The magnetotransport properties of Ni-SiO[subscritp 2] and Fe-SiO₂ granular composite systems have been systematically studied. In metallic region, when the metallic grain size becomes comparable with the mean free path of electron in the bulk metal of Ni or Fe (-5nm), the nature of magnetoresistance (MR) will change from the anisotropic MR (AMR) dominated in the homogeneous system to the negative and isotropic MR dominated as the metal volume fraction x decreases. In the transition region, when x approaches the percolation threshold x_c a giant enhancement of the extraordinary Hall resistivity ρ_xys (GHE) is observed in both the Ni-SiO₂ and the Fe-SiO₂ system. This giant enhancement cannot be explained via a simple percolation theory. Also, the scattering mechanism in such systems, which...[ Read more ]

The magnetotransport properties of Ni-SiO[subscritp 2] and Fe-SiO₂ granular composite systems have been systematically studied. In metallic region, when the metallic grain size becomes comparable with the mean free path of electron in the bulk metal of Ni or Fe (-5nm), the nature of magnetoresistance (MR) will change from the anisotropic MR (AMR) dominated in the homogeneous system to the negative and isotropic MR dominated as the metal volume fraction x decreases. In the transition region, when x approaches the percolation threshold x_c a giant enhancement of the extraordinary Hall resistivity ρ_xys (GHE) is observed in both the Ni-SiO₂ and the Fe-SiO₂ system. This giant enhancement cannot be explained via a simple percolation theory. Also, the scattering mechanism in such systems, which is characterized by ρ_xys[proportional to]ρⁿ with n [almost equal to] 0.8, is different from that in the homogeneous ferromagnetic metals, which follows ρ_xys[proportional to]ρⁿ with n =I or 2. Temperature dependent resistivity studies on both the as-deposited and the annealed films in these two systems show that there is a general correlation between the GHE and a large resistivity with a -logT like temperature dependence. Also in this region, the absolute value of magnetoresistivity, -δρ, whose nature is dominated by the negative and isotropic MR, follows -δρ[similar to] -ρ^m with m[almost equal to]1.33 in both systems as x → x_c. This relation, along with the relation ρ_xys[similar to]ρⁿwith n[almost equal to] 0.8, confirms that there is a common spin-dependent scattering mechanism for these granular composite systems in the transition region. In the insulating region, a tunnelling type magnetoresistance (TMR) is observed. Systematic studies of the temperature dependence of the TMR show that the existing spin-dependent tunnelling models describe the TMR behavior qualitatively but not quantitatively. Furthermore, a possible way to improve the existing models is suggested.