K. Pi

Tunneling Spin Injection into Single Layer Graphene

We achieve tunneling spin injection from Co into single layer graphene (SLG) using TiO₂ seeded MgO barriers. A nonlocal magnetoresistance (ΔR(NL)) of 130  Ω is observed at room temperature, which is the largest value observed in any material. Investigating ΔR(NL) vs SLG conductivity from the transparent to the tunneling contact regimes demonstrates the contrasting behaviors predicted by the drift-diffusion theory of spin transport. Furthermore, tunnel barriers reduce the contact-induced spin relaxation and are therefore important for future investigations of spin relaxation in graphene.

Electron-Hole Asymmetry of Spin Injection and Transport in Single-Layer Graphene

Spin-dependent properties of single-layer graphene (SLG) have been studied by nonlocal spin valve measurements at room temperature. Gate voltage dependence shows that the nonlocal magnetoresistance (MR) is proportional to the conductivity of the SLG, which is the predicted behavior for transparent ferromagnetic-nonmagnetic contacts. While the electron and hole bands in SLG are symmetric, gate voltage and bias dependence of the nonlocal MR reveal an electron-hole asymmetry in which the nonlocal MR is roughly independent of bias for electrons, but varies significantly with bias for holes.

Effect of cluster formation on graphene mobility

We investigate the effect of gold (Au) atoms in the form of both pointlike charged impurities and clusters on the transport properties of graphene. Cryogenic deposition (18 K) of Au decreases the mobility and shifts the Dirac point in a manner that is consistent with scattering from pointlike charged impurities. Increasing the temperature to room temperature promotes the formation of clusters, which is verified with atomic force microscopy. We find that for a fixed amount of Au impurities, the formation of clusters enhances the mobility and causes the Dirac point to shift back toward zero.

Metallic and insulating adsorbates on graphene

We directly compare the effect of metallic titanium (Ti) and insulating titanium dioxide (TiO2) on the transport properties of single layer graphene. The deposition of Ti results in substantial n-type doping and a reduction in graphene mobility by charged impurity scattering. Subsequent exposure to oxygen largely reduces the doping and scattering by converting Ti into TiO2. In addition, we observe evidence for short-range scattering by TiO2 impurities. These results illustrate the contrasting scattering mechanisms for identical spatial distributions of metallic and insulating adsorbates.

Oscillatory spin polarization and magneto-optical Kerr effect in Fe₃O₄ thin films on GaAs(001).

The spin dependent properties of epitaxial Fe₃O₄ thin films on GaAs(001) are studied by the ferromagnetic proximity polarization (FPP) effect and magneto-optical Kerr effect (MOKE). Both FPP and MOKE show oscillations with respect to Fe₃O₄ film thickness, and the oscillations are large enough to induce repeated sign reversals. We attribute the oscillatory behavior to spin-polarized quantum well states forming in the Fe₃O₄ film. Quantum confinement of the t(2g) states near the Fermi level provides an explanation for the similar thickness dependences of the FPP and MOKE oscillations.

Enhanced spin injection into single layer graphene with atomically smooth MgO barrier

Single layer graphene (SLG) is a promising material for spintronics due to predictions of long spin life time, and unusual spin dependent behaviors such as half-metallic ferromagnetism for graphene nanoribbons. Previously, we fabricated SLG spin valves utilizing transparent contacts Co/SLG with MgO masking layer, and observed gate tunable spin transport at room temperature (RT) [1]. For the non-local MR measurement (Figure 1a), spin is injected at electrode E2 and detected at E3. Figure 1b shows the non-local MR loop for a SLG spin valve with transparent contacts as the magnetic field is swept up (black curve) and swept down (red curve).

Bias and gate control of graphene spin valves

We have fabricated and utilized bias current and gate voltage to control the room temperature (RT) single layer graphene (SLG) spin valves [1]. The SLG spin valves are fabricated by electron-beam lithography with transparent SLG/Co contact junctions (Figure 1).