Frank Volmer

Toward Wafer Scale Fabrication of Graphene Based Spin Valve Devices

We demonstrate injection, transport, and detection of spins in spin valve arrays patterned in both copper based chemical vapor deposition (Cu-CVD) synthesized wafer scale single layer and bilayer graphene. We observe spin relaxation times comparable to those reported for exfoliated graphene samples demonstrating that chemical vapor deposition specific structural differences such as nanoripples do not limit spin transport in the present samples. Our observations make Cu-CVD graphene a promising material of choice for large scale spintronic applications.

Nanosecond spin lifetimes in single- and few-layer graphene-hBN heterostructures at room temperature.

We present a new fabrication method of graphene spin-valve devices that yields enhanced spin and charge transport properties by improving both the electrode-to-graphene and graphene-to-substrate interface. First, we prepare Co/MgO spin injection electrodes onto Si(++)/SiO2. Thereafter, we mechanically transfer a graphene-hBN heterostructure onto the prepatterned electrodes. We show that room temperature spin transport in single-, bi-, and trilayer graphene devices exhibit nanosecond spin lifetimes with spin diffusion lengths reaching 10 μm combined with carrier mobilities exceeding 20,000 cm(2)/(V s).

Spin Lifetimes Exceeding 12 ns in Graphene Nonlocal Spin Valve Devices

We show spin lifetimes of 12.6 ns and spin diffusion lengths as long as 30.5 μm in single layer graphene nonlocal spin transport devices at room temperature. This is accomplished by the fabrication of Co/MgO-electrodes on a Si/SiO2 substrate and the subsequent dry transfer of a graphene-hBN-stack on top of this electrode structure where a large hBN flake is needed in order to diminish the ingress of solvents along the hBN-to-substrate interface. Interestingly, long spin lifetimes are observed despite the fact that both conductive scanning force microscopy and contact resistance measurements reveal the existence of conducting pinholes throughout the MgO spin injection/detection barriers. Compared to previous devices, we observe an enhancement of the spin lifetime in single layer graphene by a factor of 6. We demonstrate that the spin lifetime does not depend on the contact resistance area products when comparing all bottom-up devices indicating that spin absorption at the contacts is not the predominant source for spin dephasing.

Role of MgO barriers for spin and charge transport in Co/MgO/graphene nonlocal spin-valve devices

We investigate spin and charge transport in both single and bilayer graphene non-local spin-valve devices. Similar to previous studies on bilayer graphene, we observe an inverse dependence of the spin lifetime on the carrier mobility in our single layer devices. This general trend is only observed in devices with large contact resistances. Furthermore, we observe a second Dirac peak in devices with long spin lifetimes. This results from charge transport underneath the contacts. In contrast, all devices with low ohmic contact resistances only exhibit a single Dirac peak. Additionally, the spin lifetime is significantly reduced indicating that an additional spin dephasing occurs underneath the electrodes.

Suppression of contact-induced spin dephasing in graphene/MgO/Co spin-valve devices by successive oxygen treatments

By successive oxygen treatments of graphene nonlocal spin-valve devices we achieve a gradual increase of the contact-resistance\char21{}area products

Contact-induced charge contributions to non-local spin transport measurements in Co/MgO/graphene devices

({R}_{c}A)

Intervalley dark trion states with spin lifetimes of 150 ns in WSe2

of Co/MgO spin injection and detection electrodes and a transition from linear to nonlinear characteristics in the respective differential

Dry-transferred CVD graphene for inverted spin valve devices

dV\text{\ensuremath{-}}dI

Simulations on the Influence of Spatially Varying Spin Transport Parameters on the Measured Spin Lifetime in Graphene Non-Local Spin Valves

curves. With this manipulation of the contacts, both spin lifetime and the amplitude of the spin signal can significantly be increased by a factor of seven in the same device. This demonstrates that contact-induced spin dephasing is the bottleneck for spin transport in graphene devices with small

Observation of long spin-relaxation times in bilayer graphene at room temperature.

{R}_{c}A