Properties and microstructure of ultrathin (La,Ca)MnO3 films under different conditions of strain

Abstract

Ultrathin films of La1-xCaxMnO3 (x≅0.33) in a thickness range below 10 nm were deposited by direct current (dc) magnetron sputtering on different substrates, (001)SrTiO3 (STO), (001)LaAlO3 (LAO), and (110)NdGaO3 (NGO) in order to study the effects of strain in this thickness regime. The combined study of physical properties and microstructure by high-resolution electron microscopy (HREM) reveals that ultrathin (≲ nm) strained films on STO can be grown in a body-centered-tetragonal structure not recognized previously. Such films do not show an insulator-metal (IM) transition. At slightly larger thickness, around 6 nm, the microstructure is different. Depending on the growth parameters we find either a structure involving MnO6-octahedra tilted around two axes, in combination with the occurrence of twin boundaries as reported previously; or the bulk orthorhombic Pnma structure. In the latter case an IM transition is found at temperatures close to the bulk value. On lattice-matched NGO, the IM transition and bulk-like magnetoresistance effects are observed down to the lowest thickness investigated (3 nm). On LAO, which compresses the lattice, no IM transition is found even at a thickness of 15 nm, which is due to island-like growth, as confirmed by HREM observations.