Finding the right substrate support for magnetic superatom assembly from density functional calculations

In order to find the right support to form assemblies of superatoms with useful magnetic properties, adsorption of the magnetic superatom ${\mathrm{FeCa}}_8$ on different types of substrates is studied using first-principles electronic structure methods. ${\mathrm{FeCa}}_8$ interacts strongly with oxide and metal substrates and its structure and magnetic properties are destroyed. Interaction between ${\mathrm{FeCa}}_8$, and graphene and hexagonal BN (h-BN) are weak, and its structure is retained on these substrates. Dimers of ${\mathrm{FeCa}}_8$ are also found to be stable on both h-BN and graphene. While the magnetic interaction between two ${\mathrm{FeCa}}_8$ units deposited on h-BN involves only direct exchange, substrate mediated RKKY interaction also plays a role on graphene. Consequences of these for the magnetic properties of the assemblies are discussed.

Figure 1

The lowest-energy structure of ${\mathrm{FeCa}}_8$ using (a) vdW-DF2 and (b) LSDA.

Figure 2

The MO's, and corresponding charge densities of ${\mathrm{FeCa}}_8$ using (a) vdW-DF2 and (b) LSDA. Degeneracy of the MO's are also indicated.

Figure 3

The lowest-energy structure of ${\mathrm{FeCa}}_8$ supported over alumina (a) and calcium (b) substrates. In (b), the substrate Ca atoms are shown with a darker (blue) color to distinguish them from the Ca atoms of the superatom.

Figure 4

The lowest-energy structure of ${\mathrm{FeCa}}_8$ supported over h-BN sheet (a) and Graphene (b) using vdW-DF2. First column: top view; second column: side view.

Figure 5

Total density of states, and atom projected partial density of states of h-BN supported ${\mathrm{FeCa}}_8$.

Figure 6

Difference in the adsorption energies (circle) of ${\mathrm{FeCa}}_8$ having magnetic moment of 4 ${\mu }_B$ (fixed-moment) and no magnetic moment (nonmagnetic) on a graphene sheet represented by supercells of various sizes; and the moment (square) obtained through self-consistent calculations (M).

Figure 7

Dimer of ${\mathrm{FeCa}}_8$ deposited over h-BN sheet using vdW-DF2.

Figure 8

Lowest-energy structure of ${\mathrm{FeCa}}_8$ supported over h-BN sheet (a) and graphene (b) using LSDA method. First column-top view; second column-side view.

Figure 9

DOS of ${\mathrm{FeCa}}_8$ deposited over BN-sheet using LSDA.

Figure 10

DOS of ${\mathrm{FeCa}}_8$ deposited over graphene using LSDA.

Figure 11

Adsorption sites of alumina and calcium surface. (Top) Alumina surface (top and side views). (Lower) Calcium surface (top and side views). Different layers are represented with different colors.

Figure 12

Density of states of ${\mathrm{FeCa}}_8$ deposited over alumina surface.

Figure 13

Density of states of ${\mathrm{FeCa}}_8$ deposited over calcium surface.

Figure 14

Some high-energy isomers of ${\mathrm{FeCa}}_8$ deposited over BN-sheet using vdW-DF2.

Figure 15

Spin density isosurface of ${\mathrm{FeCa}}_8$ deposited over h-BN using vdW-DF2 (plotted at isovalue 115 a.u.).

Figure 16

Scatter plot of adsorption energy and magnetic moment for various structures of ${\mathrm{FeCa}}_8$ on h-BN. The two structures with moment 4 ${\mu }_{\mathrm{B}}$ are the lowest-energy one and the one shown in Fig. 14. The one with a moment of 2 ${\mu }_{\mathrm{B}}$ is the one in Fig. 14. The structure having no magnetic moment is a still higher energy one with large distortion of the antiprism cage.

Figure 17

Considering the two ${\mathrm{FeCa}}_8$ units are in the ground state over substrate and diffusing towards each other, we can guess two possible structures of a dimer, (a) each atom of triangular face is connected to only one atom of the other triangular face, and (b) one atom of each triangular face is connected to two atoms of the other triangular face.