Nanosecond spin lifetimes in bottom-up fabricated bilayer graphene spin-valves with atomic layer deposited Al$_2$O$_3$ spin injection and detection barriers

TL;DR

This study demonstrates nanosecond spin lifetimes in bottom-up fabricated bilayer graphene spin-valves with improved Al$_2$O$_3$ barriers, showing potential for advanced spintronic applications.

Contribution

It introduces a bottom-up fabrication method with atomic layer deposition of Al$_2$O$_3$ barriers, achieving more homogeneous barriers and enhanced spin lifetimes in graphene devices.

Findings

Spin lifetimes reach up to 2 ns after oxygen treatment.

Atomic layer deposition produces more uniform barriers than molecular beam epitaxy.

Voltage pulses can partially break oxide barriers to enable spin transport.

Abstract

We present spin transport studies on bi- and trilayer graphene non-local spin-valves which have been fabricated by a bottom-up fabrication method. By this technique, spin injection electrodes are first deposited onto Si$^{++}$/SiO$_2$ substrates with subsequent mechanical transfer of a graphene/hBN heterostructure. We showed previously that this technique allows for nanosecond spin lifetimes at room temperature combined with carrier mobilities which exceed 20,000 cm$^2$/(Vs). Despite strongly enhanced spin and charge transport properties, the MgO injection barriers in these devices exhibit conducting pinholes which still limit the measured spin lifetimes. We demonstrate that these pinholes can be partially diminished by an oxygen treatment of a trilayer graphene device which is seen by a strong increase of the contact resistance area products of the Co/MgO electrodes. At the same time, the spin lifetime increases from 1 ns to 2 ns. We believe that the pinholes partially result from the directional growth in molecular beam epitaxy. For a second set of devices, we therefore used atomic layer deposition of Al$_2$O$_3$ which offers the possibility to isotropically deposit more homogeneous barriers. While the contacts of the as-fabricated bilayer graphene devices are non-conductive, we can partially break the oxide barriers by voltage pulses. Thereafter, the devices also exhibit nanosecond spin lifetimes.