Logo: to the web site of Uppsala University

We demonstrate ultrafast graphene spin-field-effect junctions, where gate-tunable superdiffusive spin currents across graphene-ferromagnet interfaces enable electric field control of magnetization dynamics in the ferromagnet. By electrostatically tuning the Fermi level in graphene underlying a cobalt thin film, we modulate the ultrafast spin transport across graphene-cobalt interfaces, reducing femtosecond laser-induced demagnetization time from 203 fs in bare cobalt thin films to 93 fs, a more than 100% increase in the rate of magnetization quenching. Supported by superdiffusive spin transport calculations, our findings unlock field-tunable magnetic speeds in devices, paving the way for innovations in subpicosecond spintronic memory-logic operations. Furthermore, this work creates new possibilities for electrical modulation of spin dynamics and ultrafast spin injection into two-dimensional quantum materials, with potential for nextgeneration quantum sensors and faster magnetic technologies.

The high current-carrying capacity of graphene is essential for its use as an interconnect in electronic and spintronic circuits. At the same time, knowing the breakdown limits and mechanism under high fields can enable new device design strategies. In this work, we push the current carrying capacity of the scalable form of chemical vapor deposited (CVD) graphene employing a high-thermal conducting single crystalline diamond substrate. Our experiments on CVD graphene reveal extremely high current densities > 10⁹ A/cm² in graphene on the diamond with both ohmic (low-resistive) and tunneling tunnel (high-resistive) contacts. Measurements on ferromagnetic (TiO_x/Co) and metallic (Ti/Au) contacts demonstrate current densities of ∼1.16×10⁹ A/cm² and ∼1.7×10⁹ A/cm², respectively. The tunnel (high-resistive) contacts exhibit a shunting of graphene under high currents via the bottom graphitized diamond, resulting in dielectric breakdown and via alternative conducting paths. Electrical measurements show a distinct threshold for conducting paths of graphitized diamond, in tune accordance with Middleton-Wingreen's theory. Our results of high current densities achieved in CVD graphene, with distinct dependence on ohmic and tunneling, contact resistance, and the observed breakdown mechanism, provide new insights for enabling high-current all carbon circuits.

Achieving enhanced and stable electrical quality of scalable graphene is crucial for practical graphene device applications. Accordingly, encapsulation has emerged as an approach for improving electrical transport in graphene. In this study, we demonstrate high-current treatment of graphene passivated by AlOx nanofilms as a new means to enhance the electrical quality of graphene for its scalable utilization. Our experiments and electrical measurements on large-scale chemical vapor-deposited (CVD) graphene devices reveal that high-current treatment causes persistent and irreversible de-trapping density in both bare graphene and graphene covered by AlOx. Strikingly, despite possible interfacial defects in graphene covered with AlOx, the high-current treatment enhances its carrier mobility by up to 200% in contrast to bare graphene samples, where mobility decreases. Spatially resolved Raman spectroscopy mapping confirms that surface passivation by AlOx, followed by the current treatment, reduces the number of sp3 defects in graphene. These results suggest that for current treated-passivated graphene (CTPG), the high-current treatment considerably reduces charged impurity and trapped charge densities, thereby reducing Coulomb scattering while mitigating any electromigration of carbon atoms. Our study unveils CTPG as an innovative system for practical utilization in graphene nanoelectronic and spintronic integrated circuits.