Effects of thickness on the cation segregation in epitaxial (001) and (110) La2/3Ca1/3MnO3 thin films

Abstract

Electron-energy-loss spectroscopy is used to map composition and electronic states in epitaxial La 2/3 Ca 1/3 MnO 3 ͑LCMO͒ films of various thicknesses grown on SrTiO 3 ͑001͒ and ͑110͒ substrates. For relatively thick films ͑Ն20 nm͒, epitaxial tensile strain in ͑001͒ films promotes a compositional La/Ca gradient across the film thickness, being the interface La rich, while the relaxed ͑110͒ films are chemically homogeneous. In contrast, much thinner ͑001͒ and ͑110͒ LCMO films display a different La/Ca distribution, being La rich at the free surface. The observed distinct thickness-dependent composition gradient behavior reflects a balance between strain-induced elastic energy minimization and kinetic effects during growth. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3211130͔Mixed-valence ferromagnetic manganite films have been the object of much attention in recent years due to their potential applications in spintronics. 1,2 However, expectations have been lowered by the negligible room-temperature magnetoresistance observed in tunnel junctions. 3 Although the reasons for this behavior are not yet fully known, it has been suggested that they may be linked to electronic phase separation. 4 Whether the latter is a pure electronic effect or related to inhomogeneous chemical distributions can be suitably explored by electron energy-loss spectroscopy ͑EELS͒. For instance, electronic states can be mapped out by direct determination of local Mn oxidation state at the nanometric scale. [5][6][7][8] On the other hand, mappings of the chemical composition can be achieved offering chemical information at the nanometric scale [5][6][7][8][9] We recently reported 8 a comparative characterization, both from the microstructural and chemical points of view, of relatively thick ͑ϳ80 nm͒ LCMO layers grown on STO in order to determine which differences accounted for the fact that ͑110͒ LCMO films display enhanced magnetic properties when compared to their ͑001͒ counterparts. While homogeneous chemical composition was associated to the contribution of plastic defects as strain-relieve mechanism in the more relaxed ͑110͒ films, a Ca 2+ ion migration toward free surface, and the concomitant Mn m+ oxidation state variation was observed in the more stressed ͑001͒ films.In this letter, we have extended the characterization of the microstructure, local stoichiometry variations, and Mn oxidation state of ͑001͒ and ͑110͒ LCMO films within a range of layer thicknesses between 3.5 and 40 nm. The results confirm the same trend for LCMO films with t ϳ 40 nm as those with t ϳ 80 nm previously reported. 8 However, most importantly, we assess that for ultrathin LCMO films ͑t Ͻ 20 nm͒, a new scenario is revealed with significant differences regarding cation distribution across the film in comparison with thicker layers, while keeping an orthorhombic crystalline phase in contrast with structural changes reported by Qin et al. 10 The ͑001͒ and ͑110͒ LCMO films with thicknesses ͑t͒ ranging from 3.5 to 82 nm were grown by rf sputtering at 8...

Ferroelectric domain structure of anisotropically strained NaNbO3 epitaxial thin films

“…This deviation points to a slight oxygen deficiency in these films 55,56 and is attributed to the beneficial effect of the formation of oxygen vacancies in order to reduce the lattice mismatch for films which grow under tensile lattice strain. 57 Partially relaxed films on ReScO 3 with Re 5 Dy, Tb, Gd PFM data in Figs. 5(a)-5(c) show that domain formation critically depends on the magnitude of lattice mismatch.…”

Impact of Sr segregation on the electronic structure and oxygen reduction activity of SrTi1−xFexO3 surfaces

“…40, 41 An increase in Fe content may make it easier to equilibrate a larger amount of (positively charged) oxygen vacancies near the surface that electrostatically prefers Sr 2+ ions more than the Ti 4+ or Fe 3+/4+ ions. Second is the elastic interactions related to large size of Sr, which can drive the Sr to 20 free surfaces to minimize the elastic energy of the system 42,43 (note that the lattice parameter of STF decreases with increasing Fe content 18 ). In the authors' ongoing research, these driving forces for cation segregation phenomenon are being assessed quantitatively using both experiments and computation.…”

Factors that Influence Cation Segregation at the Surfaces of Perovskite Oxides

“…In our recent work, we showed that cation segregation can be described by the elastic and electrostatic interactions of the dopant with the surrounding lattice in perovskite oxides. (1) The specific mechanisms include the size mismatch between the dopant and host cations and the associated elastic energy minimization by pushing the dopant to free surfaces or interfaces (18)(19)(20)(21), and the charged defect interactions, such as a strong association of dopant cations with oxygen vacancies, which can drive the dopants to positively charged interfaces where oxygen vacancies are in abundance (22) as well as with polar surfaces. Based on this understanding, we propose that deficiency of the A-site cations on the perovskite lattice should suppress the dopant segregation on the A-site by allowing for more space in the bulk and reducing the elastic energy.…”