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Register a new userConductivity and magnetoresistance of La$_{0.7}$Ce$_{0.3}$MnO$_3$ thin films under photoexcitation
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La$_{0.7}$Ce$_{0.3}$MnO$_3$ thin films of different thicknesses, degrees of CeO$_2$-phase segregation and oxygen deficiency, grown on SrTiO$_3$ single crystal substrates, were comparatively investigated with respect to both their spectral and temperature-dependent photoconductivity (PC) and their magnetoresistance (MR) behaviour under photoexcitation. While as-grown films were insensitive to optical excitation, oxygen reduction appeared to be an effective way to decrease the film resistance, but the film thickness was found to play a minor role. However, from the evaluation of the spectral behaviour of the PC and the comparison of the MR of the LCeMO/substrate-samples with a bare substrate under illumination we find that the photoconductivity data reflects not only contributions from (i) photogenerated charge carriers in the film and (ii) carriers injected from the photoconductive substrate (as concluded from earlier works), but also (iii) a decisive parallel photoconduction in the SrTiO$_3$ substrate. Furthermore -- also by analyzing the MR characteristics -- the unexpected occurence of a strong electroresistive effect in the sample with the highest degree of CeO$_2$ segregation and oxygen deficiency could be attributed to the electroresistance of the SrTiO$_3$ substrate as well. The results suggest a critical reconsideration and possibly a reinterpretation of several previous photoconductivity and electroresistance investigations of manganite thin films on SrTiO$_3$.
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Cerium-doped manganite thin films were grown epitaxially by pulsed laser deposition at $720 ^circ$C and oxygen pressure $p_{O_2}=1-25 $Pa and were subjected to different annealing steps. According to x-ray diffraction (XRD) data, the formation of CeO$_2$ as a secondary phase could be avoided for $p_{O_2}ge 8 $Pa. However, transmission electron microscopy shows the presence of CeO$_2$ nanoclusters, even in those films which appear to be single phase in XRD. With O$_2$ annealing, the metal-to-insulator transition temperature increases, while the saturation magnetization decreases and stays well below the theoretical value for electron-doped La$_{0.7}$Ce$_{0.3}$MnO$_3$ 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-to-insulator 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.
Films of cerium-doped LaMnO$_3$, which has been intensively discussed as an electron-doped counterpart to hole-doped mixed-valence lanthanum manganites during the past decade, were analyzed by x-ray photoemission spectroscopy with respect to their manganese valence under photoexcitation. The comparative analysis of the Mn 3s exchange splitting of La$_{0.7}$Ce$_{0.3}$MnO$_3$ (LCeMO) films in the dark and under illumination clearly shows that both oxygen reduction and illumination are able to decrease the Mn valence towards a mixed 2$+$/3$+$ state, independently of the film thickness and the degree of CeO$_2$ segregation. Charge injection from the photoconductive SrTiO$_3$ substrate into the Mn e$_g$ band with carrier lifetimes in the range of tens of seconds and intrinsic generation of electron-hole pairs within the films are discussed as two possible sources of the Mn valence shift and the subsequent electron doping.
The magnetic dead layers in films a few nanometers thick are investigated for La$_{0.7}$Sr$_{0.3}$MnO$_3$ on (001)-oriented SrTiO$_3$ (STO), LaAlO$_3$ (LAO) and (LaAlO$_3$)$_{0.3}$(Sr$_2$TaAlO$_6$)$_{0.7}$ (LSAT) substrates. An anomalous moment found to persist above the Curie temperature of the La$_{0.7}$Sr$_{0.3}$MnO$_3$ films is not attributed to the films, but to oxygen vacancies at or near the surface of the substrate. The contribution to the moment from the substrate is as high as 20 $mu$B/nm$^2$ in the case of STO or LSAT. The effect is increased by adding an STO cap layer. Taking this d-zero magnetism into account, extrapolated magnetic dead layer thicknesses of 0.8 nm, 1.5 nm and 3.0 nm are found for the manganite films grown on LSAT, STO and LAO substrates, respectively. An STO cap layer eliminates the LSMO dead layer.
Charge transfer induced interfacial ferromagnetism and its impact on the exchange bias effect in La$_{0.7}$Sr$_{0.3}$MnO$_3$/NdNiO$_3$ correlated oxide heterostructures were investigated by soft x-ray absorption and x-ray magnetic circular dichroism spectra in a temperature range from 10 to 300 K. Besides the antiferromagnetic Ni$_3^+$ cations which are naturally part of the NdNiO$_3$ layer, Ni$_2^+$ ions are formed at the interface due to a charge transfer mechanism involving the Mn element of the adjacent layer. They exhibit a ferromagnetic behavior due to the exchange coupling to the Mn$_4^+$ ions in the La$_{0.7}$Sr$_{0.3}$MnO$_3$ layer. This can be seen as detrimental to the strength of the unidirectional anisotropy since a significant part of the interface does not contribute to the pinning of the ferromagnetic layer. By analyzing the line shape changes of the x-ray absorption at the Ni L$_{2,3}$ edges, the metal-insulator transition of the NdNiO$_3$ layer is resolved in an element specific manner. This phase transition is initiated at about 120 K, way above the paramagnetic to antiferromagnetic transition of NdNiO$_3$ layer which measured to be 50 K. Exchange bias and enhanced coercive fields were observed after field cooling the sample through the Neel temperature of the NdNiO$_3$ layer. Different from La$_{0.7}$Sr$_{0.3}$MnO$_3$/LaNiO$_3$, the exchange bias observed in La$_{0.7}$Sr$_{0.3}$MnO$_3$/NdNiO$_3$ is due to the antiferromagnetism of NdNiO$_3$ and the frustration at the interface. These results suggest that reducing the interfacial orbital hybridization may be used as a tunable parameter for the strength of the exchange bias effect in all-oxide heterostructures which exhibit a charge transfer mechanism.
Presently, cerium-doped LaMnO$_3$ is vividly discussed as an electron-doped counterpart prototype to the well-established hole-doped mixed-valence manganites. Here, La$_{0.7}$Ce$_{0.3}$MnO$_3$ thin films of different thicknesses, degrees of CeO$_2$ phase segregation, and oxygen deficiency, grown on SrTiO$_3$ single crystal substrates, are compared with respect to their resistance-vs.-temperature (R vs. T) behavior from 300~K down to 90~K. While the variation of the film thickness (and thus the degree of epitaxial strain) in the range between 10~nm and 100~nm has only a weak impact on the electrical transport, the degree of oxygen deficiency as well as the existence of CeO$_2$ clusters can completely change the type of hopping mechanism. This is shown by fitting the respective textit{R-T} curves with three different transport models (adiabatic polaron hopping, Mott variable-range hopping, Efros-Shklovskii variable-range hopping), which are commonly used for the mixed-valence manganites. Several characteristic transport parameters, such as the hopping energies, the carrier localization lengths, as well as the Mn valences are derived from the fitting procedures.
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