Tunable Spin Injection in High-Quality Graphene with One-Dimensional Contacts.

Spintronics involves the development of low-dimensional electronic systems with potential use in quantum-based computation. In graphene, there has been significant progress in improving spin transport characteristics by encapsulation and reducing impurities, but the influence of standard two-dimensional (2D) tunnel contacts, via pinholes and doping of the graphene channel, remains difficult to eliminate. Here, we report the observation of spin injection and tunable spin signal in fully encapsulated graphene, enabled by van der Waals heterostructures with one-dimensional (1D) contacts. This architecture prevents significant doping from the contacts, enabling high-quality graphene channels, currently with mobilities up to 130 000 cm2 V-1 s-1 and spin diffusion lengths approaching 20 µm. The nanoscale-wide 1D contacts allow spin injection both at room and at low temperature, with the latter exhibiting efficiency comparable with 2D tunnel contacts. At low temperature, the spin signals can be enhanced by as much as an order of magnitude by electrostatic gating, adding new functionality.

Magnetoresistance in Co-hBN-NiFe Tunnel Junctions Enhanced by Resonant Tunneling through Single Defects in Ultrathin hBN Barriers.

Hexagonal boron nitride (hBN) is a prototypical high-quality two-dimensional insulator and an ideal material to study tunneling phenomena, as it can be easily integrated in vertical van der Waals devices. For spintronic devices, its potential has been demonstrated both for efficient spin injection in lateral spin valves and as a barrier in magnetic tunnel junctions (MTJs). Here we reveal the effect of point defects inevitably present in mechanically exfoliated hBN on the tunnel magnetoresistance of Co-hBN-NiFe MTJs. We observe a clear enhancement of both the conductance and magnetoresistance of the junction at well-defined bias voltages, indicating resonant tunneling through magnetic (spin-polarized) defect states. The spin polarization of the defect states is attributed to exchange coupling of a paramagnetic impurity in the few-atomic-layer thick hBN to the ferromagnetic electrodes. This is confirmed by excellent agreement with theoretical modeling. Our findings should be taken into account in analyzing tunneling processes in hBN-based magnetic devices. More generally, our study shows the potential of using atomically thin hBN barriers with defects to engineer the magnetoresistance of MTJs and to achieve spin filtering, opening the door toward exploiting the spin degree of freedom in current studies of point defects as quantum emitters.