Role of Coulomb blockade and spin-flip scattering in tunneling magnetoresistance of FeCo-Si-O nanogranular films

In this work we report the effect of FeCo atomic fraction (0.33 < x < 0.54) and temperature on the electrical, magnetic and tunneling magnetoresistance (TMR) properties of FeCo-Si-O granular films prepared by atom beam sputtering technique. GAXRD and TEM studies reveal that films are amorphous in nature. The dipole-dipole interactions (particle-matrix mixing) is evident from Zero-field cooled (ZFC) and field-cooled (FC) magnetic susceptibility measurements and the presence of oxides (mainly Fe- related) is observed by XPS analysis. The presence of Fe-oxides is responsible for the observed reduction of saturation magnetization and rapid increase in coercivity below 50 K. TMR has been observed in a wide temperature range and a maximum TMR of -4.25 % at 300 K is observed for x = 0.39 at a maximum applied field of 60 kOe. The fast decay of maximum TMR at high temperatures and lower TMR values at 300 K as compared to , where PFeCo is the spin polarization of FeCo are in accordance with a theoretical model that includes spin-flip scattering processes. The temperature dependent study of TMR effect reveals a remarkably enhanced TMR at low temperatures. The TMR value varies from -2.1% at 300 K to -14.5% at 5 K for x = 0.54 and a large MR value of -18.5% at 5 K for x = 0.39 is explained on the basis of theoretical models involving Coulomb blockade effects. Qualitatively particle-matrix mixing and presence of Fe-oxides seems to be the source of spin-flip scattering, responsible for fast decay of TMR at high temperatures. A combination of higher order tunneling (in Coulomb blockade regime) and spin flip scattering (high temperature regime) explains the temperature dependent TMR of these films.