Effect of on-site correlation on the structure, stability, and magnetic properties of (FeCl2)3 trimer

In a recent experiment, Kresin and co-workers, in addition to validating an earlier theoretical prediction by Pradhan and Jena that the Fe atoms in the (FeCl2)2 dimer are coupled antiferromagnetically, found that the magnetic moment of the (FeCl2)3 trimer is same as that of its monomer, namely, 4 μB. This is in contrast to the FeCl2 monolayer where the coupling between the Fe atoms is ferromagnetic. Calculations using density functional theory showed that the ground state of the trimer is ferromagnetic with a total magnetic moment of 12 μB and its structure mimics that of the H-phase of the monolayer. Molecular dynamics simulations showed that the ferromagnetic isomer is unable to overcome an energy barrier at liquid helium temperature. However, when on-site correlation was included, which was earlier found to be unimportant for the magnetic configuration of the (FeCl2)2 dimer, the ground state geometry of the trimer assumed linear chain geometry, sharply different from what would be expected from the H- or T-phases of its monolayer. Interestingly, the preferred magnetic moment of the chain-like trimer is found to be 4 μB, in agreement with experiment. These results demonstrate the importance of including on-site correlation in each calculation involving transition metals. The intricate relationship between structure and magnetic properties of clusters can be used to tailor metastable materials that nature does not prefer.