An experimental investigation of spin polarized transport in carbon nanotubes

In this Thesis, we report on electrical spin injection measurements on Multi-Wall Carbon Nanotubes (MWNTs) as well as on Single-Wall Carbon Nanotubes (SWNT) . We use a ferromagnetic alloy Pd1−x Nix with x ≈ 0.7 which allows to obtain devices with resistances as low as 5.6 kΩ at 300 K. The yield of device resistances below 100 kΩ, at 300 K, is around 50%. We measure atK a hysteretic magneto-resistance due to the magnetization reversal of the ferromagnetic leads. We present the results of tunnelling magnetoresistance (TMR) measurements in carbon nanotubes as a function of electrical gate voltage. We have used ferromagnetic contacts to inject and detect spins in carbon nanotubes in a spin field-effect transistor geometry. In the linear regime, the sign and the amplitude of the relative difference between the resistance in the antiparallel and the parallel magnetization configurations is tunable with the gate voltage. We attribute this effect to resonant tunneling through the nanotube where the transmission probabilities of up and down spins are modulated differently. In addition to this, we study also the bias-voltage dependence of the TMR in ferromagnetically contacted carbon nanotube (CNT) devices. Experiments performed on PdNi/CNT/PdNi double tunnel junctions show that both positive and negative values of TMR are possible, depending on the applied bias-voltage. We show that the TMR is strongly enhanced by charging energy in Coulomb Blockade (CB) regime. A large negative TMR of up to −33 % is observed atK. We investigate the effect of temperatute on TMR. For both high bias voltage and high temperature TMR reduces remarkably.