Physical Review B - CMMP

Influence of defects on structural and magnetic properties of multifunctional La2 NiMn O6 thin films

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Thin films of the double perovskite La2 NiMn O6 (LNMO) have been grown on various lattice-matched substrates (SrTi O3, LaAl O3, NdGa O3, and MgO) by pulsed laser deposition under different oxygen background pressure (25-800 mTorr) conditions. The out-of-plane lattice constant of the LNMO film decreases with increasing pressure, which is likely caused by reduction in the defect concentration and improved structural ordering, before leveling off at higher oxygen concentrations. The scanning transmission electron microscopy results confirm that the films are epitaxial, and the interface is sharp and coherent. While few defects are observed in a film grown at a high oxygen pressure (800 mTorr), a film grown at a lower pressure (100 mTorr) clearly shows the formation of defects that extend throughout the thickness except for a very thin layer near the interface. The Raman spectra of the films are dominated by two broad peaks at around 540 and 685 cm-1, which are assigned to the antisymmetric stretching and symmetric stretching modes of Mn O6 and Ni O6 octahedra, respectively. The Raman peaks of the LNMO thin films grown in the 800 mTorr background O2 are blueshifted in comparison to those of bulk LNMO, and the shift increases with decreasing film thickness, indicating the increased influence of strain. The critical thickness for strain relaxation, as determined from the Raman spectra, is between 40 and 80 nm. However, the strain is observed to have negligible influence on the magnetic properties of films grown at high oxygen pressures. In contrast, films grown at low pressures exhibit degraded magnetic properties, which can be attributed to a combination of increased B -site cation disorder and the concentration of Mn3+ and Ni3+ Jahn-Teller ions caused by oxygen or cation related defects. With increasing oxygen pressure during growth, the paramagnetic-ferromagnetic transition temperature (∼280 K) becomes sharper and the saturation magnetization at low temperatures is enhanced. Based on the electron energy loss spectroscopy studies, the Mn and Ni ions in LNMO thin films are determined to be mixed-valent Mn3+ Mn4+, and charge transition disproportionation of the Mn4+ + Ni2+ → Mn3+ + Ni3+ type likely occurs with increasing oxygen deficiency. © 2008 The American Physical Society.