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 an d 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 p articles. A structural diameter has been determined from EXAFS and anomalou s small-angle X-ray scattering measurements. NiAg alloys exhibit original m agnetic properties characterized by magnetic fluctuations both at the scale of the particles as in usual superparamagnetism, and also at the atomic sc ale, within each particle. The as-deposited Ni20Ag80 and Ni37Ag63 samples s how intra-particle magnetic ordering temperature lower than room temperatur e (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 larg est amplitude of magnetoresistance (15% at room temperature) is observed fo r Co37Ag63 annealed at 350 degrees C. In this alloy, the average diameter o f the granules is about 43 Angstrom. CoAg alloys always present larger MR a mplitudes 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 magnetor esistance (according to Vedyayev theory [A. Vedyayev et al., J. Magn. Magn. Mater. 164 (1996) 91]). (C) 1999 Elsevier Science B.V. All rights reserved .