Anomalous Behavior of the Tunneling Magnetoresistance in (CoFeB)(x)(LiNbO3)(100-)(x)/Si Nanocomposite Film Structures Below the Percolation Threshold: Manifestations of the Cotunneling and Exchange Effects

A strongly nonmonotonic temperature dependence of the magnetoresistance in (CoFeB)(x)(LiNbOy)(100 -)(x) film nanocomposites (x & AP; 40-48 at %) is observed in the temperature range of 3-250 K at the magnetic field up to 14 T near the percolation threshold on its insulating side. The magnetoresistance has a minimum at 40 K and increases steeply on cooling. Such behavior of the magnetoresistance is attributed to the coexistence of superferromagnetic regions with exchange-coupled granules separated by regions with superparamagnetic granules in the nanocomposite. In this case, an increase in the negative magnetoresistance at T > 40 K is due to the destruction of superferromagnetic ordering, whereas an increase in the magnetoresistance at T < 40 K is related to the processes involving simultaneous elastic tunneling via the chains of granules. At the saturation of the magnetization, an additional negative contribution arises, which is probably due to the quantum interference effects. At T < 4 K, a double-well shape of the field dependence of the magnetoresistance is observed, which could be attributed to the effect of a positive contribution that competes with the negative magnetoresistance.

Automated summary

A strongly nonmonotonic temperature dependence of magnetoresistance is observed in (CoFeB)(x)(LiNbOy)(100 -)(x) film nanocomposites, with a minimum value at 40 K and a steep increase on cooling. This behavior is attributed to the coexistence of superferromagnetic and superparamagnetic regions in the nanocomposite. At T > 40 K, an increase in negative magnetoresistance is due to the destruction of superferromagnetic ordering, while at T < 40 K, an increase in magnetoresistance is related to elastic tunneling via chains of granules. An additional negative contribution arises at the saturation of magnetization, possibly due to quantum interference effects. A double-well shape of the field dependence of magnetoresistance is observed at T < 4 K, which could be explained by the competition between positive and negative magnetoresistance contributions.