Room-temperature anomalous Hall effect in graphene in interfacial magnetic proximity to EuO grown by topotactic reduction

We show that thin layers of EuO, a ferromagnetic insulator, can be achieved by topotactic reduction under titanium of a Eu$_2$⁢O$_3$ film deposited on top of a graphene template. The reduction process leads to the formation of a 7-nm-thick EuO smooth layer, without noticeable structural changes in the underlying chemical vapor deposited graphene. The obtained EuO films exhibit ferromagnetism, with a Curie temperature that decreases with the initially deposited Eu$_2$⁢O$_3$ layer thickness. By adjusting the thickness of the Eu$_2$⁢O$_3$ layer below 7 nm, we promote the formation of EuO at the very graphene interface: the EuO/graphene heterostructure demonstrates the anomalous Hall effect (AHE), which is a fingerprint of proximity-induced spin polarization in graphene. The AHE signal moreover persists above 𝑇𝑐 up to 350 K due to a robust super-paramagnetic phase in EuO. This original high-temperature magnetic phase is attributed to magnetic polarons in EuO: we propose that the high strain in our EuO films grown on graphene stabilizes the magnetic polarons up to room temperature. This effect is different from the case of bulk EuO in which polarons vanish in the vicinity of the Curie temperature 𝑇𝑐=69 K.