (H)-R(square) (0)]/R(square) (0), are of the order of a tenth of a percent or smaller, depending on the values of R(square). 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.