Spin-spin interactions in organic magnetoresistance probed by angle-dependent measurements.

The dependence of organic magnetoresistance (OMAR) on the orientation of the magnetic field has been investigated. In contrast with previous claims, a finite and systematic change in magnitude is observed when the orientation of the field is changed with respect to the sample. It is demonstrated that, to explain these effects, spin-spin interactions have to be included in the models previously suggested for OMAR. Dipole coupling and exchange coupling are introduced in combination with either an anisotropy of the orientation of the spin pairs or an anisotropy in the hyperfine fields.

Magnetoresistance in hybrid organic spin valves at the onset of multiple-step tunneling.

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.

Sign inversion of magnetoresistance in space-charge limited organic devices.

In this Letter, we explain the puzzling sign change of organic magnetoresistance in space-charge limited devices by device physics. We prove analytically and numerically that in the case of bipolar conduction with an Ohmic majority carrier and an injection limited minority carrier contact, a decrease in minority carrier mobility may give rise to an increase in the device current. It is shown that when the magnetic field acts to decrease the mobility of both carriers, a sign change in the magnetoconductivity as a function of applied bias may result. This behavior is in agreement with experimental observations.

Theory for spin diffusion in disordered organic semiconductors.

We present a theory for spin diffusion in disordered organic semiconductors, based on incoherent hopping of a charge carrier and coherent precession of its spin in an effective magnetic field, composed of the random hyperfine field of hydrogen nuclei and an applied magnetic field. From Monte Carlo simulations and an analysis of the waiting-time distribution of the carrier we predict a surprisingly weak temperature dependence, but a considerable magnetic-field dependence of the spin-diffusion length. We show that both predictions are in agreement with experiments on organic spin valves.

Extremely large magnetoresistance in boron-doped silicon.

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.

Separating positive and negative magnetoresistance in organic semiconductor devices.

We study the transition between positive and negative organic magnetoresistance (OMAR) in tris-(8 hydroxyquinoline) aluminium (Alq_{3}), in order to identify the elementary mechanisms governing this phenomenon. We show how the sign of OMAR changes as function of the applied voltage and temperature. The transition from negative to positive magnetoresistance (MR) is found to be accompanied by an increase in slope of log(I) versus log(V). ac admittance measurements show this transition coincides with the onset of minority charge (hole) injection in the device. All these observations are consistent with two simultaneous contributions with opposite sign of MR, which may be assigned to holes and electrons having different magnetic field responses.