Every single layer matters

  Nanophysics

SrRuO₃ belongs to a class of complex perovskite oxides that display an interesting interplay of spin, charge, and orbital degrees of freedom, and may provide alternatives to conventional silicon-based electronics. While bulk SrRuO₃ is a metallic ferromagnet at low temperatures, thin films of SrRuO₃ undergo a metal-to-insulator transition and exhibit intriguing changes in the magnetic order as a function of thickness. A detailed understanding of how the thickness of the films affects the underlying physics is, however, absent.

In a Rapid Communication published in Physical Review B, Wolter Siemons and Gertjan Koster of the University of Twente in the Netherlands, and Jing Xia, Malcolm Beasley, and Aharon Kapitulnik of Stanford University study the transport and magnetic properties of thin films of SrRuO₃ grown on a SrTiO₃ substrate. They pin the critical thickness for the metal-insulator transition to four layers, observing eight orders of magnitude increase in resistance when the thickness decreases from four to three layers. In addition, they find that for two and three layers, the axis of the magnetic moment associated with ferromagnetism collapses to the plane of the films, compared to the bulk case where the alignment is close to perpendicular. The authors propose that reconstruction of the crystal lattice at the interface with the SrTiO₃ substrate creates an antiferromagnetic layer and induces the insulating state in the two and three layer case. The implication is that a thickness of four layers is adequate to overcome the interface influence, resulting in a phase transition to the bulk properties of SrRuO₃.– Alex Klironomos