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B-site ordered thin films of double perovskite Sr 2 CoIrO 6 were epitaxially grown by a metal-organic aerosol deposition technique on various substrates, actuating different strain states. X-ray diffraction, transmission electron microscopy and polarized far-field Raman spectroscopy confirm the strained epitaxial growth on all used substrates. Polarization dependent Co L 2,3 X-ray absorption spectroscopy reveals a change of the magnetic easy axis of the antiferromagnetically ordered (high-spin) Co 3+ sublattice within the strain series. By reversing the applied strain direction from tensile to compressive, the easy axis changes abruptly from in-plane to out-of-plane orientation. The low-temperature magnetoresistance changes its sign respectively and is described by a combination of weak anti-localization and anisotropic magnetoresistance effects. PACS numbers: 73.50.-h,75.47.-m,75.70.-i arXiv:1801.03853v2 [cond-mat.str-el]

Polarization‐dependent tip‐enhanced Raman spectroscopy (TERS) was carried out on double perovskite La2CoMnO6 (LCMO) thin films to determine the contributions of the enhancement by the excitation of surface plasmon polariton and tip‐induced polarization effects. Due to B‐site ordering and the resulting monoclinic P121/n1 structure, the spectra exhibit a strong Ag mode around 644 cm−1 originated from (Co/Mn)O6 stretching vibrations. The TERS contrast C, defined as the ratio of the total‐field (the sum of the near field and the far field) to the far‐field intensity, up to C ~60–110 for a cross‐polarized configuration (incident light polarization perpendicular to the detected one), reveals a possible breakdown of polarization‐dependent selection rules for the Raman scattering due to a tip‐induced near‐field depolarization component. We disentangled a significant enhancement for all observed modes in a parallel configuration with a TERS contrast of C ~2–5 due to general near‐field amplification, strongly affected also by the film thickness. With B‐site ordering and highly dielectric behavior, LCMO therefore provides an excellent test system to study TERS phenomena on oxide surfaces. Copyright © 2016 John Wiley & Sons, Ltd.

We report direct evidence of the theoretically predicted electron-rich surface of doped perovskite manganites La 0.7 A 0.3 MnO 3 (A = Ca, Sr) by means of surface-enhanced Raman spectroscopy. The required Au nanoparticles were grown on top of thin manganite films by the metalorganic aerosol deposition technique, which provides a stable oxygen atmosphere and prevents deoxygenation effects. The acquired surface-enhanced Raman spectra of rhombohedral La 0.7 Sr 0.3 MnO 3 thin films reveal the symmetry-forbidden Jahn-Teller stretching modes demonstrating the predicted symmetry breaking at the surface and fallback into the orthorhombic (Pnma) structure. This results in a unique Jahn-Teller reconstructed surface structure, which limits the capabilities of mixed-valence manganites in spintronic applications, but will be highly favorable for catalytic reactions.

A long-standing issue in the physics of the colossal magnetoresistance is the role of electron-phonon coupling, which manifests itself as Jahn-Teller polarons. The origin and architecture of polarons makes it possible to study their behavior by Raman spectroscopy, which allows to analyze the polaronic behavior in an applied magnetic field. We performed magnetic-field-dependent Raman spectroscopy on thin films of (La0.6Pr0.4)0.7Ca0.3MnO3 in a range of H = 0–50 kOe and compared the obtained Raman spectra with the magnetic field behavior of the electrical resistivity. In the vicinity of the Curie temperature, TC = 197 K, the intensity of the Jahn-Teller stretching mode at 614 cm−1 and of the bending mode at 443 cm−1 was found to be suppressed and enhanced, respectively. This observed behavior has a remarkable similarity with the field and temperature dependence of the colossal magnetoresistance in (La0.6Pr0.4)0.7Ca0.3MnO3. Our work provides direct evidence that the reduction of the amount of Jahn-Teller polarons at the phase transition is the main mechanism underlying the colossal magnetoresistance.