By combining experiments with simple model calculations, we obtain new insight in spin transport through hybrid, CoFeB/Al2O3(1.5 nm)/tris(8-hydroxyquinoline)aluminium (Alq3)/Co spin valves. We have measured the characteristic changes in the I-V behavior as well as the intrinsic loss of magnetoresistance at the onset of multiple-step tunneling. In the regime of multiple-step tunneling, under the condition of low hopping rates, spin precession in the presence of hyperfine coupling is conjectured to be the relevant source of spin relaxation. A quantitative analysis leads to the prediction of a symmetric magnetoresistance around zero magnetic field in addition to the hysteretic magnetoresistance curves, which are indeed observed in our experiments.

Boron-doped Si-SiO2-Al structures are fabricated to study extremely large magnetoresistance (MR) effects. Current-voltage characteristics show a nonlinear behavior, dominated by an autocatalytic process of impact ionization. At low temperatures, the magnetic field postpones the onset of impact ionization to higher electric fields. This results in a symmetric positive MR of over 10,000% at 400 kA/m. Applying a magnetic field leads to an increase of the acceptor level compared to the valence band as deduced by admittance spectroscopy. A macroscopic transport model is introduced to describe how the MR is controlled by voltage, electrode spacing, and oxide thickness.

In this experimental study, we confirm that the magnetoresistance effect measured in boron-doped Si–SiO2–Al structures originates from bulk silicon, and prove that the magnetic field non-uniformly squeezes the acceptor wave functions. In line with the expectation that the effect of a magnetic field on an acceptor wave function becomes stronger with increasing distance from the impurity centre, we show that the magnetoresistance increases for decreasing doping concentration, although the role of background impurities in this process cannot be completely excluded. When using an ultrathin insulating layer instead of ohmic contacts in the latter devices, large constant-voltage magnetoresistances (MR > 1000% at 1 MA m−1) are obtained in low-doped silicon at room temperature.

A magnetic-field-dependent resistance change of eight orders of magnitude is observed in boron-doped Si-SiO2-Al structures. In order to identify the elementary mechanisms governing this phenomenon, the thickness of the oxidic layer, which is used as an interface energy barrier, has been varied by changing the exposure time to an oxygen plasma. Next, the chemical composition has been monitored by in situ x-ray photoelectron spectroscopy measurements. From current-voltage measurements, we observe that at low temperatures, an ultrathin SiO2 layer provides the kinetic energy to trigger an autocatalytic process of impact ionization. A magnetic field suppresses the onset of impact ionization to higher electric fields, resulting in a large magnetoresistance.

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