3.1 Structural characterization
The cross sections of the deposited La1 − xSrxMnO3 (LSMO, where x = 0, 0.1, 0.3 and 0.5) thin films on the single crystal SrTiO3 (001) substrate were examined by TEM. A typical TEM image of the La0.7Sr0.3MnO3 thin film annealed at 900°C is shown in Fig. 2a. It can be seen that the obtained film is homogeneously deposited on the substrate with a thickness of ~ 30 nm, having a well-defined interface and uniform and crack-free surfaces. EDS elemental maps corresponding to the area presented in the TEM micrograph (Fig. 2b) indicate a uniform distribution of elements over the film cross section. Besides, some amounts of Sr can be seen in the film structure depicted by the red arrow in Fig. 2b.
XRD analyses were used to follow the structural changes of the deposited LSMO films on single crystal SrTiO3 (001) during annealing. The room temperature XRD patterns of the as-prepared LSMO thin films (heated at 750°C for 1 h) confirm preferential c-axis film orientation along with the crystallographic [001] direction of the STO substrate (as an example, the XRD pattern of the La0.7Sr0.3MnO3 film is shown in Fig. 3 - the logarithmic scale of the y-axis was used to better observe the possible presence of XRD peaks of a secondary phase). The presence of small amount of non-stoichiometric LMO phase [14,21] can be associated with low-intensity XRD peak at ~ 22.5° (indicated with an arrow in Fig. 3). The appearance of this XRD peak could be related to incomplete film structure formation and the incorporation of a higher amount of different defects, causing a slight deviation from perfect epitaxial growth. This distorted perovskite structure consists of corner-sharing MnO6 octahedra and La (Sr) cations occupying the 12-fold coordination site formed in the center of eight such octahedra [22]. In theory, La-manganite can be considered as a structure derived from the elementary cubic cell with a lattice parameter of ~ 3.944 Å [23], where La3+ and Mn3+ are slightly displaced from their ideal positions, but the precise definition of phase is usually complicated. Thus, for simplicity, the structure of La1 − xSrxMnO3 films on STO substrate is usually defined as pseudocubic.
For the obtained LSMO films it is expected that the in-plane lattice constants a and b are well strained to the STO substrate resulting in a change of lattice constant in the out-of-plane direction [24]. An increase in the out-of-plane lattice constant c characterizes the undoped LMO films, having larger lattice parameter of pseudocubic phase (~ 3.944 Å) than cubic STO substrate (3.905 Å) and a lattice mismatch of δSTO = − 1.0%. Substitution of La3+ (1.36 Å) with larger Sr2+ (1.44 Å) ions in LMO is accompanied by the conversion of manganese ions from Mn3+ to Mn4+ and contraction of the unit cell since the ionic radius of Mn4+ (0.53 Å) is much smaller than that of Mn3+ (0.65 Å) and the Jahn-Teller distortion of MnO6 octahedra due to the presence of Mn4+ ions is less pronounced [25,26]. Thus, bulk La0.7Sr0.3MnO3 has a rhombohedral perovskite structure that can be approximated as pseudocubic, with unit cell parameter a = 3.873 Å and α = 90.26° [27,28], and due to the corresponding lattice mismatch of δSTO = + 0.8% the contraction of the unit cell in the out-of-plane direction (decreasing of lattice constant c) is expected in LSMO film with a higher amount of Sr, deposited at STO (001) substrate. The calculated out-of-plane lattice constant c of the as-prepared La0.7Sr0.3MnO3 thin film (heated at 750°C) is ~ 3.88 Å and corresponds to the lattice parameter of bulk La0.7Sr0.3MnO3. However, the calculated lattice constant c of the as-prepared La0.7Sr0.3MnO3 thin film is almost the same as that for the as-prepared pure LaMnO3 thin film (heated at 750°C) [13] even though it is mentioned that Sr-addition contracts the LMO lattice. Thus, the reason might be the formation of an imperfect crystal structure with some disoriented areas, non-stoichiometric LMO phases and defects created just to support the epitaxial growth of the as-prepared LSMO films.
Growth by using vapor deposition techniques (PLD, MBE etc.), in which atoms from the gas phase are transported and deposited onto a substrate to form a thin film, is usually a high-energy process carried out under lower oxygen content in comparison to chemical solution deposition [26,29]. In the modified sol-gel method, so-called polymer assisted deposition the polymers are used to form covalent complexes with metal cations and thus inactivate them. The coordination between the polymers and the metal ions prevents the nucleation of LMO-based films before the decomposition of the polymers and, in this way, controls the film deposition process. The used polymers are decomposed into volatile species, usually at temperatures up to 600°C, and the released cations diffuse towards the substrate and incorporate into the film structure. Crystallization and the formation of epitaxial LMO-based films is complicated and relatively slow process [30,31]. Thus, the formation of an imperfect crystal structure at lower temperatures (up to 750°C) can be expected, since different defects (such as point defects, planar defects, dislocations etc.) can be incorporated during this low-energy process [31–33]. This kind of structure is schematically shown in Supplementary material (Fig. S1a), where disordered regions are marked with red rectangles. It was already mentioned [13,31,34] that epitaxial growth of LaMnO3 film on top of the STO (001) substrate could produce energetically unfavorable LMO cell compression in the ab plane, which causes the formation of a non-stoichiometric phase that allows relaxation of this stress through introducing La3+ and/or O2− vacancies and the transition of Mn3+ to Mn4+.
Rearrangement of the obtained film structures, improved crystallinity and the achievement of high-quality epitaxial (or highly oriented) films can be expected only at higher temperatures [35]. In the first step at intermediate temperatures (i.e. 800–850°C) the non-stoichiometry of LaMnO3 film can be partially reduced (Supplementary material, Fig. S1b) by eliminating potential La-vacancies according to the following equation:
Complete structural rearrangements are expected with further temperature increase (Supplementary material, Fig. S1c), which was confirmed by the movement of the characteristic XRD peak from ~ 46.8° in the as-prepared LMO film to ~ 45.8° in the LMO film annealed at 900°C [13].
In accordance with the above mentioned the as-prepared LSMO films were annealed in air at 800, 850 and 900°C. The epitaxial nature of the deposited LSMO thin films was confirmed with HRTEM analysis. Figure 4 shows HRTEM micrographs of the thin LaMnO3 and La0.7Sr0.3MnO3 films deposited by PAD on STO (001) substrates annealed at 900°C. A highly oriented structure along the direction [001] with clean and well-defined interfaces between the deposited film and substrate is noticeable.
XRD patterns (selected 2θ ranges) of the epitaxial La0.7Sr0.3MnO3 films annealed at different temperatures are shown in Fig. 5. After annealing at 800°C, the high orientation of the LSMO films is preserved, but the XRD peaks of the non-stoichiometric phase gradually disappears, the intensity of the XRD peak at ~ 46.8° (c = 3.88 Å) decreases and XRD peak at ~ 46.0° (c = 3.93 Å) appears indicating obvious structural rearrangements. Additional thermal treatment at 800°C provided more energy to rearrange the structure and improve cation stoichiometry [13]. It seems that this structure has a lower amount of defects and is close to the orthorhombic (Pbnm) LMO phase that was also proposed for some epitaxial LaMnO3 films annealed at lower temperatures [33]. In addition, it is interesting that similar changes were observed for the undoped LMO film after annealing at 800°C [13] and could indicate that the rearrangements at this stage are not influenced dominantly by Sr-doping. This is unexpected since it is well known that substitution of La3+ with Sr2+ causes the transition of Mn3+ to Mn4+, changing the Jahn-Teller distortion and decreasing lattice volume [23].
The observed increase of the lattice parameter after annealing at 800 and 850°C can be explained by the slow reduction of the non-stoichiometry through the elimination of potential La-vacancies according to Eq. 1, but also by the limited formation of Mn4+ ions (which is expected in Sr-doped LMO structures). This is due to the fact that Sr-doping in this stage favors the formation of defected structures with O2− vacancies (Eq. 2) instead of the simple conversion of Mn3+ to Mn4+ ions (Eq. 3):
Thus, at higher temperatures, the structure is rearranged slowly decreasing first the non-stoichiometricity without increasing Mn4+ content (schematically presented in Supplementary material, Figs. S2a and S2b). These changes are correlated with the movement of the characteristic XRD peak from ~ 46.8° in the as-prepared LSMO film to ~ 45.8° in the annealed LSMO film at 850°C. The expected decrease of the lattice parameter (i.e. movement of XRD peak to higher angles) was observed after annealing at 900°C (Fig. 5). The reason is the elimination of O2− vacancies and the conversion of Mn3+ to Mn4+ according to the following equation:
These changes are schematically shown in Supplementary material, Fig. S2c.
Since the observed structural changes during annealing at temperatures from 750 to 900°C should have influence on electric properties, the electrical resistivity (sheet resistance) of the prepared films was measured (Fig. 6). It is obvious that the resistivity decrease with the increase of annealing temperature what is an indication of structural rearrangement. Very steep decrease can be seen for the La0.5Sr0.5MnO3 film (Fig. 6), which also has the lower crystallinity at 750°C in comparison to other Sr-doped films. Thus, it can be concluded that the as-deposited films with higher Sr content have more disordered structure. The LaMnO3 film annealed at 900°C has the highest resistivity and, as expected, resistivity decreases with Sr addition for lower Sr-content (the La0.9Sr0.1MnO3 and La0.7Sr0.3MnO3 films) due to the hoping induced by higher amount of Mn4+ ions. Resistivities of the sample are in the range with the values for the corresponding epitaxial thin films obtained by vapor deposition techniques [36,37].
It is important to underline that with the increase in Sr-content bulk La1 − xSrxMnO3 undergoes an orthorhombic-to-rhombohedral phase transition for x > 0.17 and the appearance of the rhombohedral (R-3c) LMO phase can be expected in the bulk La0.7Sr0.3MnO3 sample [38,39]. The presence of orthorhombic and coexistence of orthorhombic and rhombohedral phases was also reported for Sr-doped LMO films [33,38].
The effect of Sr-doping is clearly visible only for the samples annealed at 850 and 900°C (Figs. 7 and 8). XRD patterns (selected 2θ ranges) of the thin La0.7Sr0.3MnO3 and La0.5Sr0.5MnO3 films annealed at 850°C (Fig. 7) reveal the increased intensity of the XRD peak at ~ 47° which could be an indication of an orthorhombic-to-rhombohedral phase transition. In addition, there are obvious differences in XRD patterns of the LSMO films with different Sr-content. The position of the characteristic XRD peak of LMO-phase is at ~ 46.1° (c = 3.93 Å) for the LaMnO3 and La0.9Sr0.1MnO3 samples and shifts to ~ 47.1° (c = 3.87 Å) for the La0.7Sr0.3MnO3 and La0.5Sr0.5MnO3 films (Fig. 8). The obtained values are very close to the pseudocubic lattice parameters of bulk LaMnO3 and La0.7Sr0.3MnO3, respectively [28,40]. However, those characteristic peaks are accompanied with a small secondary XRD peak (on the opposite side with respect to the STO substrate peak - see red arrows in Fig. 8). This second peak is clearly visible in all XRD patterns of the LSMO films annealed at 900°C, which indicates the possible coexistence of orthorhombic and rhombohedral phases. Indeed, the orthorhombic phase seems to be the dominant one in the thin LaMnO3 and La0.9Sr0.1MnO3 films, but the rhombohedral phase is more pronounced in the samples with higher Sr-content. These films are constrained in ab plane (in-plane) due to the high lattice constant of the cubic STO substrate forcing the structural accommodation in the out-of-plane direction. This led to a decrease in lattice parameter c and a decrease in LSMO lattice volume, which could explain the formation of the phases with lower symmetry.
3.2 Magnetic properties
Figure 9a shows the measured hysteresis loops of M(H) at 5, 100 and 250 K of the epitaxial La0.9Sr0.1MnO3 film deposited on STO (001) substrate, which look typical for LSMO system with such Sr concentration. Well-defined hysteresis loops showing ferromagnetic behavior are observed at the lower temperatures (5 and 100 K). Increasing the temperature up to 250 K causes a considerable decrease in magnetization and coercivity, due to approaching the transition to paramagnetic state. This is in agreement with the literature data for bulk lanthanum manganite, since bulk La0.9Sr0.1MnO3 is paramagnetic at room temperature [41]. Indeed, the temperature dependence of magnetic moment M(T) points to the magnetic ordering temperature of 265 ± 4 K. Zero field cooled (ZFC) and field cooled (FC) curves measured in three different magnetic fields are shown in Fig. 9b. For lower fields the ZFC-FC splitting is characteristic for ferromagnet, while there is no irreversibility in the full temperature interval for field of 1 kOe. Despite the coercive field of 300 Oe, the reversibility appears for field less than 1 kOe, due to the steep rise on hysteresis loops at small fields.
The epitaxial La0.7Sr0.3MnO3 thin film was investigated in more detail, since it is ferromagnetic at ambient condition that is appropriate for possible applications. The first of all, measured hysteresis loops of the epitaxial La0.7Sr0.3MnO3 film at 5, 100 and 300 K when the magnetic field was applied parallel (in-plane) and perpendicular (out-of-plane) to the film surface are shown in Fig. 10. It is obvious that the La0.7Sr0.3MnO3 thin film is ferromagnetic at room temperature. Assuming that the thicknesses of the films La0.9Sr0.1MnO3 and La0.7Sr0.3MnO3 are approximately similar as well as the surface areas, we can see that La0.7Sr0.3MnO3 has somewhat smaller saturation magnetization. But, this is opposite to the already known behavior of bulk LSMO system, where higher amount of Sr should increase the magnetization, which therefore demands for further explanation. The observation tells that La0.9Sr0.1MnO3 film has actually bigger volume than La0.7Sr0.3MnO3. Nevertheless, for the La0.7Sr0.3MnO3 the measured magnetic moment divided by volume and density the film gives the mass magnetization in saturated state of 85 ± 7 emu/g that fits perfectly within the saturation magnetization values of bulk compound produced with different synthesis methods (75–100 emu/g). The produced films have magnetic properties in agreement with the known phase diagram and the literature data [9,41,42], and from the observed magnetic transition temperatures, 265 ± 4 K and 360 ± 4 K for the La0.9Sr0.1MnO3 and La0.7Sr0.3MnO3 films, respectively, it is confirmed that Sr concentration incorporated during preparation is very near to the desired one, and La0.7Sr0.3MnO3, is tuned to the maximal magnetic transition temperature within LSMO system.
LSMO members are usually very soft ferromagnets in bulk form, so that the increased coercive field of 140 Oe shows that thin film geometry contributes to the difficulty of the magnetization reversal through formation of magnetic hard axis. Indeed, measurements of magnetization in direction parallel and perpendicular to the thin film surface, as shown in Fig. 10, suggests the presence of an easy plane magnetization and a hard axis perpendicular to the thin film. The difference of magnetization between two directions is very pronounced as can be seen in insets of each panel through small slope without open hysteresis (black curves) for out-of-plane direction in strong contrast to the in-plane steep hysteresis curves. Such large anisotropy can be explained only with considerable magnetocrystalline anisotropy that favors in-plane magnetization. It was already confirmed that anisotropic magnetization can be induced by strain [43], which is here achieved with matching the lattice of the film with the substrate lattice beneath. It should be noted that demagnetization factor is very far below the value needed for such large observed anisotropy, as well as intrinsic magnetocrystalline anisotropy of the bulk LSMO, making the induced magnetocrystalline anisotropy as the reliable source of the observed phenomena.
The field cooled (FC) and zero field cooled (ZFC) magnetization as a function of the temperature from 5 to 400 K under applied in-plane and out-of-plane magnetic fields of 0.1 and 1 kOe are shown in Fig. 11. The observed sharp rise in magnetization due to the paramagnetic to ferromagnetic transition with cooling corresponds to the Curie temperature (TC) of ~ 360 K. For the in-plane magnetic field of 1 kOe, both ZFC and FC magnetization curves coincide well with each other, but a bifurcation between them is clearly observed for the field of 0.1 kOe (typical behavior for doped lanthanum manganites [44]). However, inset of Fig. 11 shows much lower magnetization in the out-of-plane direction, only 5% of the in-plane in small fields, with the well observable ZFC-FC splitting in broad temperature range from 2 up to 300 K. This confirms again the presence of a magnetocrystalline anisotropy in the prepared epitaxial La0.7Sr0.3MnO3 thin film. With increasing field, the in-plane and out-of-plane ZFC-FC curves become closer, and they would become the same at 15 kOe as is visible on hysteresis loops, which points to considerable anisotropy field induced by strain in natively soft ferromagnet.
More detailed insight into magnetic anisotropy is obtained with measuring the angle dependence of the magnetic moment, shown in Fig. 12. The presented results for room temperature indicating, a strong relationship between the value of the magnetic moment and the measurement angle (Fig. 12a), confirms directly the preferential orientation in plane of the prepared La0.7Sr0.3MnO3 film, with a clear indication of single hard axis perpendicular to the film. Additionally, the hysteresis loops at room temperature obtained by VSM presented in Fig. 12b also show an obvious difference of the in-plane and out-plane magnetization. High density of points enables to see the open hysteresis with much smaller coercive field at room temperature, as well as coercivity in out-of-plane direction. These properties are favorable for application in spintronics. Moreover, the difference between in-plane and out-of-plane magnetizations related to anisotropy in LSMO film structure is essential for achieving high-density, high-stability and low-power-consumption spintronic devices [45].