Since the discovery of colossal magnetoresistivity (CMR) effect in perovskite manganites with general formula A_1-xB_xMnO₃ (where A is a lanthanum (La) and B an alkaline-earth (Ba, Ca, Sr) element), the magnetic and transport properties of these materials and their relations to microstructures have attracted much interest. The CMR effect occurs in the 0.2< x <0.5 doping range. Among these materials La_1-xSr_xMnO₃ (La_0.7Sr_0.3MnO₃, La_0.66Sr_0.33MnO₃) group has been intensively studied because of their high Curie temperature (T_C) (higher than room temperature), making these materials suitable for uncooled bolometric detectors, sensors, etc. At this temperature the materials undergo a ferromagnetic-paramagnetic transition and simultaneously, in most cases, a metal-insulator transition- when they reach the resistivity  maximum at a temperature T_MI.  For possible industrial applications, these materials have to be prepared in thin film form. Various single crystalline substrates, such as dielectric (SrTiO₃, LaAlO₃, NdGaO₃, YSZ, MgO) or semiconducting (Si, GaAs) are used for the La_0.66Sr_0.33MnO₃ (LSMO) films. Beside preferred substrates with a perfect matching to the LSMO significant results on MgO substrates or MgO buffer layers have also been obtained with a T_C=365 K [1], T_MI = 416 K [2] and a  T_MI = 382 K [3], respectively.

LSMO has a perovskite structures with a rhombohedral distortion. In the pseudocubic description the lattice parameter and the unit cell angle are 0.3873 nm and 90.26°, respectively. The MgO substrate has the rock-salt cubic structure with a lattice parameter of 0.421 nm. The lattice mismatch between the LSMO film and the MgO substrate is 8 %.

The LSMO films were deposited using two different deposition techniques, on axis dc magnetron sputtering or pulsed laser deposition (PLD) onto a one-side polished MgO (001) substrate. The thickness of the films was about 50 nm, the growth rate of the sputtered and the laser ablated LSMO were 1.2 nm/min and 6.5 nm/min, respectively.

Both types of LSMO films exhibited a significant increased temperature of metal-insulator transition, measured using a standard four-point method, T_MI of about 400-458 K.

X-ray diffraction (XRD) analyses were carried out using Bruker D8 DISCOVER diffractometer equipped with X-ray tube with rotating Cu anode operating at 12 kW. We revealed different shape of the rocking curves measured on the (004) diffractions of the LSMO  - in case of the sputtered LSMO film two distinct maxima of the rocking curve, while in case of the laser ablated LSMO film simple shape of the rocking curve with one maximum were observed. To explain the origin of the rocking curve splitting reciprocal space maps were recorded around the (004) and (204) Bragg reflections. The LSMO films prepared by sputtering exhibited splitted Bragg reflections in the direction parallel to the sample surface, indicating a distorted orthorombic LSMO unit cell. On the other hand, no splitting of the Bragg reflections was observed in case of laser ablated films. These films possess a pseudocubic structure.

The microstructure of the films was investigated by the use of transmission electron microscopy (TEM) Jeol 1200EX. Cross-section and planar view TEM samples were prepared in the conventional way with a final ion milling process. TEM analyses revealed an epitaxial growth of both types of the LSMO films. From the selected area diffraction pattern we determined a cube on cube relationship of the LSMO film to the MgO substrate. Cross-sectional TEM micrograph demonstrates a columnar epitaxial structure of the films with Moiré fringes clearly visible in the blocks.

XRD and TEM analyses have revealed that the LSMO films grown on the MgO are completely relaxed with respect to the substrate. The LSMO lattice parameters calculated from the reciprocal space maps are different from the LSMO bulk value, indicating a presence of some strains in the LSMO films. However, these strains are created during the film growth and are brought into by the adjusting of the deposition conditions of the films. In this paper we discuss the influence of the deposition parameters on the increased temperature of metal-insulator transition.  

[1]        S.Y.Yang et al., J. of Magnetism and Magnetic Materials 226-230 (2001) 690.

[2]        Š. Chromik et al., Applied Surface Science 269 (2013) 98.

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