Antiferromagnetism and insulating nature of ultrathin films of SrRuO3

Metallic oxides form an integral part of oxide-based technologies, constituting the connecting electrode material. In this context SrRuO3 is a material that has been widely studied as it in addition to being metallic is also ferromagnetic. Recent experiments have found that ultrathin films of SrRuO$_3$ are insulating and hence 5-6 monolayers are required before metallic character is observed. In this work we theoretically examine the origin of the insulating state with first principle GGA+U calculations. The value of U is calculated from first principles. Ru has a formal configuration of d$^4$ in SrRuO$_3$. In bulk SrRuO$_3$ this translates into a low spin state with an electronic configuration of $t_{2g\uparrow}t_{2g\downarrow}$. Hence at the surface/ultrathin film limit one expects the observed insulating nature to come from a transition into the nonmagnetic state with the lowest crystal-field levels contributed by $d_{yz}$ and $d_{xz}$ orbitals being completely filled. However one finds that the system undergoes an unusual structural distortion which is accompanied by a spin state transition. This spin state transition is accompanied by a transition into an antiferromagnetic state which drives the system insulating.