Effects of film thickness on surface flatness and physical properties in La1−xSrxMnO3 thin films investigated by scanning tunneling microscopy

We have used scanning tunneling microscopy (STM) to investigate the effects of lattice strain on the surface flatness and physical properties of La1−xSrxMnO3 (LSMO) thin films, and scanning tunneling spectroscopy (STS) to investigate the change of surface density of state (DOS) caused by lattice strain. For relatively thin films (50 Å), lattice strain causes the surface to become extremely rough, whereas it is atomically flat with film thicknesses of greater than 200 Å. The filmʹs electrical and magnetic properties are similar to bulk-like ferromagnetic metal whose TC is 370 K when the film thickness is 2000 Å. However, when the film thickness is less than 200 Å, LSMO films exhibit insulating properties with lower TC than that of bulk LSMO at RT due to lattice strain. The value of surface DOS measured by STS is consistent with the filmʹs electrical properties. These results suggest that strain effect on physical properties are larger than that on the surface flatness.