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 (105 %) 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 in the vicinity of the spin-dependent potential barrier formed in the semiconductor near the interface. Kinetic energy of electrons, which pass through the barrier and trigger the avalanche process, is reduced by the applied magnetic field. This electron energy suppression postpones the onset of the impact ionization to higher electric fields and results in the giant magnetoresistance. The spin-dependent potential barrier is due to 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 and the magnetoresistance effect. Spin injector and spin-valve structure on the base of ferromagnet / semiconductor heterostructures with quantum wells with spin-polarized localized electrons in the semiconductor at the interface are considered.