Transmission electron microscopy and x-ray structural investigation of La_0.7Ca_0.3MnO₃ thin films

Abstract

The structural changes and magnetoresistance (MR) properties of as-grown and post-annealed La0.7Ca0.3MnO3 films were investigated by transmission electron microscopy (TEM) and x-ray diffraction (XRD). The data for the films were compared to that for bulk La0.7Ca0.3MnO3 post-annealed under the same conditions. The main structure of the as-grown films was face-centered pseudo-cubic with a doubled perovskite unit cell, of size ∼2ap × ∼2ap × 2ap, where ap is the single perovskite parameter. The phase showed a cube-on-cube epitaxy with the underlying LaAlO3 substrate. Upon annealing to a saturation point, a minor primitive pseudo-tetragonal structure evolved, of cell parameters . A total of four possible orientations of the two structures was observed by TEM, comprised of one orientation of the ∼ 2ap × ∼ 2ap × ∼ 2ap cell, i.e., the cube-on-cube epitaxy, giving rise to (00l) peaks in x-ray, and three orientations of the cell, giving rise to a single (00l)/(hk0) peak in x-ray. The bulk La0.7Ca0.3MnO3 sample also contains the × structure. The difference between the bulk and the film and the effects of annealing on films can be ascribed to the influence of strain between the film and substate, induced by lattice mismatch.

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

“…Reconstruction of these five ED patterns suggests a body-centred cubic lattice structure in reciprocal space, corresponding approximately to a face-centred pseudo-cubic lattice in real space with a unit cell of 2a p  2a p  2a p . It is noteworthy that this pseudo fcc structure has also been reported in as-grown LCMO films on LaAlO 3 substrates [19].…”

Magnetic and transport properties of La0.8Sr0.2MnO3/La0.8Ca0.2MnO3 bilayer

“…The aforementioned analysis of x-ray data showed that the out-of-plane lattice parameter c is larger for LSM/LAO. It is well known that LSM thin films grown on LAO substrates exhibit an out-of-plane uniaxial tensile strain and, correspondingly, an in-plane biaxial compression [9,10]. Assuming that the film is strained from the ideal bulk structure and that the structure is a single perovskite, the in-plane lattice parameter of film can be estimated from the unit cell volume in the bulk.…”

Magnetic and transport properties controlled by structural disorder in La0.7Ca0.3MnO3 films

“…The LCM2 film shows weak Bragg peaks on a background of the same smooth plateau. The out-of-plane lattice parameter for the LCM2 film, c ; 0.3866 nm, is very close to that for the bulk [13,14]. This suggests that the incipient crystalline phase grows in equilibrium conditions without a significant influence of the substrate and is formed through the amorphous state.…”

“…However, together with the perpendicular crystal axes, a tetragonal distortion of the lattice (c a ; 1.026) is also found in inset B. The estimated in-plane lattice parameter for the LCM3 film, a ; 0.3826 nm, is smaller than that for the bulk compound, a ; 0.3858 nm [14]. This difference leads to for- ; -0.83 and e ; 1.79%.…”

“…Ca 0 3 . MnO 3 compound is T C ( ) e = 0 ; 265 K [14,19] and using the obtained values of e 100 and e 001 and T C for LCM3, we determined D to be about 950, which is of the same order of magnitude as the prediction of the theoretical model [17]. A possible explanation for this discrepancy is that the lattice mismatch between LCM3 and LAO is so large that the effect of structural disorder becomes dominant over the strain effect and that the role of Jahn-Teller distortion in formation of the ferromagnetic ordering is distinctly decreased.…”