The Effects of Substrate-Induced Strains on the Charge-Ordering Transition in Nd 0.5 Sr 0.5 MnO 3 Thin Films
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structure with a=3.79 A. The laser energy density on the target was about 1.5 J/cm 2, and the deposition rate was 10 Hz. The LaAIO 3 substrate was kept at a constant temperature of 820 °C during the deposition which was carried out at a pressure of 400 mTorr of flowing oxygen. After deposition, the samples were slowly cooled to room temperature at a pressure of 400 Torr of oxygen. Further details of the target preparation and the deposition procedure are given elsewhere [5]. The structural study was done, at room temperature, by X-ray diffraction (XRD) using a Rigaku diffractometer. Low temperature XRD experiments were performed with a Siemens Kristalloflex X-ray diffractometer. DC resistivity was measured by a four-probe method and magnetization was measured using a Quantum Design MPMS SQUID magnetometer. The composition analysis, checked by Rutherford Backscattering Spectroscopy (RBS), indicate a stochiometric composition within error limits. RESULTS AND DISCUSSION Fig. la shows the room temperature 0-20 scan of the films for different thicknesses (in the region 45°-50'). A peak is observed at 46.5' for all thicknesses which corresponds to an outof-plane parameter of 1.955 A. Another diffraction peak appears gradually for thicknesses above 1000 A. This latter peak corresponds to a lattice parameter of 1.92 A for a 2000 A film. 2,xx0'l0
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700as-grown 'C,10h
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485000 200
eq 4
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Fig. 1a : Room temperature XRD for asgrown films. Peaks labeled S and * are due to the substrate and the sample holder.
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Fig. l b Room temperature XRD for annealed films. Peaks labeled S and * are due to the substrate and the sample holder.
These two peaks correspond to two phases which we will call phases A and B (where A indicates the phase with the larger lattice parameter). We also plot the effects of a post-annealing under 02 for a 1500 A film in Fig.lb made at different temperatures during 10 hours. The structure of bulk Ndo. 5Sr 0.5 MnO 3 is orthorhombic (Pnma) with a=5.43153 A, b=7.63347 'A and c=5.47596 A [9]. The out-of-plane parameter of the B-phase is equal to the d20 2 of bulk of Ndo. 5Sro.5MnO 3. Since this phase, which corresponds to the bulk value, appears for larger thicknesses, we assume that it is the relaxed part of the film with less influence from the substrate. The A-phase is always present, even in the thinnest film, which strongly suggests that it represents the strained phase of the initial layers of the film. The B-phase appears as the thickness is increased, which implies that the film is relaxed after a critical thickness of about 500 'A (see inset of Fig. I a). Under the oxygen annealing, the peak corresponding to the B-phase is increasing indicate that the film is relaxing along the out-of-plane direction. The effect has already be seen in previous studies of manganite thin films [15]. Moreover, the film is [101]-oriented, i.e. with
232
the [101] axis perpendicular to the substrate
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