Giant magnetoresistance in semiconductor/granular film heterostructures with cobalt nanoparticles

We have studied the electron transport in SiO2(Co)/GaAs and SiO2(Co)/Si heterostructures, where the SiO2(Co) structure is the granular SiO2 film with Co nanoparticles. In SiO2(Co)/GaAs heterostructures giant magnetoresistance effect is observed. The effect has positive values, is expressed when electrons are injected from the granular film into the GaAs semiconductor, and has the temperature-peak-type character. The temperature location of the effect depends on the Co concentration and can be shifted by the applied electrical field. For the SiO2(Co)/GaAs heterostructure with 71 at. % Co the magnetoresistance reaches 1000 (10(5)%) at room temperature. On the contrary, for SiO2(Co)/Si heterostructures magnetoresistance values are very small (4%) and for SiO2(Co) films the magnetoresistance has an opposite value. High values of the magnetoresistance effect in SiO2(Co)/GaAs heterostructures have been explained by magnetic field-controlled process of impact ionization. The spin-dependent potential barrier is formed in the accumulation electron layer in the semiconductor near the interface. The impact ionization induced by injected electrons produces holes, which move, are accumulated in the region of the potential barrier and lower the barrier height. The height decrease grows the electron current flowing through the barrier and leads to the enhancement of the avalanche. Due to the formed positive feedback small variations in the barrier height lead to great changes in the current. The applied magnetic field increases the height and reduces the transparency of the barrier. As a result, small feedback reduction suppresses the onset of the impact ionization. The spin-dependent potential barrier is formed by the exchange interaction between electrons in the accumulation electron layer in the semiconductor and d electrons of Co. Existence of spin-polarized localized electron states in the accumulation layer results in the temperature-peak-type character of the barrier, which is manifested in the magnetoresistance effect. Spin injectors and efficient magnetic sensors on the base of ferromagnet/semiconductor heterostructures with holes traps and quantum wells containing spin-polarized localized electrons in the semiconductor at the interface are considered.