Structural and magnetic properties of ultrathin epitaxial La0.7Ca0.3MnO3 manganite films: Strain versus finite size effects

Abstract

We present a structural and magnetic study of La0.7Ca0.3MnO3 epitaxial films (the thickness ranges from 2.4 to 80 nm) on SrTiO3. Their structure, as obtained by x-ray diffraction, is orthorhombic with a 45° rotated 2×2 square lattice, referred to the SrTiO3 substrate, in the film plane. The 2.4 nm film adopts a different structure as evidenced by the extinction of particular diffraction peaks and supported by the behavior of the size of the in-plane structural domains. While no important structural changes are detected for the other films as the layer thickness is reduced, Tc decreases drastically while the remanent magnetization is constant. Tc follows the functional dependence of a mean field approximation for the limitation of the divergence of the spin–spin correlations by the film thickness.

Effect of size reduction on the ferromagnetism of the manganite La1−xCaxMnO3(x=0.33)

“…Finite-size effects have been investigated in oxide films [38] From these data we find that λ = 0.6 and ξ 0 = 3.4 nm. This value of λ, although smaller than other systems, is very close to the theoretical predictions of λ ≈ 0.7 [35].…”

Crystal structure of (La,Ca)MnO3ultrathin films deposited on SrTiO3substrates

“…For example, only integer peaks in the (1, 0, l ) scan and half-integer peaks in the (1.5, 0.5, l ) scan appeared for the 2.4-nm film. According to de Andre´s et al [21], the 2.4-nm film follows the STO parameter in the film plane, while the out of plane parameter is slightly reduced. Remarkable is that the 2.4-nm film shows a slightly larger out of plane parameter than that of the thicker films, probably due to the structural phase transition.…”

“…Extrapolated to the 3 nm films, an out of plane parameter of 0.382 nm would be obtained. However, recently de Andre´s et al [21] studied the crystal structure of LCMO films with thicknesses ranging from 2.4 to 80 nm by X-ray diffraction. The diffractometer at the DUBBLE-BM26 beamline at the Europen Synchronton Radiation Facility allowed these authors to obtain the diffraction patterns from the thinnest 2.4-nm films.…”

“…Following Woodward [23], the space group and unit cell associated with each tilting system can be deduced. Considering the fourfold symmetry of the (001) STO plane of the STO substrate and the thickness of the films, the most likely tilting system would be a one-tilt or a two-tilt system, whose space group and unit cell are listed in the 2.4-nm film of de Andre´s et al [21] have either a bct I4/mcm structure or a bco Imma structure. The tetragonal I4/mcm structure is a result of rotation of the MnO 6 octahedra around the out of plane axis, while the orthorhombic Imma structure arises from the rotations of the MnO 6 octahedra around the two perovskite cubic axes in the film plane.…”

“…Remarkable is that the 2.4-nm film shows a slightly larger out of plane parameter than that of the thicker films, probably due to the structural phase transition. In this paper, we will adopt the unit-cell parameters from de Andre´s et al [21], a ¼ 0.3905 nm and c ¼ 0.387 nm, leading to an axis ratio c/a ¼ 0.991, where a and c are the in plane and out of plane parameters of the simple perovskite, respectively. Because of this tetragonal distortion, the 2a p  2a p  2a p fcc structure is actually a bct or bco structure with a unit cell of $2 ½ a  2 ½ a  2c.…”