Unusual structure and magnetism in manganese oxide nanoclusters

We report an unusual evolution of structure and magnetism in stoichiometric MnO clusters based on an extensive and unbiased search through the potential-energy surface within density functional theory. The smaller clusters, containing up to five MnO units, adopt two-dimensional structures; and regardless of the size of the cluster, magnetic coupling is found to be antiferromagnetic in contrast to previous theoretical findings. Predicted structure and magnetism are strikingly different from the magnetic core of Mn-based molecular magnets, whereas, they were previously argued to be similar. Both of these features are explained through the inherent electronic structures of the clusters.

Figure 1

Geometrical evolution shows a 2D to three-dimensional (3D) transition at $(\mathrm{MnO}){}_6$. Minimum-energy structures are always found to be AFM. The up (down) Mn atoms are shown with yellow/light gray (green/gray) color. Oxygen atoms are shown in red/dark gray. The Mn-O (1.80–2.18 Å) and Mn-Mn (2.55–3.10 Å) distances and the Mn-O-Mn angles (${78}^{°}\text{−}{108}^{°}$) in these clusters are comparable to the Mn-based molecular magnets.[1, 2]

Figure 2

(a) The trend in binding energy shows that the clusters adopt 2D structures until they contain five MnO units, above which they are 3D. The Mn-Mn magnetic coupling is always AFM for the most stable solution. Moreover, the AFM coupling is always favorable over the FM coupling, in both 2D and 3D. (b) Total hybridization index $\mathcal{H}={\mathcal{H}}_{\mathit{sp}}+{\mathcal{H}}_{\mathit{pd}}+{\mathcal{H}}_{\mathit{sd}}$ shows a direct correspondence to the energy.

Figure 3

Orbital projected density states summed over all the atoms in the cluster [for 2D and 3D $(\mathrm{MnO}){}_4$ and $(\mathrm{MnO}){}_6$ clusters in their AFM state; FM state for 3D $(\mathrm{MnO}){}_6$ is also shown] show that the minimum-energy structures (both geometric and magnetic) correspond to larger energy spread of the orbitals and higher hybridization. Gaussian smearing (0.1 eV) has been used.