Magnetic tunnel junctions (MTJ) with perpendicular magnetic anisotropy (PMA) provide better thermal stability and lower switching current compared to in-plane MTJs. Traditional approach for PMA engineering is to use FM/oxide interfaces or multilayer structures comprising two FM or FM/nonmagnetic metal interfaces . Here, they propose a Co-graphene-based system to realize electrodes with giant PMA that might be of high importance for both traditional and graphene spintronic.
In this work, we investigated magnetocrystalline anisotropy of graphene-Co structures from both first-principles calculations and experiments. We demonstrate that graphene coating on Co films can dramatically enhance the PMA up to twice that of pristine Co films depending on Co thickness (c. f. Figure 1 [d]). Moreover, graphene can increase the film effective PMA and stabilize an out-of-plane magnetization easy axis for a FM layer thickness up to 25 Å, which is much larger than that of the intensively studied Fe/MgO structure. In addition, our layer-resolved analysis reveals that the interfacial three Co layers play a decisive role in system’s anisotropy and can be dramatically affected by the proximity of graphene. Furthermore, our orbital hybridization- resolved analysis unveils the origin of PMA enhancement, which is attributed to a change of anisotropy contribution from hybridization between dz2 and dyz orbitals when coating Co with graphene. Finally, on the basis of the anatomy of Co-graphene PMA, we propose Co-graphene heterostructures stabilized by superexchange interaction, which are demonstrated by our first-principles calculations to possess a linearly increasing surface anisotropy and constant effective anisotropy as a function of film thickness[3]. These findings point toward possibilities to engineer graphene/ferromagnetic metal heterostructures with giant magnetic anisotropy more than 20-times larger compared to conventional multilayers, which constitutes a hallmark for future graphene and traditional spintronic technologies.
Figure 1 Top and side view of (a) bare Co slab, (b) Co on graphene, and (c) Gr/Co/Gr, respectively. (d) Magnetocrystalline anisotropy energy as function of Co thickness N (monolayers). (e) Effective anisotropy Keff*t as a function of Co thickness for bare Co film (Co), one surface of Co film coated by graphene (Co/Gr), and both surfaces of Co film coated by graphene (Gr/Co/Gr), respectively. (b) Thickness dependence of the M⊥/M// ratio for bare Co and for graphene/Co films obtained from experiment.
[1] Phys. Rev. B. 90 064422 (2014).
[2] Phys. Rev. B. 88 184423 (2013).
[3] Nano Lett. 16, 145 (2015).