Evidence of Orbital Reconstruction at Interfaces in UltrathinLa0.67Sr0.33MnO3Films

Abstract

Linear dichroism (LD) in x -ray absorption, diffraction, transport and magnetization measurements on thin La 0.7 Sr 0.3 MnO 3 films grown on different substrates, allow identification of a peculiar interface effect, related just to the presence of the interface. We report the LD signature of preferential 3d-e g (3z 2 -r 2 ) occupation at the interface, suppressing the double exchange mechanism.This surface orbital reconstruction is opposite to that favored by residual strain and is independent of dipolar fields, the chemical nature of the substrate and the presence of capping layers.Interfaces between perovskite oxides display unexpected properties. The roles of chemistry, polarization and strain may be singled out by selective experiments, e.g.Ref.[11], where an engineered interface obtained by intercalating two LMO unit cells (u.c.) between the LSMO and the STO has been shown to recover the LSMO bulk properties even at room temperature. The role of strain on preferential orbital occupation in transition metal oxides has been widely studied [12]. The anisotropy of d-orbitals influences the electron correlation effects in an orbital direction-dependent manner, thus giving rise to the anisotropy of the electron-transfer and eventually destroying the DE order of unstrained half-metallic LSMO (Fig.1, center). The strain effect on orbital physics can be understood on the basis of the experimental phase diagram proposedby Konishi et al.,[ 13] and explained theoretically by Fang et al.[14]. Spin ordering in strained manganite is influenced by orbital ordering and several anti-ferromagnetic (AF) insulating JahnTeller distorted phases are observed: the strain induced elongation or compression of the MnO 6 octahedra leads to crystal field splitting of the e g levels, lowering either i) the (3z 2 -r 2 ) state which favors the C -type AF structure (Fig.1, left) or ii) the (x 2 -y 2 ) state which stabilizes the A -type structure ( Fig.1, right resonant transition. Polarization effects arise when the polarization vector is set parallel to t he c crystallographic axis or perpendicular to it (I c and I ab respectively). The LD is the difference between the two spectra (I ab -I c ) and gives a direct insight of the empty Mn 3dstates: a LD which is on average positive (negative) indicates a majority o f off-plane (in-plane) empty 3d states. Considering the crystal field splitting, the effect can be mainly related to the occupation of the two e g states (3r 2 -z 2 and x 2 -y 2 ) with majority spin: a LD which is on average positive (negative) is due to a preferential occupation of the in-plane x 2 -y 2 (out-of-plane 3r 2 -z 2 )orbital.Magnetization measurements were carried out by a SQUID magnetometer. Further experimental details are given in ref.[16] and [26].In Fig.2 Fig.4(b), revealing opposite signs for these two cases.Although the comparison with experiments can only be qualitative and a proper fit is not feasible, the sign reversal is observed in the experimental spectra of Fig.4(a) for energies above the E˜644 ...

Origin of insulating weak-ferromagnetic phase in ultra-thin La0.67Sr0.33MnO3 films on SrTiO3 substrate

“…We use Slater integrals reduced to 70% and a tetragonally distorted octahedral crystal field (D4h symmetry), with Ds = ±0.186 eV, Dt = ±0.025 eV, 10Dq = 1.1 eV for the cubic crystal field splitting and three valence states (Mn 3+ , 0.7Mn 3+ + 0.3Mn 4+ , Mn 4+ ). 10 The preferential occupation of the d3z 2 (dx 2 -y 2 ) orbital is induced by the positive (negative) signs of Ds and Dt, which is corresponding to an in-plane compressive (tensile) strain. Among the simulated spectra the two with compressive strain and Mn 3+ and 0.7Mn 3+ + 0.3Mn 4+ valence states are more similar to the experimental spectra of the 3, 6 and 8-u.c.…”

“…One of the most interesting examples is phase transition from metal in bulk form to insulator in thin film, as exhibited by several metallic oxides, such as SrVO 3 , 1 LaNiO 3 , 2,3 SrRuO 3 , 4 and La 1x Sr x MnO 3 . [5][6][7][8][9][10][11] Researchers have worked to minimize several extrinsic effects of interfacial chemical or oxygen disorders by optimizing growth conditions, but the dead layer in which electrical conductivity or ferromagnetism disappear still remains.…”

Unraveling how electronic and spin structures control macroscopic properties of manganite ultra-thin films

“…The spectral difference (I(E||a)-I(E||c), linear dichroism) implies a preferential occupancy of the 3d x 2 Ày 2 orbital at both 80 and 300 K. The result is in good agreement with the theoretically calculated result for LSMO film under tensile strain. 18,24 X-ray magnetic circular dichroism measurements…”

Interfaces and Nanostructures of Functional Oxide Octahedral Framework Structures

“…In the La 1-x Sr x MnO 3 solid solution, the electron occupancy of the e g doublet depends on the strength of the JahnTeller distortion, which in turn is determined by the hole doping level, x (Tokura and Nagaosa, 2000). In epitaxial thin films, however, similarly to the Jahn-Teller distortion, misfit strains have the ability to break the x 2 − y 2 /3z 2 − r 2 orbital degeneracy (Fang et al, 2000) as recently confirmed by X-ray linear dichroism investigations (Huijben et al, 2008;Tebano et al, 2008;Pesquera et al, 2012). A thorough investigation taking into account the attenuation of the signal with the depth in the sample, suggests that the observed X-ray linear dichroism is rather a consequence of the superposition of interfacial and free surface effects (Pesquera et al, 2012).…”