Towards graphene-based spintronics

As a prototypical two-dimensional quantum system, graphene displays a combination of exceptional properties including; large charge carrier mobility, high thermal conductivity, strong mechanical strength, excellent optical characteristics, electrically tuneable band gap, as well as the recently discovered long spin coherence length. The revolutionary nature of graphene makes it a prime candidate to become a key material for the proposed spin transistors, in which the generation and tuning of spin-polarised currents are prerequisites¹. In its pristine state, graphene exhibits no signs of conventional spin-polarisation and so far no experimental signature shows a ferromagnetic phase of graphene. This gap is now being filled up by combined efforts in multi-disciplinary research.

Figure 1: Illustration of the Fe-graphene-based spin field effect transistor and its interface.

The ferromagnetic metal (FM)/graphene heterojunction is one of the most promising avenues to realise efficient spin injection into graphene. In any proposed graphene-based transistors, the best opportunity for spin transport could only be achieved when no magnetic dead layer exists at the FM/graphene interface. Previous studies on various FM/semiconductor (FM/SC) heterojunctions revealed the possibility that the magnetic ordering near a region of the surface or interface of FM/SC may be modified due to interdiffusion, termination and hybridisation; and controversial reports make this issue rather complex².

Fundamentally all the intriguing spintronic phenomena observed in the FM/graphene heterojunctions strongly depend upon the interfacial hybridisation and magnetic exchange interaction. A direct demonstration of the magnetic and electronic state of the FM/graphene interface down to monolayer (ML) scale remains a nontrivial task, even today, due to the inaccessibility of the buried layer between the topmost atoms and substrate. Moreover, while the FM atoms reduce to a minute amount, many global detection techniques become invalid, as the experimental conditions like vacuum, sensitivity, cryogen etc. must be simultaneously satisfied at a high level. To overcome these obstacles, we employed a unique FM₁/FM₂/SC structure (in this work, FM1 = 30 ML Ni, FM₂ = 1 ML Fe, and SC = graphene)³. The thick FM₁ layer provides the FM₂ ML with a source of exchange interaction and these two together restore the interfacial behavior of the thick ferromagnetic FM₂/ SC. Combined with the unique elemental selectivity of XMCD, such a structure allows direct observation of the magnetisation of FM₂ at the FM₂/SC interface.

The single layer graphene used in this study was prepared by chemical vapor deposition (CVD) on Cu foil and then transferred on top of Si substrate. After one hour of annealing at 200 °C, 1 ML Fe was grown on the graphene by molecular beam epitaxy (MBE) using an e-beam evaporator. 30 ML Ni was then deposited onto the Fe and the Ni/Fe/graphene was finally capped by 15 ML Cr for easy transport to the synchrotron facility. Raman scattering measurements were performed on the FM/graphene and an as-grown area of the sample without FM deposition. The obtained characteristic spectra of graphene suggest that its structure was well maintained after the transfer, annealing, and the FM deposition processes.

The XMCD of the Fe and Ni L_2,3 absorption edges were performed on beamline I10. Circularly polarised X-rays with 100% degree of polarisation were used in normal incidence with respect to the sample plane and parallel with the applied magnetic field (Fig. 1), in order to minimise the nonmagnetic asymmetries and saturation effects. The XAS spectra of the interface Fe and the stabilising layer Ni for both left- and right-circularly polarised X-rays show a white line at each spin-orbit split core level without prominent splitting of the energy, suggesting that the sample has been well protected from oxidation. The spin (m_spin) and orbital magnetic moment (m_orb) were calculated by applying the sum rules to the integrated XMCD and summed XAS spectra of the Fe L_2,3 edges⁴. The m_total of Fe displays a decreasing trend with the increasing temperature from 1.23 μ_B/atom at 5 K to 1.12 μ_B/atom at 300 K, i.e. ∾9% reduction, pointing to a TC close to bulk-like Fe. At the lowest temperature (5 K), we obtained m_spin = (1.06 ± 0.1) μ_B/atom, which is ∾50% reduced from the bulk-like Fe (whose m_spin = 2.2 μ_B/atom) and m_orb = (0.18 ± 0.02) μ_B/atom, corresponding to an enhancement of ∾200% compared to m_orb = 0.085 μ_B/ atom of that in the bulk. The calculated m_spin, m_orb, and m_total of the Ni stabilising layer are in good agreement with the previous reports of metallic Ni. The sumrules derived m_total of Ni exhibits a trend of slight decrease with the increasing temperature from (0.63 ± 0.06) μ_B/atom at 5 K to (0.52 ± 0.06) μ_B/atom at 300 K, pointing to a T_C close to the bulk-like Ni. Unlike the interfacial Fe, whose m_orb shows a significant enhancement compared to that of bulk-like Fe, the Ni stabilising layer shows a nearly quenched m_orb. This is expected for bulk-like Ni, where the effect of the crystal field plays a dominant role⁵.

Figure 2: Spin-resolved band structures for a freestanding Fe ML (left column) and the ML Fe^top/graphene (right column), respectively, together with their corresponding partial density of states.

The first-principle calculations (Fig. 2) were performed using the projector augmented wave methods implemented in the Vienna ab initio simulation package. In order to determine the electronic and magnetic ground state of the Fe ML on graphene, first it must be addressed where Fe likes to reside with respect to the C atoms. It was found that the Fe atoms deposited on graphene prefer to follow fcc(111) stacking and there exist three inequivalent positions of the fcc Fe allocations on graphene among which the top configuration is most energetically favourable. Consistent with the experiment, the theoretical simulation also reveals a ∾50% reduction in the magnetic moment of Fe. Although Fe^top was found the most stable geometry on graphene according to the calculation, the observed numeric results from XMCD, i.e., m_spin = (1.06 ± 0.1) μB/atom, is more likely a mixture of Fe^top (m_spin = 1.23 μ_B/atom) and Fe^bridge (m_spin = 0.67 μ_B/atom), given the small energy difference (ΔE = 43 meV) of their calculated total energies.

To summarise, we have demonstrated an unambiguous ferromagnetic epitaxial Fe/graphene interface by XMCD. This was aided by the inclusion of a Ni stabilising layer atop the Fe film, which facilitated the study of the temperature dependence of the Fe/graphene interface. Our study restores a realistic scenario of the proposed graphene-based transistors and addresses the open question of the magnetic and electronic nature of the Fe/graphene interface. This approach represents a valuable model of exploring the interfacial magnetism of FM on graphene and the results are encouraging based on the contemporary sample preparation technique. Future work to explore the tuning of the spin polarised band structure of both the FM and graphene via the interface engineering will be of great interest and have strong implications for both fundamental physics and the emerging spintronics technology.

References:

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3. Liu, W. et al. Probing the Buried Magnetic Interfaces. ACS Applied Materials and Interfaces 8, 5752-5757, doi:10.1021/acsami.5b11438 (2016).
4. Thole, B. T., Carra, P., Sette, F. & Vanderlaan, G. X-ray circular dichroism as a probe of orbital magnetization. Physical Review Letters 68, 1943-1946, doi:10.1103/PhysRevLett.68.1943 (1992).
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Funding acknowledgements:
This work is supported by the State Key Program for Basic Research of China (Grants No. 2014CB921101 and 2011CB921404), NSFC (Grants No. 61274102 and 21473168), and UK STFC.

Corresponding authors:
Dr Wenqing Liu, University of York, [email protected]; Professor Yongbing Xu, University of York.