Ion-beam modification of Co/Ag multilayers II: Variation of structural andmagnetic properties with Co layer thickness

The structural, magnetic and transport properties of rf sputtered Co/Ag mul tilayers with Co-layer thicknesses ranging from 1 to 14 Angstrom have been studied by a combination of x-ray diffraction, magnetic and transport measu rements. The magnetoresistance at room temperature has a maximum value of m ore than 12% for a Co-layer thickness around 5 Angstrom. Magnetic measureme nts demonstrate that samples near this Co-layer thickness are in the transi tion region from superparamagnetic to ferromagnetic behavior. X-ray analysi s indicates that, during deposition, a significant quantity of Co is disper sed throughout a highly textured Ag matrix. Upon irradiation with 1 MeV Si ions up to a dose of 5x10(16) Si+/cm(2), an initial demixing of the Co is followed by segregation into grains with the same texture as the Ag. The re sulting changes in the magnetization and magnetoresistance are characterize d on the basis of a log-normal distribution of the volume of the magnetic p articles. As the particle sizes increase, a systematic evolution towards fe rromagnetic behavior for films initially in the superparamagnetic and trans ition regions results. (C) 2000 American Institute of Physics. [S0021-8979( 00)08912-X].