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

Abstract

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. Glancing angle x-ray diffraction and TEM studies reveal that films are amorphous in nature. The dipole–dipole interactions (particle–matrix mixing) is evident from zero-field cooled and field-cooled magnetic susceptibility measurements and the presence of oxides (mainly Fe-related) is observed by x-ray photoelectron spectroscopy 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 when compared to PFeCo2/(1+PFeCo2), 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 the 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.

Stabilization Of FeCo Alloy Phase In FeCo-SiO2 nanocomposites

“…FeCo-SiO 2 granular films were prepared by FAB sputtering technique using 1.5 KeV energy neutral Ar atoms, as described elsewhere [10]. An Ar atom source was mounted at 45 o facing towards a 3 in.…”

“…Out of these, sputtering is used on industrial scale due to uniform film thickness and large area deposition. Recently, atom beam source based sputtering method, called fast atom beam (FAB) sputtering has been employed by our group to synthesize FeCo-SiO 2 granular films under ambient conditions and its structural, electrical, magnetic and temperature dependent TMR properties were studied [10]. The structural characterization of the as-deposited FeCo-SiO 2 granular films was carried out using XRD and selected area electron diffraction, cannot prove the formation of FeCo alloy due to amorphous nature of the films.…”

Nanostructure characterization of Co–Pd–Si–O soft magnetic nanogranular film using small-angle X-ray and neutronscattering

“…In order to resolve those issues and obtain optimal soft magnetic properties, knowledge of the nanostructure of the film is required. In previous studies of nanogranular films, a possible intermixing of magnetic elements into the amorphous matrix was often noted [7][8][9][10]. A magnetic element dispersed in the matrix can influence the magnetic properties via a reduction of magnetization, a modulation of magnetic coupling, and/or a change in transport properties [11].…”

Structural Characteristics and Magnetic Properties of Al2O3 Matrix-Based Co-Cermet Nanogranular Films

“…Second, this reason can occur simultaneously with the first reason but can also be more dominant: the shift of the Al 2 O 3 humps as mentioned above, the size of Co clusters is very small, and/or the Co atoms are broadly dispersed into the Al 2 O 3 matrix. Such similar feature of no diffraction peaks in the XRD has been also observed for amorphous or nanocrystalline CoFe clusters in a SiO 2 silica matrix, with cluster sizes of about 1 nm-4 nm observed and estimated based on their TEM images [23]. The Co particle sizes of the Co-cermet films are hard to evaluate from the XRD data by using Scherrer's formula because of the very weak X-ray reflection of Co.…”