Determination of para- and ferromagnetic components of magnetization and magnetoresistance of granular Co/Ag films (invited)

Several series of thin films (∼3000 Å) were fabricated by coevaporation of Co and Ag in a dual e-beam UHV system at substrate temperatures, TS, between 300 and 500 K. The composition was varied between 23 and 54 at. % Co. The maximum measured magnetoresistance was 31% at room and 67% at nitrogen temperature. Magnetization and Rutherford backscattering measurements showed that the Co and Ag atoms are completely segregated for films deposited at TS≥400 K. The magnetoresistance (MR) and magnetization curves have been analyzed taking into account both the paramagnetic (PM) and ferromagnetic (FM) contributions that are observed to be present. The films having ≤32 at. % Co are entirely paramagnetic. The fraction of Co atoms in the ferromagnetic component increases monotonically with increasing TS and/or at. % Co. The average radii of the PM granules in the films having an MR effect of 25%–31% are ∼20–22 Å with a minimum average separation of ∼10 Å. The large MR values attained in both the PM and FM components are attributed to the small effective domain sizes, ∼40–50 Å. The MR values are large because the MR effect varies inversely as the mean-free-path for magnetic boundary scattering which is the average distance between domain boundaries. The variation of the resistive and magnetic behavior of the films was also studied as a function of the annealing time for an annealing temperature of 300 °C. A striking behavior seen in these films is the long relaxation times when a FM component is present.