Magnetic properties and magnetoresistance of as-deposited and annealed CoxAg1-x and NixAg1-x (x=0.2, 0.37) heterogeneous alloys

We have measured the magnetization and magnetoresistance of as-deposited and annealed CoxAg1-x and NixAg1-x (x = 0.2, 0.37) heterogeneous alloys. From the magnetization curves, we have estimated the magnetic diameter of the particles. A structural diameter has been determined from EXAFS and anomalous small-angle X-ray scattering measurements. NiAg alloys exhibit original magnetic properties characterized by magnetic fluctuations both at the scale of the particles as in usual superparamagnetism, and also at the atomic scale, within each particle. The as-deposited Ni20Ag80 and Ni37Ag63 samples show intra-particle magnetic ordering temperature lower than room temperature (70 K for Ni20Ag80 and 200 K < T-C < 300 K for Ni37Ag63). Model has been developed to explain the special magnetic behavior of NiAg alloys. The largest amplitude of magnetoresistance (15% at room temperature) is observed for Co37Ag63 annealed at 350 degrees C. In this alloy, the average diameter of the granules is about 43 Angstrom. CoAg alloys always present larger MR amplitudes than the NiAg alloys of same volume fraction of magnetic element. In the as-deposited and annealed CoAg series, a maximum is observed in the relative MR amplitude as a function of particle diameter. This indicates a significant contribution of bulk spin-dependent scattering to the magnetoresistance (according to Vedyayev theory [A. Vedyayev et al., J. Magn. Magn. Mater. 164 (1996) 91]). (C) 1999 Elsevier Science B.V. All rights reserved.