Transport, magnetic, and structural properties ofLa0.7Ce0.3MnO3thin films: Evidence for hole-doping

Abstract

Cerium-doped manganite thin films were grown epitaxially by pulsed laser deposition at 720 • C and oxygen pressure pO 2 = 1 − 25 Pa and were subjected to different annealing steps. According to x-ray diffraction (XRD) data, the formation of CeO2 as a secondary phase could be avoided for pO 2 ≥ 8 Pa. However, transmission electron microscopy shows the presence of CeO2 nanoclusters, even in those films which appear to be single phase in XRD. With O2 annealing, the metal-toinsulator transition temperature increases, while the saturation magnetization decreases and stays well below the theoretical value for electron-doped La0.7Ce0.3MnO3 with mixed Mn 3+ /Mn 2+ valences. The same trend is observed with decreasing film thickness from 100 to 20 nm, indicating a higher oxygen content for thinner films. Hall measurements on a film which shows a metal-toinsulator transition clearly reveal holes as dominating charge carriers. Combining data from x-ray photoemission spectroscopy, for determination of the oxygen content, and x-ray absorption spectroscopy (XAS), for determination of the hole concentration and cation valences, we find that with increasing oxygen content the hole concentration increases and Mn valences are shifted from 2+ to 4+. The dominating Mn valences in the films are Mn 3+ and Mn 4+ , and only a small amount of Mn 2+ ions can be observed by XAS. Mn 2+ and Ce 4+ XAS signals obtained in surface-sensitive total electron yield mode are strongly reduced in the bulk-sensitive fluorescence mode, which indicates hole-doping in the bulk for those films which do show a metal-to-insulator transition.

Quantifying the electrical transport characteristics of electron-doped La0.7Ce0.3MnO3 thin films through hopping energies, Mn valence, and carrier localization length

“…Concerning the discrepancy of the XPS-and the transport related Mn 3+ concentrations of sample C one could argue that there is a gradient of the Mn 3+ concentration from the film surface towards deeper regions of the film and that XPS as a surface-sensitive method naturally detects only the value correct for the first few nanometers beneath the surface. In other words, the films are stronger oxygen-deficient at the surface, which is indeed in agreement with the former x-ray absorption (XAS) results of comparable LCeMO films [16]. Furthermore, equation (12) is only valid when we assume the equivalence of Mn 2+ and Mn 4+ with respect to the double exchange mechanism, as proposed in the theoretical work by Schlottmann [12], which might also be questionable due to the high ionic radius of Mn 2+ and the subsequent dramatic decrease of the Mn-O-Mn bond angle.…”

“…Obviously, the low oxygen partial pressure during the deposition of film D is the origin for the deviating R-T characteristics. As proven previously on an equivalent film [16], the MIT can be recovered by appropriate annealing under oxygen atmosphere.…”

“…between the Mn 3+ concentration c(Mn 3+ ) and the activation energy E h for the APH process, we obtain a Mn 3+ concentration of 78% for samples A, B, C. Since the as-prepared films had been shown to suffer from overoxygenation and concomitant hole doping [14,16], it is reasonable to assume that the residual 22% of Mn ions have a valence of 4+. The corresponding average Mn valence is 3.22, being dramatically larger than the nominal Mn valence of 2.7, but being in good accordance with former results obtained on 10-nm thick as-prepared LCeMO films [14].…”

“…The exact growth conditions and the results of the structural analysis can be found in reference [16], the most important parameters (sample label / film thickness / oxygen partial pressure during PLD / result of the x-ray diffraction analysis) are listed here:…”

“…Since lattice movements are slow compared to the hopping velocities, the the term adiabatic is used for this process. ‡ Though appearing single-phasic in the XRD analysis, a transmission electron microscopy study showed the existence of nanoscopic CeO 2 clusters for samples equivalent to B and C [16].…”