Search for Exchange Fields in Graphene
Proximity based interactions between nearby systems (i.e. films, adsorbates, molecules) and graphene could lead to the realization of novel phenomena. Graphene, a single atomic layer of sp2 bonded carbon with conducting "pi" orbitals that extend out of the plane, is ideal for investigating proximity related behavior. Such effects, which rely on atomic orbital overlap, require highly controlled surfaces and interfaces. Therefore, we take advantage of molecular beam epitaxy (MBE) which offers control over growth at the atomic scale. In order to understand the effect of adsorbates on spin properties, we have performed systematic in-situ deposition of adatoms onto graphene non-local spin valves at cryogenic temperatures.
This approach directly probes the spin-degree of freedom locally on the graphene flake and can be properly characterized before and after the adsorbates are introduced. In particular, we have deposited atomic hydrogen and Mg adsorbates. We have found that atomic hydrogen can induce magnetic moments in graphene and that these moments couple via exchange to the injected spin current. This coupling results in an exchange field which causes the spins to precess rapidly with an observed effective g-factor as large as ~20. Beyond adatom doping, there have been many theoretical works suggesting that when a magnetic insulator is brought into contact with graphene, that many novel applications can arise including spin filtering, gate tunable manipulation of spin transport and magnetoresistance, spin torque, and even the long searched for quantized anomalous hall effect. However, oxide growth on graphene is notoriously difficult due to graphene's high surface energy. Recently, we have integrated the ferromagnetic insulator EuO onto graphene by MBE and the resulting films are shown to be crystalline and very at with magnetic properties typical for EuO.