Proximity-induced ferromagnetism in graphene revealed by anomalous Hall effect (1412.1521v1)

Abstract: We demonstrate the anomalous Hall effect (AHE) in single-layer graphene exchange-coupled to an atomically flat yttrium iron garnet (YIG) ferromagnetic thin film. The anomalous Hall conductance has magnitude of ~0.09(2e2/h) at low temperatures and is measurable up to ~ 300 K. Our observations indicate not only proximity-induced ferromagnetism in graphene/YIG with large exchange interaction, but also enhanced spin-orbit coupling which is believed to be inherently weak in ideal graphene. The proximity-induced ferromagnetic order in graphene can lead to novel transport phenomena such as the quantized AHE which are potentially useful for spintronics.

• The paper demonstrates that proximity coupling with YIG induces ferromagnetism in graphene, evidenced by an anomalous Hall effect of ~0.09(2e/h) from low to room temperatures.
• The experimental methodology employs a transfer technique to place graphene on atomically flat YIG, preserving high electron mobility (>10,000 cm²/V·s).
• The findings pave the way for spintronic devices by enabling tunable magnetic properties in graphene without detrimental doping effects.

Proximity-Induced Ferromagnetism in Graphene: An Overview of Anomalous Hall Effect Observations

The paper "Proximity-induced ferromagnetism in graphene revealed by anomalous Hall effect" investigates the intriguing phenomenon of inducing ferromagnetism in graphene through proximity coupling with yttrium iron garnet (YIG) and its consequent effect on electron transport properties. This paper, conducted by Wang et al., demonstrates clear evidence of the anomalous Hall effect (AHE) in single-layer graphene interfaced with a YIG ferromagnetic substrate.

Key Experimental Observations

The researchers use a sophisticated transfer technique to position prefabricated graphene devices onto atomically flat YIG films, ensuring minimal defects or disturbances that could impair the intrinsic electronic properties of graphene. This technique capitalizes on graphene's π-orbitals hybridizing with spin-polarized d-orbitals of the YIG, thereby introducing ferromagnetic order in an otherwise non-magnetic graphene layer.

The experimental findings reveal:

• The manifestation of proximity-induced ferromagnetism in graphene, evidenced by AHE at a magnitude of ~0.09(2e/h) at low temperatures, and detectable up to ~300 K.
• Enhanced spin-orbit coupling (SOC), which is inherently weak in pristine graphene structures, contributing to the observed AHE.

The paper reports robust numerical results, particularly the maintenance of AHE signals over a broad temperature range, implying substantial interactions between graphene and the underlying YIG. The process preserves graphene's innate high carrier mobility, with mobilities exceeding 10,000 cm²/V·s at low temperatures, comparable to or surpassing graphene on common SiO₂ substrates.

Implications and Theoretical Considerations

The implications of this research extend into the field of spintronics, where ferromagnetic graphene could unlock novel electronic devices leveraging spin properties. The ability to induce ferromagnetism without detrimental doping processes that often compromise electronic performance is significant for future graphene-based applications.

From a theoretical standpoint, the research explores spin-orbit coupling's role in the AHE by discounting linear contributions from ordinary Hall effects. The nonlinear Hall signal is attributed to spin-polarized carriers, supported by gate-tunability which demonstrates that AHE persists irrespective of carrier type. This outcome hints at a scattering-independent AHE mechanism indicated by a quadratic relationship between AHE resistance and longitudinal resistance (R$_{AHE}$ ∝ R$_{xx}^2$).

Challenges and Future Directions

Despite these promising results, the paper acknowledges that the quantized AHE (QAHE) regime remains unobserved, arguably due to insufficient Rashba SOC strength. Future efforts might focus on optimizing device quality or increasing SOC via external means, such as substrate engineering or chemical functionalization, to achieve QAHE. Investigating the Fermi level's influence on AHE conductivity via gate modulation may offer insights that advance understanding of 2D Dirac fermion systems under ferromagnetic influences.

In summary, this paper contributes valuable insights into the proximity-induced magnetism in graphene systems, opening the door to potential innovations in electronic devices with tunable magnetic properties. The precise control over induced magnetic effects and retention of graphene's superior electronic properties are crucial strides towards the practical realization of graphene-based spintronic technologies.