Field-regulated switching of the magnetization of Co-porphyrin on graphene

Different magnetic coupling mechanisms have been identified for a few monolayers of Co-porphyrin molecules deposited on a graphene-covered Ni(111) single crystal. A relatively strong antiferromagnetic coupling of the first molecular layer via graphene to the Ni crystal in comparison to a weaker intermolecular coupling gives rise to a complex field-dependent response of this hybrid system. By continuously increasing the magnetic field strength, the net magnetization of the molecular system switches from antiparallel to parallel to the field direction at 2.5 T. Utilizing x-ray absorption spectroscopy and x-ray magnetic circular dichroism, the element-specific magnetization and field dependence was probed. The nature of the magnetic couplings is identified by means of density functional theory and orbital-dependent susceptibilities.

Figure 1

Top: Schematic images of the sample: 3 ML CoOEP/graphene/Ni(111). The white and black arrows indicate the directions of the magnetic moments of Ni (blue) and Co (red) in a low magnetic field of 500 mT (left) and in a high magnetic field of 5 T (right). Bottom: XMCD at the Co L${}_{2,3}$ edges of the CoOEP molecules in the low magnetic field (left) and in the high magnetic field (right). Insets show the XMCD at the L${}_{2,3}$ edges of the saturated Ni crystal at 500 mT and at 5 T. All spectra are recorded at $T=2.8$ K and $\Theta$ = 70${}^{\circ }$.

Figure 2

Magnetization curves between $-2$ and +2 T for Ni (blue) and between $-6$ and +6 T for Co (red), measured at $\Theta ={70}^{\circ }$ and $T=2.8$ K. The continuous line shows the fitted Co magnetization (see text for details).

Figure 3

Out-of-plane contribution to the DOS, resolved on C, N, and Co states projected onto the Co-porphyrin molecule, labeled (CoP)-Co, (CoP)-C, (CoP)-N. We also show a projection of graphene C atoms, labeled (Graphene)-C atoms. Projections are done on specific $m_l$ projected orbitals. Inset: Magnetization density cross section, which depicts antiferromagnetic coupling between Ni and Co along with graphene sublattice magnetization. Red and blue colors indicate positive and negative densities on CoP (on top), graphene (middle layer), and the first Ni layer of our unit cell.

Figure 4

Orbital-dependent susceptibility as a function of Fermi energy. Note the dominance of the $d_z^2$ orbital at the Fermi energy $E_F=0$, relevant for our present system. Inset: Distance- and angle- (between two CoP molecules) dependent exchange energies. Schematic views of relative separation and rotation between two CoP molecules are given in the two insets.