Substrate-Induced Strain Effect on Structural and Magnetic Properties of La0.5Sr0.5Coo3 Films

nanomaterials

Article Substrate-Induced Strain Effect on Structural and Magnetic Properties of La0.5Sr0.5CoO3 Films

Miriam Sánchez-Pérez 1, Juan Pedro Andrés 1,2 , Juan Antonio González 1,2 , Ricardo López Antón 1,2 , Marco Antonio López de la Torre 1,3 and Oscar Juan Dura 1,3,*

1 Department of Applied Physics, University of Castilla-La Mancha, 13071 Ciudad Real, Spain; [email protected] (M.S.-P.); [email protected] (J.P.A.); [email protected] (J.A.G.); [email protected] (R.L.A.); [email protected] (M.A.L.d.l.T.) 2 Instituto Regional de Investigación Cientíﬁca Aplicada (IRICA), University of Castilla-La Mancha, 13071 Ciudad Real, Spain 3 Instituto de Investigaciones Energéticas y Aplicaciones Industriales (INEI), University of Castilla-La Mancha, 13071 Ciudad Real, Spain * Correspondence: [email protected]

Abstract: We present a detailed study about the substrate-induced strain and thickness effects on the

structure and magnetic properties of La0.5Sr0.5CoO3 ﬁlms. The in-plane tensile or compressive strain imposed by four different substrates conﬁgures an in-plane or out-of-plane easy axis, respectively. The presence of a soft magnetic phase at the interface is also conditioned by the type of strain. The obtained results are discussed in terms of the different anisotropies that participate and control the

ﬁnal magnetic behavior. The relevance of these results lies in the feasibility of La0.5Sr0.5CoO3 in memory applications and spintronic devices.

Citation: Sánchez-Pérez, M.; Andrés, Keywords: thin films; ferromagnetism; oxides J.P.; González, J.A.; López Antón, R.; López de la Torre, M.A.; Juan Dura, O. Substrate-Induced Strain Effect on Structural and Magnetic Properties of 1. Introduction La0.5Sr0.5CoO3 Films. Nanomaterials 2021, 11, 781. http://doi.org/ Perovskite complex oxides have received great attention in the last decades because 10.3390/nano11030781 of their potential implementation in diverse technological applications as ferromagnetic electrodes, gas separation systems, electrochemical reactors, multiferroic and thermoelec- Academic Editor: Anna Vilà tric applications, or spintronic devices [1,2]. Among them, strontium doped lanthanum cobaltite La1−xSrxCoO3 (LSCO) is one of the most studied complex oxides due to its in- Received: 18 February 2021 triguing fundamental behavior. The first works about the fundamental properties of this Accepted: 16 March 2021 system were performed in the 1950s by G. Jonker and J. Van Santen [3], and J. Goode- Published: 18 March 2021 nough [4]. Without strontium doping, the lanthanum cobaltite LaCoO3 is an insulating nonmagnetic compound with a rhombohedral structure [5]. In this case, a magnetic tran- Publisher’s Note: MDPI stays neutral sition was observed around 50–90 K, in addition to a metal–insulator transition around with regard to jurisdictional claims in 500–600 K [6,7]. These transitions can be understood in terms of spin transitions, whose na- published maps and institutional affil- ture is still debated [7–13]. These spin transitions can be also reached by strontium-induced iations. hole doping, stabilizing the spin states of the system. The substitution of La3+ by Sr2+ induces the oxidation of the CoO3 matrix, giving rise to a semiconducting behavior of the La1−xSrxCoO3 compound [14–16]. Its phase diagram is still being studied [5,14,17–20], although the current consensus indicates that a doping of x = 0.18 is the critical point for Copyright: © 2021 by the authors. a metal–insulator transition [17]. Under light doping conditions (x < 0.18), magnetoelec- Licensee MDPI, Basel, Switzerland. tronic phase separation (MEPS) has been proposed to unveil its complex phase diagram. This article is an open access article MEPS is defined as the spatial coexistence of multiple electronic and magnetic phases distributed under the terms and without chemical segregation [16,17,21–24]. Hence, there is a coexistence of ferromagnetic conditions of the Creative Commons clusters embedded on a nonmagnetic matrix, following the cluster-glass ferromagnetic Attribution (CC BY) license (https:// model. When the hole doping increases, the cluster density is increased, leading to a creativecommons.org/licenses/by/ percolation transition to a metallic and long-range ferromagnetic order state above the 4.0/).

Nanomaterials 2021, 11, 781. https://doi.org/10.3390/nano11030781 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 781 2 of 13

critical point (x = 0.18). For high strontium doping (x > 0.18), the Curie temperature increases, reaching a maximum of around 240 K at x = 0.5, which corresponds to LSCO with a pseudocubic structure (a = 3.84 Å) [17,20,21]. This maximum in Tc can be interpreted in terms of the 1:1 ratio of Co3+:Co4+, maximizing the double-exchange interaction [17]. A clear understanding of these changes in the electric and magnetic properties as a function of Sr doping is still under consideration [22,25,26]. While several authors indicated that double-exchange interaction (Co3+-Co4+) could be the responsible [18,27], other works present the competition of superexchange (Co3+-Co3+, Co4+-Co4+) and double-exchange interactions as the reason of these phenomena [28,29]. The study of this compound in thin film form has also received remarkable attention. Firstly, the confinement of the magnetoelectronic phase separation, or its emergence in thin film samples, can lead to changes in the properties of the compound, e.g., increasing strongly the coercivity with potential applications in permanent magnets [16,26]. It is also of interest to study the influence of the film thickness on the magnetic properties of ultrathin films, with relevant applications in data storage devices [30]. The role of the substrate- induced strain and its relaxation in the perpendicular direction to the sample when the film thickness increases is to provide tunability to the properties of the compound. Hence, strain engineering can be a potential tool to modify the state and properties of a compound as, for example, the magnetic properties. An excellent example of the power of the strain control can be the insulating nonmagnetic LaCoO3 compound, which can present strain- stabilized ferromagnetic behavior in thin film form [31,32]. Another example showing the importance of strain control is the possibility to modify or tune the magnetic anisotropies in the sample. Special attention should be given to the magnetocrystalline and magnetoelastic anisotropies induced by the substrate strain. Different effects of substrate strain have been reported recently, such as the change of the easy axis as a result of the strain [33,34] or the strong dependence of the coercivity on the magnetocrystalline anisotropy controlled via strain [35–37]. In this work, we present an analysis of the substrate-induced strain influence on the structural and magnetic properties of the perovskite complex oxide LSCO films. We focus on x = 0.5 LSCO, which displays a cubic structure (lattice parameter, a = 3.84 Å) and ferromagnetic behavior. For this, we grow LSCO films on four different substrates that impose both in-plane tensile and compressive strains by means of its different lattice in-plane parameters, varying from 3.75 Å to 3.90 Å. The effect of the relaxation in the direction perpendicular to the sample is concomitantly considered by means of the growth of LSCO films with different thicknesses ranging between 7 and 40 nm. X-ray reflectivity and diffraction as well as magnetometry measurements were performed to characterize the structural and magnetic properties of the films. The measurements show a dependence of the magnetic easy axis with the type of strain imposed and point to the competition between magnetoelastic and magnetocrystalline anisotropies as the main source. The ability to stabilize the easy axis and understanding its mechanism results in great interest, both fundamental and applied. These results could pave the way for advanced applications, for example, in magnetic and magneto-optical storage media [38,39]; fuel cells batteries, electrochemical, and water splitting processes [40]; or in the hot pursuit of spintronics devices [41].

2. Experimental Section

LSCO ﬁlms were deposited by reactive magnetron sputtering from La0.5Sr0.5CoO3 target compound. The LSCO target was prepared by solid-state reaction from SrCO3, Co3O4, and La2O3 starting powders. These powders were mixed and cold pressed in a 2-inch di- ameter disk and sintered at 1100 ◦C and 1200 ◦C for 48 h and 24 h, respectively [17,21,22,42]. X-ray diffraction, energy-dispersive X-ray spectroscopy, and magnetometry measurements were performed to check the stoichiometry and ferromagnetic behavior of the target material. Commercial single crystal substrates were obtained from Mateck GmbH. Four different substrates were used while attending to the lattice parameter to impose tensile or com- Nanomaterials 2021, 11, 781 3 of 13

pressive strain. Therefore, considering that the bulk (i.e., relaxed) lattice parameter of the LSCO is 3.84 Å, we selected SrTiO3 (STO; a = 3.90 Å) and Sr0.3La0.7Al0.65Ta0.35O3 (LSAT; a = 3.87 Å) for applying the tensile in-plane strain and LaAlO3 (LAO; a = 3.82 Å) and Sr0.3La0.7AlO4 (SLAO; a = 3.75 Å) for applying the in-plane compressive strain. LSCO films were simultaneously grown on these four substrates with fixed sputtering conditions in order for us to compare the substrate-induced strain effect more accurately. In addition to this, LSCO films with different thicknesses were obtained by varying the deposition time in order to explore the effect of the film relaxation in the growth direction. Hence, four representative thicknesses, i.e., 7 nm, 14 nm, 28 nm, and 40 nm, were obtained. The growth of all LSCO films was on-axis, with the four substrates placed on a heater at a source-to-substrate distance of 11 cm approximately. Prior to deposition, the substrates were heated in a working atmosphere at 800 ◦C for 10 min. The working atmosphere was a combination of oxygen and argon gases with a total pressure of 9·10−2 mbar. The ratio of partial pressures ◦ O2/Ar and the deposition temperature were fixed to 0.4 and 800 C, respectively, after several studies to simultaneously obtain the optimal structural and magnetic properties on the four analyzed substrates. The deposition rate was 2 Å/min approximately. During postdeposition, the LSCO films were cooled in the working atmosphere down to 500 ◦C. At this point, to avoid the formation of oxygen vacancies, the samples were annealed in 700 mbar of oxygen atmosphere at 500 ◦C for 10 min, and finally cooled in oxygen atmosphere down to room temperature. The final grow conditions were selected after many rounds of trial-and-error while following scanning electron microscopy (SEM), electrical resistivity, and magnetometry properties. The final reproducibility obtained was very high. X-ray reflectivity (XRR) and diffraction (XRD) measurements were performed with a Cu Kα Philips X’Pert MRD diffractometer by scanning 2θ from 0.4◦ to 9◦ and from 20◦ to 60◦ respectively (figures only show data in the range of interest for a better comparison). The analysis of the spacing between Kiessig fringes in the XRR scans allowed us to estimate the thickness of the films. Additionally, the crystal domain size in the out-of-plane direction was calculated from the XRD peaks by using the Scherrer relation, once the instrumental broadening was discounted. Magnetometry measurements were performed on a commercial Evercool MPMS-XL Superconducting Quantum Interference Device system (Quantum Design). The magnetization versus temperature (MT) curves were obtained under an applied magnetic field of 100 Oe between 5 and 300 K. All MT curves were measured with the magnetic field direction parallel to the film surface (in-plane). The hysteresis loops (MH) were measured at 5 K under applied magnetic fields between 50 and −50 kOe (again, figures only show data in the range of interest for a better comparison), after field-cooling from room temperature under a magnetic field of 10 kOe. The MH curves were measured in both in-plane and out-of-plane dispositions (i.e., magnetic field direction parallel and perpendicular to the film surface, respectively). Clean substrates were fully characterized to allow for a better correction of its diamagnetic signal, which was considered in the magnetic measurement of the LSCO films.

3. Results and Discussion 3.1. Structural Characterization The X-ray reflectivity (XRR) measurements shown in Figure1 were performed on LSCO films grown on STO and SLAO substrates, with an approximate thickness of 14 nm. They display well-defined oscillations with slight differences between the analyzed substrates due to the chemical contrast. These measurements allow us to calculate the thickness corresponding to each sample [43–45], with an error within the range of 0.1–0.4 nm. The thicknesses calculated for films grown simultaneously on different substrates, i.e., samples with the same deposition time, are very similar, as we expected. Hereafter, the thicknesses will be used to refer to each sample. Nanomaterials 2021, 11, x FOR PEER REVIEW 4 of 13

Nanomaterials 2021, 11, x FOR PEER REVIEW 4 of 13

samples with the same deposition time, are very similar, as we expected. Hereafter, the samplesthicknesses with the will same be depositionused to refer time, to each are sample.very similar, as we expected. Hereafter, the Nanomaterials 2021, 11, 781 thicknesses will be used to refer to each sample. 4 of 13

Figure 1. X-ray reflectivity measurements of La1−xSrxCoO3 (LSCO) films grown on SrTiO3 (STO) (a) and Sr0.3La0.7AlO4 (SLAO) (b) substrates for one representative thickness (about 14 nm). Figure Figure1. X-ray 1. reflectivityX-ray reﬂectivity measurements measurements of La1− ofxSr LaxCoO1−xSr3 (LSCO)xCoO3 (LSCO)films grown ﬁlms on grown SrTiO on3 (STO) SrTiO 3(a(STO)) (a) and Sr0.3andLa0.7 SrAlO0.3La4 (SLAO)0.7AlO4 (SLAO)(b) substrates (b) substrates for one representa for one representativetive thickness thickness (about 14 (about nm). 14 nm). X-ray diffraction measurements were performed in θ–2θ geometry on all the sam- X-ray diffraction measurements were performed in θ–2θ geometry on all the samples. ples.X-ray Figure diffraction 2 shows measurements the region of were interest performed of these scansin θ–2 onθ geometryall the samples on all studied. the sam- These Figure2 shows the region of interest of these scans on all the samples studied. These ples. scansFigure are 2 shows sensitive the toregion the crystallographic of interest of these stru scanscture on in all the the direction samples normal studied. to These the plane scans are sensitive to the crystallographic structure in the direction normal to the plane (z). scans(z). are Thus, sensitive the LSCOto the filmscrystallographic grown on the stru in-pcturelane in tensile the direction substrates normal (STO to and the LSATplane sub- Thus, the LSCO films grown on the in-plane tensile substrates (STO and LSAT substrates) (z). Thus,strates) the display LSCO filmsa lattice grown parameter on the in-psmallerlane tensilethan the substrates bulk value, (STO whereas and LSAT LSCO sub- films display a lattice parameter smaller than the bulk value, whereas LSCO films grown on in- strates)grown display on in-plane a lattice compressive parameter substratessmaller than (LAO the and bulk SLAO value, substrates) whereas display LSCO largerfilms lat- plane compressive substrates (LAO and SLAO substrates) display larger lattice parameter, growntice on parameter, in-plane compressive as expected. substrates (LAO and SLAO substrates) display larger lat- as expected. tice parameter, as expected.

Figure 2. a Figure 2. X-rayX-ray diffraction diffraction scans scans for LSCO for LSCOfilms of films various of variousthicknesses thicknesses grown on grown STO (a ),on STO ( ), SrSr0.30.3LaLa0.7Al0.70.65AlTa0.650.35TaO0.353 (LSAT)O3 (LSAT) (b), (LaAlOb), LaAlO3 (LAO)3 (LAO) (c) and (c) and SLAO SLAO (d) (substrates.d) substrates. Dashed Dashed lines lines indicate indicate Figure 2. X-ray diffraction scans for LSCO films of various thicknesses grown on STO (a), thethe position position of of the the (200) (200) peak peak corresponding corresponding to to LSCO LSCO bulk. bulk. Sr0.3La0.7Al0.65Ta0.35O3 (LSAT) (b), LaAlO3 (LAO) (c) and SLAO (d) substrates. Dashed lines indicate the position Forof the LSCO (200)films peak withcorresponding thicknesses to LSCO below bulk. 30 nm, the scans performed for the four differ- For LSCO films with thicknesses below 30 nm, the scans performed for the four dif- ent substrates show only (h00) reflections, which indicates that samples grow along the ferent substrates show only (h00) reflections, which indicates that samples grow along the Forz direction, LSCO films keeping with thicknesses this family below of planes 30 nm, (h00). the In scans reference performed to the for thickest the four LSCO dif- films, z direction, keeping this family of planes (h00). In reference to the thickest LSCO films, ferent thesubstrates same family show only of planes (h00) isreflections, observed which for samples indicates grown that samples on STO grow and LAO along substrates. the the same family of planes is observed for samples grown on STO and LAO substrates. z direction,However, keeping in the this case family of LSAT of planes and SLAO(h00). substrates,In reference different to the thickest peaks (notLSCO shown) films, corre- However, in the case of LSAT and SLAO substrates, different peaks (not shown) corre- the samesponding familyto of the planes family is (110)observed were fo alsor samples observed. grown Additionally, on STO and for LAO LSCO substrates. films grown on sponding to the family (110) were also observed. Additionally, for LSCO films grown on However,SLAO in substrate,the case of a relaxationLSAT and ofSLAO the film substrates, in the z-direction different peaks is evidenced (not shown) by a splitting corre- and spondingdisplacement to the family of the (110) peak were as thealso thickness observed. increases, Additionally, approaching for LSCO the films (200) peakgrown position on of LSCO bulk. Although this is also hinted in the sample deposited on the LAO substrate, this behavior is clearly observed only on films grown on SLAO substrate and is most probably due to the largest strain value imposed by the SLAO (2.3%). However, it is noteworthy that for the thinnest film grown on SLAO, the film appears strained. On the other hand, a Nanomaterials 2021, 11, x FOR PEER REVIEW 5 of 13

SLAO substrate, a relaxation of the film in the z-direction is evidenced by a splitting and displacement of the peak as the thickness increases, approaching the (200) peak position of LSCO bulk. Although this is also hinted in the sample deposited on the LAO substrate, this behavior is clearly observed only on films grown on SLAO substrate and is most Nanomaterials 2021, 11, 781 probably due to the largest strain value imposed by the SLAO (2.3%). However, it is note-5 of 13 worthy that for the thinnest film grown on SLAO, the film appears strained. On the other hand, a peak corresponding to La2O3 oxide was observed only for the thickest film grown onpeak the correspondingLSAT substrate. to Nevertheless, La2O3 oxide as was it is observed further discussed, only for the no thickest effect of film this grown compound on the LSAT substrate. Nevertheless, as it is further discussed, no effect of this compound on the on the physical properties of the film was found. physical properties of the film was found. The out-of-plane pseudocubic lattice parameter (az) of each film was calculated from The out-of-plane pseudocubic lattice parameter (a ) of each film was calculated from the (200) peak, and their thickness dependence is shownz in Figure 3a. For the thinnest the (200) peak, and their thickness dependence is shown in Figure3a. For the thinnest LSCO LSCO films, there is a clear trend obeying the substrate strain: The LSCO film with the films, there is a clear trend obeying the substrate strain: The LSCO film with the largest largest lattice parameter az corresponds to the lowest in-plane lattice parameter substrate, lattice parameter a corresponds to the lowest in-plane lattice parameter substrate, i.e., the i.e., the most in-planez compressive substrate (SLAO substrate), whereas the sample with most in-plane compressive substrate (SLAO substrate), whereas the sample with the lowest the lowest az value matches the largest in-plane lattice parameter substrate, i.e., the most az value matches the largest in-plane lattice parameter substrate, i.e., the most in-plane in-plane tensile substrate (STO substrate). This situation would suggest epitaxial growth tensile substrate (STO substrate). This situation would suggest epitaxial growth for the four for the four different substrates, at least for the thinnest films. However, as the film thick- different substrates, at least for the thinnest films. However, as the film thickness increases, nesstwo increases, different behaviorstwo different were behaviors observed, were depending observed, on thedepending compressive on the or compressive tensile substrate or tensilestrain. substrate For LSCO strain. films For obtained LSCO onfilms SLAO obta andined LAO on SLAO in-plane and compressive LAO in-plane substrates, compressive there substrates,is a generally there downward is a generally trend downward of the film tren latticed of the parameter film lattice as parameter the thickness as the increases, thick- nessapproaching increases, approaching the bulk lattice the parameterbulk lattice valueparameter more value markedly more markedly for the SLAO for the case. SLAO On case.the otherOn the hand, other for hand, LSCO for films LSCO grown films on gr theown LSAT on the and LSAT STO in-planeand STO tensilein-plane substrates, tensile substrates,we observed we anobserved approximately an approximately constant behavior constant of behavior the lattice of parameter the lattice when parameter the film whenthickness the film increases, thickness which increases, is 3.80 which Å and is 3.79 3.80 Å, Å respectively. and 3.79 Å, respectively. All this suggests All this a greater sug- gestscapability a greater of thiscapability material of tothis support material tensile to support than compressive tensile than strains compressive⊥. strains.

FigureFigure 3. 3. ThicknessThickness and and substrate-induced substrate-induced strain strain dependencies dependencies of lattice parameter ((aa)) andand estimatedesti- matedcrystal crystal size (bsize) in (b the) in z-direction the z-direction of LSCO of LSCO ﬁlms films grown grown on STO, on STO, LSAT, LSAT, LAO, LAO, and SLAOand SLAO substrates. substrates.The dashed The dashed line in line (a) in indicates (a) indicates the latticethe lattice parameter parameter value value of LSCOof LSCO bulk bulk (3.84 (3.84 Å). Å). Inset Inset shows showsout-of-plane out-of-plane lattice lattice strain strain versus versus in-plane in-plane lattice lattice strain, strain, with a with linear a linear ﬁt presented fit presented by the by dashed the line dashed line (see text). (see text).

WeWe compared compared the the elastic behaviorbehavior inin the the inset inset of of Figure Figure3b by3b plottingby plotting the out-of-planethe out-of- planelattice lattice strain, strain,εzz = Ɛ (azzz =− (azc )/− aacc)/,a versusc, versus the the in-plane in-plane lattice lattice strain, strain,εxx Ɛ=xx ( a= −(a a−c )/ac)/aca.c. Here, Here,a c acstands stands for for the the bulk bulk lattice lattice parameter, parameter, and anda ais is the the substrate substrate lattice lattice parameter, parameter, assumed assumed as asthe the in-plane in-plane lattice lattice parameter parameter for for the the thinnest thinnest LSCO LSCO film. film. Thus, Thus, the lineal the lineal relation relation shown shownas dashed as dashed line in line the figurein the confirmsfigure confirms the expected the expected Poisson Poisson ration ν ration= 1/3, ν and = 1/3, it suggests and it suggestsan epitaxial an epitaxial growth growth of the LSCO of the films LSCO [32 films]. [32]. AsideAside from from this, this, using using the the full full width width at at half half maximum maximum (FWHM) (FWHM) of of the the (200) (200) peak peak correspondingcorresponding to to LSCO LSCO films, films, we we esti estimatedmated the the crystalline crystalline domain domain size size (D (Dc)c )for for each each film film in the out-of-plane direction. The thickness dependence of Dc for the group of LSCO films is shown in Figure3b, indicating again two different trends related to the compressive and tensile substrate strain. While in LSCO films grown on in-plane tensile substrates, the crystal size linearly increases as the thickness does (the crystal grains reaching almost the maximum size allowed by the thickness of the film), those films grown on in-plane compressive substrates behave differently. The increase of Dc is fairly gentler in the case of Nanomaterials 2021, 11, x FOR PEER REVIEW 6 of 13

in the out-of-plane direction. The thickness dependence of Dc for the group of LSCO films is shown in Figure 3b, indicating again two different trends related to the compressive and tensile substrate strain. While in LSCO films grown on in-plane tensile substrates, the crystal size linearly increases as the thickness does (the crystal grains reaching almost the maximum size allowed by the thickness of the film), those films grown on in-plane com- Nanomaterials 2021, 11, 781 pressive substrates behave differently. The increase of Dc is fairly gentler in the case of6 ofthe 13 LAO substrate, while there is almost no increase for the case of the SLAO substrate, indicating that the films grown on the SLAO substrate would remain more relaxed. Hence, thethe evolution LAO substrate, of both while the lattice there parameter is almost noand increase crystal forsize the suggests case of that the the SLAO tensile substrate, strain isindicating maintained that up the to films larger grown thicknesses. on the SLAO It indicates substrate a greater would feasibility remain more to support relaxed. the Hence, in- planethe evolution tensile strain of both by the LSCO, lattice which parameter agrees and with crystal previous size studies suggests performed that the tensile on thin strain films is [46].maintained up to larger thicknesses. It indicates a greater feasibility to support the in-plane tensile strain by LSCO, which agrees with previous studies performed on thin films [46]. To analyze the surface morphology of the films, SEM images were obtained. In Fig- To analyze the surface morphology of the films, SEM images were obtained. In Figure4 ure 4 the surface of two samples corresponding to both tensile and compressive cases can the surface of two samples corresponding to both tensile and compressive cases can be be compared. The SEM inspection allowed us to exclude the presence of breaks, defects, compared. The SEM inspection allowed us to exclude the presence of breaks, defects, or or inhomogeneities thorough the samples’ surface, which could influence the subsequent inhomogeneities thorough the samples’ surface, which could influence the subsequent magnetic characterization. magnetic characterization.

Figure 4. Scanning electron microscope images of LSCO films grown on STO (a ) and LAO (b) Figuresubstrates 4. Scanning for the ~28 electron nm thickness. microscope images of LSCO films grown on STO (a) and LAO (b) substrates for the ~28 nm thickness. 3.2. Magnetic Properties 3.2. MagneticIn view Properties of the differences observed in the structural characterization we analyzed its effectIn view on of the the magnetic differences behavior. observed Figure in the5 shows structural the field-cooledcharacterization (FC) we magnetization analyzed its effectcurves on performed the magnetic on LSCO behavior. films grown Figure on 5 STO, shows LSAT, the LAO,field-cooled and SLAO (FC) substrates magnetization for two representative thicknesses, around 14 nm (a) and 28 nm (b). These data were taken under curves performed on LSCO films grown on STO, LSAT, LAO, and SLAO substrates for an in-plane magnetic field of 100 Oe between 5 and 300 K. The curves obtained evidence two representative thicknesses, around 14 nm (a) and 28 nm (b). These data were taken of ferromagnetic behavior on all studied films, showing a decrease of the magnetization under an in-plane magnetic field of 100 Oe between 5 and 300 K. The curves obtained as we approach the Curie temperature, T . The shoulder observed nearly below T in evidence of ferromagnetic behavior on all studiedC films, showing a decrease of the mag-C some curves, as seen for example, in the film grown on LSAT (15.2 nm), is related with netization as we approach the Curie temperature, TC. The shoulder observed nearly below the Hopkinson effect [47,48]. This appears when magnetic anisotropy decreases with the TC in some curves, as seen for example, in the film grown on LSAT (15.2 nm), is related temperature faster than magnetization does, and it was confirmed in these samples when with the Hopkinson effect [47,48]. This appears when magnetic anisotropy decreases with we observed that the feature disappears when the field applied during the measurement the temperature faster than magnetization does, and it was confirmed in these samples increases. We estimated the Curie temperature of each sample as the maximum of the first when we observed that the feature disappears when the field applied during the meas- derivative of the curves, and accordingly we obtained the thickness dependence depicted urement increases. We estimated the Curie temperature of each sample as the maximum in Figure5a. The obtained values for the T C are below the corresponding value to LSCO ofbulk the (around first derivative 240 K). Similar of the curves, findings and have accordingly been already we reported obtained in the the thickness literature, depend- and they encewere depicted mainly relatedin Figure to the5a. The finite obtained size effect values and/or for the T oxygenC are below stoichiometry the corresponding [30,49,50]. value to LSCO bulk (around 240 K). Similar findings have been already reported in the However, an ascendant tendency of the TC values as the film thickness increases was literature,observed and for thethey four were studied mainly substrates, related to the approaching finite size effect the LSCO and/or bulk the value.oxygen Again, stoichi- a ometrydifferent [30,49,50]. trend was However, observed an depending ascendant on tendency the compressive of the TC or values tensile as substrate the film strain.thickness For increases was observed for the four studied substrates, approaching the LSCO bulk value. LSCO films grown under tensile strain, on the STO and LSAT substrates, the TC value Again,moderately a different increases trend as was the thicknessobserved depend does, anding it on reaches the compressive an almost stableor tensile value substrate around strain.30 nm. For In LSCO the compressive films grown cases, under LSCO tensile films strain, grown on the on theSTO LAO and substrateLSAT substrates, display the the TlargestC value rise, moderately whereas increases samples grown as the onthickness the SLAO does, substrate and it reaches show a an sharp almost rise stable up to 30value nm, decreasing strongly afterwards. This behavior totally agrees with the structural details obtained by XRD, as LSAT is the case of the largest strain and is where more relaxation was seen when the thickness increases.

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around 30 nm. In the compressive cases, LSCO films grown on the LAO substrate display around 30 nm. In the compressive cases, LSCO films grown on the LAO substrate display the largest rise, whereas samples grown on the SLAO substrate show a sharp rise up to 30 the largest rise, whereas samples grown on the SLAO substrate show a sharp rise up to 30 nm, decreasing strongly afterwards. This behavior totally agrees with the structural de- nm, decreasing strongly afterwards. This behavior totally agrees with the structural details obtained by XRD, as LSAT is the case of the largest strain and is where more relaxa- tails obtained by XRD, as LSAT is the case of the largest strain and is where more relaxation was seen when the thickness increases. Nanomaterials 2021, 11, 781 tion was seen when the thickness increases. 7 of 13

FigureFigure 5. 5. Field-cooledField-cooled magnetization magnetization curves curves under under in-p in-planelane magnetic magnetic field field of of 100 100 Oe Oe performed performed on on Figure 5. Field-cooled magnetization curves under in-plane magnetic field of 100 Oe performed on LSCO films grown on STO, LSAT, LAO, and SLAO substrates for two representative thicknesses, LSCOLSCO films films grown grown on onSTO, STO, LSAT, LSAT, LAO, LAO, and and SLAO SLAO substrates substrates for fortwo two repres representativeentative thicknesses, thicknesses, around 14 nm (a) and 28 nm (b). aroundaround 14 14 nm nm (a) ( aand) and 28 28 nm nm (b ().b ). From the hysteresis loops (MH), the magnetization values at 30 kOe were obtained FromFrom the the hysteresis hysteresis loops loops (MH), (MH), the the magnet magnetizationization values values at at30 30 kOe kOe were were obtained obtained and are plotted in Figure 6b as a function of thickness. The MH curves were measured at andand are are plotted plotted in inFigure Figure 6b6b as as a afunction function of of thickness. thickness. The The MH MH curves curves were were measured measured at at 5 K with the magnetic field direction parallel to the surface of the sample (in-plane). The 5 K5 Kwith with the the magnetic magnetic field field direction direction parallel parallel to to the the surface surface of of the the sample sample (in-plane). (in-plane). The The magnetization observed in the LSCO films grown with in-plane tensile strain, i.e., the STO magnetizationmagnetization observed observed in in the the LSCO LSCO films films grow grownn with with in-plane in-plane tensile tensile strain, strain, i.e., i.e., the the STO STO andand LSAT LSAT substrates, substrates, displays displays values values around around 300 300 emu/cm emu/cm3,3 which, which are are similar similar to to those those and LSAT substrates, displays values around 300 emu/cm3, which are similar to those foundfound in in the the literature literature [21,51,52]. [21,51,52]. However, However, a adifferent different trend trend was was obtained obtained again again for for the the found in the literature [21,51,52]. However, a different trend was obtained again for the in-planein-plane compressive compressive strain strain cases, cases, i.e., i.e., the the SLAO SLAO and and LAO LAO substrates, substrates, displaying displaying values values in-plane compressive strain cases, i.e., the SLAO and LAO substrates, displaying values markedlymarkedly lower. lower. markedly lower.

FigureFigure 6. 6. (a()a )Curie Curie temperature temperature (T (TC)C dependence) dependence on on the the thickness thickness an andd the the substrate-induced substrate-induced strain; strain; Figure 6. (a) Curie temperature (TC) dependence on the thickness and the substrate-induced strain; thethe dashed dashed line line at at 250 250 K K corresponds corresponds to to the the TC T valueC value for for LSCO LSCO bulk. bulk. (b) ( bSaturation) Saturation magnetization magnetization the dashed line at 250 K corresponds to the TC value for LSCO bulk. (b) Saturation magnetization (M(Mts)ts at) at 5 5K Kfor for LSCO LSCO films ﬁlms grown grown on on STO, STO, LSAT, LSAT, LAO, LAO, and and SLAO SLAO substrates. substrates. (Mts) at 5 K for LSCO films grown on STO, LSAT, LAO, and SLAO substrates. TheThe hysteresis hysteresis loops loops measured measured on on LSCO LSCO fi ﬁlmslms grown grown are are shown shown in in Figure Figure 77 forfor the the The hysteresis loops measured on LSCO films grown are shown in Figure 7 for the fourfour studied studied substrates. substrates. The The magnetization valuesvalues werewere normalized normalized to to the the 30 30 kOe kOe value value for four studied substrates. The magnetization values were normalized to the 30 kOe value fora bettera better comparison. comparison. Additionally, Additionally, absolute absolute values values of magnetization of magnetization at 30 at kOe 30 kOe (3 T) (3 were T) for a better comparison. Additionally, absolute values of magnetization at 30 kOe (3 T) werecompared compared in Figure in Figure6b for 6b a completefor a complete characterization. characterization. were compared in Figure 6b for a complete characterization.

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FigureFigure 7. Normalized 7. Normalized in-plane in-plane hysteresis hysteresis loops loopsperfor performedmed at 5 K aton 5 LSCO K on LSCOfilms grown ﬁlms grownon LAO, on LAO, SLAO,SLAO, STO, STO, and andLSAT LSAT substrates. substrates.

Again,Again, the shape the shape of the of loops the loops points points to a todifferent a different magnetic magnetic behavior behavior of LSCO of LSCO films, films, dependingdepending on the on type the type of strain of strain imposed imposed by the by substrate the substrate and andthe thickness the thickness of the of film. the film. The TheLSCO LSCO films films grown grown on substrates on substrates under under in-plane in-plane compressive compressive strain strain (LAO (LAO and andSLAO) SLAO) displaydisplay larger larger hysteretic hysteretic behavior, behavior, with withcoercive coercive fieldsfields values values up to 5 up × 10 to4 5Oe,× 10as 4theOe, film as the thicknessfilm thickness increases. increases. In addition, In addition,they show they a slight show wasp-waist a slight wasp-waist shape in the shape thinner in the films, thinner morefilms, markedly more markedlyin the case in of the LAO case substrates. of LAO substrates. The LSCO The films LSCO grown films under grown tensile under tensilein- planein-plane strain display strain display a different a different trend. On trend. the one On thehand, one loops hand, mostly loops present mostly larger present hys- larger teretichysteretic behavior behavior than those than obtained those obtained for the substrates for the substrates imposing imposing compressive compressive strain, with strain, 4 coercivewith coercivefields values fields up values to 1.4 up × to104 1.4 Oe.× On10 theOe. other On the hand, other the hand, thickness the thickness dependence dependence of the hysteresisof the hysteresis is not the is notone theobserved one observed for the previous for the previous case. From case. the From films thegrown films on grown STO, on i.e., STO,the largest i.e., the tensile largest strain, tensile two strain, features two st featuresand out: standThe thickest out: The sample thickest shows sample the lowest shows the hysteresislowest in hysteresis comparation in comparation with the thinner with thefilms, thinner additionally films, additionally presenting a presenting well-defined a well- 4 step-likedefined shape step-like for field shape values for field below values 1 × 10 below4 Oe. 1 The× 10 step-likeOe. The feature step-like indicates feature the indicates pres- the encepresence of two different of two different magnetic magnetic phases, phases, i.e., hard i.e., and hard soft, and soft,whose whose relative relative contributions contributions dependdepend on thickness. on thickness. This This behavior behavior suggests suggests the soft the softone onecomes comes from from the interfacial the interfacial effect effect betweenbetween the thesubstrate substrate and andthe thefilm. film. Therefore, Therefore, the therelative relative contribution contribution of the of thesoft softone one decreasesdecreases as the as total the totalthickness thickness is increased, is increased, and thus and its thus presence its presence in the loops in the is loopsless evi- is less dentevident as the film’s as the thickness film’s thickness increases increases [53]. Similar [53]. Similarresults were results reported were reported in magnetic in magnetic thin films,thin and films, interesting and interesting conclusion conclusion were obtained were obtainedby Rigato by et Rigatoal., which et al., points which to an points effect to an effect associated with the strain acting on surfaces edges [54]. LSCO films grown on a LSAT associated with the strain acting on surfaces edges [54]. LSCO films grown on a LSAT substrate show a similar behavior but much less marked, pointing to the strain as the main substrate show a similar behavior but much less marked, pointing to the strain as the main source of the step-like feature. source of the step-like feature. In view of the dependence of both structural and magnetic behavior related to the sub- In view of the dependence of both structural and magnetic behavior related to the strate and strain imposed, we will consider the role played by the different anisotropies in the substrate and strain imposed, we will consider the role played by the different aniso- LSCO films. Therefore, different contributions related to shape anisotropy, magnetocrystalline tropies in the LSCO films. Therefore, different contributions related to shape anisotropy, anisotropy, and magnetoelastic anisotropy are participating simultaneously on the films, and magnetocrystalline anisotropy, and magnetoelastic anisotropy are participating simulta- they could be modified or induced by the substrate strain [33–37]. In order to elucidate neously on the films, and they could be modified or induced by the substrate strain [33– the substrate-induced strain effect and the role played by the anisotropy, hysteresis loops 37]. In order to elucidate the substrate-induced strain effect and the role played by the with the magnetic field applied in the direction that is perpendicular to the surface of the sample (out-of-plane) were analyzed. The in-plane tensile or compressive substrate strain must be considered together with the corresponding crystalline elongation or shortening in

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anisotropy, hysteresis loops with the magnetic field applied in the direction that is perpendicular to the surface of the sample (out-of-plane) were analyzed. The in-plane tensile Nanomaterials 2021, 11, 781 9 of 13 or compressive substrate strain must be considered together with the corresponding crystalline elongation or shortening in the out-of-plane direction of the film [30,55]. This crys- tallinethe out-of-plane anisotropy, direction a consequence of the film of the [30 ,substr55]. Thisate crystallinestrain, could anisotropy, lead to the a consequence change of the of magneticthe substrate response strain, in could each lead direction. to the change The out-of-plane of the magnetic (OOP) response hysteresis in each loops direction. are com- The paredout-of-plane to the in-plane (OOP) hysteresis (IP) curves loops in Figure are compared 8 for LSCO to the films in-plane with (IP)around curves 28 nm in Figure thickness8 for forLSCO all the films substrates. with around The OOP 28 nm measurements thickness for display all the substrates. some outliers The near OOP 10 measurements kOe because atdisplay those fields, some outliersthe raw near moment 10 kOe (before because removi at thoseng the fields, substrate the raw contribution) moment (before crosses removing zero, whichthe substrate makes contribution)the values noisier. crosses zero, which makes the values noisier.

FigureFigure 8. 8. In-planeIn-plane (IP) (IP) and and out-of-plane out-of-plane (OOP) (OOP) hyster hysteresisesis loops loops performed performed at 5 at K 5 on K LSCO on LSCO films ﬁlms growngrown on on the the LAO, LAO, SLAO, SLAO, STO, STO, and and LSAT LSAT substrates substrates for for the the ~28 ~28 nm nm thickness. thickness.

DependingDepending on on the the type type of of strain strain induced induced by by the the substrate, substrate, two two different different trends trends related related withwith the the magnetization magnetization values values and and the the shape shape of of the the loops loops were were observed. observed. Loops Loops for for LSCO LSCO filmsfilms grown grown on on STO STO and and LSAT LSAT substrates substrates (i.e., (i.e., the the in-plane in-plane tensile tensile strain strain cases) cases) show show OOP OOP magnetizationmagnetization values values significantly significantly lower lower than than the the IP IP case case for for the the same same applied applied magnetic magnetic field,field, and and they they show show an an almost almost negligible negligible hysteretic hysteretic behavior. behavior. In In fact, fact, this this is is the the expected expected behaviorbehavior for for thin thin films films where where the the magnetic magnetic anisotropy anisotropy induces induces an an in-plane in-plane magnetic magnetic easy easy axis.axis. However, However, for for LSCO LSCO films films grown grown on on LAO LAO and and SLAO SLAO (i.e., (i.e., under under in-plane in-plane compressive compressive strain),strain), quite quite similar magnetizationmagnetization valuesvalues were were found found for for both both IP IP and and OOP OOP configurations. configurations.Actually, Actually, the magnetization the magnetization in the OOPin the loops OOP is loops slightly is slightly larger than larger the than one displayedthe one dis- by playedthe IP loops.by the ThisIP loops. behavior, This behavior, presenting presenting a similar valuea similar of both value IP of and both OOP IP and magnetizations, OOP magnetizations,suggests an suggests easy axis an of easy the magnetizationaxis of the magnetization not in the planenot in of the the plane film butof the somewhere film but somewherecloser to the closer normal. to the Such normal. behavior Such has behavior been previously has been reported previously [33 –reported35,37]. In [33–35,37]. particular, InY. particular, Heo et al. [33Y. ]Heo found et thisal. [33] behavior found forthis LSCO behavior films for grown LSCO on films LSAT grown and LAO on LSAT substrates, and LAOand theysubstrates, correlated and they the magneticcorrelated anisotropy the magnetic with anisotropy the oxygen with octahedral the oxygen distortions octahedral in distortionsLSCO. In the in presentLSCO. In work, the present we confirm work, experimentally we confirm experimentally these results in these both LSATresults and in both LAO LSATsubstrates, and LAO and wesubstrates, go further and by we extending go further the analysisby extending for two the more analysis cases: for one two in-plane more cases:tensile one substrate in-plane (STO tensile substrate) substrate and (STO one in-planesubstrate) compressive and one in-plane case (SLAO compressive substrate). case In summary, the analysis of this set of four substrates suggests that when the strain induced (SLAO substrate). In summary, the analysis of this set of four substrates suggests that by the substrate is compressive, the magnetization tends to get out of plane to minimize when the strain induced by the substrate is compressive, the magnetization tends to get energy, while for the cases of in-plane tensile strain, the magnetization remains in-plane. With regards to the step-like feature, it presents a particular behavior depending on the substrate strain character. For LSCO films grown on the STO in-plane tensile substrate, the step-like was clearly observed in the IP hysteresis loops, and it is not present in the OOP case. However, for the samples grown on LAO and SLAO in-plane compressive substrates, Nanomaterials 2021, 11, 781 10 of 13

a well-defined step-like was observed in the OOP magnetization curves. By comparing the obtained results, we deduced that the step-like feature appears when the magnetic field direction is parallel to the elongation of the lattice (the IP loops for the samples grown on STO and LSAT substrates, and the OOP loops for the samples grown on LAO and SLAO substrates). By contrast, when the magnetic field direction is parallel to the shortening of the lattice (the OOP loops for samples grown on STO and LSAT substrates, and the IP loops for samples grown on LAO and SLAO substrates), the hysteresis loop does not show the step-like feature. Therefore, it follows that the source of the step-like signature is related with the lattice distortion imposed by the strain. The obtained results point to the relevant role of the substrate-induced strain on the magnetic anisotropy for the LSCO thin films. For films presenting an out-of-plane magnetization easy axis, the magnetic shape anisotropy KS must be overcome by other sources of anisotropy. It is convenient to compare the shape anisotropy KS with the perpendicular magnetic anisotropy K⊥, which includes both magnetocrystalline and magnetoelastic anisotropies. The values of K⊥ are obtained from the effective magnetic anisotropy Keff, which is calculated from the area enclosed between the OOP and the IP semi-loops. Thus, 2 MS K⊥ = Keff − KS, with KS = −µ0 2 , where MS is the saturation magnetization [56]. K⊥ values obtained for films with a thickness around 28 nm grown on the four different substrates are compared in Figure9. The negative K⊥ values correspond to the LSCO films in which the magnetic shape anisotropy (always negative) dominates, and the magnetization lies in-plane. This situation corresponds to the films grown on the in-plane tensile LSAT and STO substrates. On the other hand, samples displaying positive K⊥ values correspond to a situation where the magnetic shape anisotropy is surpassed by other anisotropies (this being the case for the samples grown on LAO and SLAO in-plane compressive substrates). From these measurements, both contributions to the perpendicular magnetic anisotropy, i.e., magnetocrystalline and magnetoelastic, cannot be resolved independently. However, a coarse estimation of the magnetoelastic anisotropy using the expression:

3 Y abulk − a f ilm Kmagnetoelastic = − λ100 (1) 2 1 + µ abulk

where λ100, µ, and Y denote the magnetostriction, the Poisson ratio, and the Young’s modu- lus, respectively [56], offers blurred results. This estimation points to a contribution caused by both magnetoelastic and magnetocrystalline anisotropies without a clear dominance. Considering these two anisotropies present equivalent symmetry in cubic structures [54], this analysis supports the easy axis obtained in each case. Hence, these results confirm the influence of the strain on the magnetic behavior on LSCO films, which is required for spintronic and advanced memory devices. It should be noted that for the largest in-plane compressive case (SLAO substrate), the results obtained and displayed in Figure9 are misleading, but the reason could be the large compressive strain we discussed above, which leads to show no trend in structural and magnetic characterization. Nanomaterials 2021, 11, x FOR PEER REVIEW 11 of 13

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FigureFigure 9. 9. ClosedClosed circles circles correspond correspond to to the the perpen perpendiculardicular magnetic magnetic shape shape anisotropy anisotropy (K (K )⊥ for) for LSCO LSCO filmsﬁlms grown grown on on tensile tensile and and compressive compressive substrates, substrates, whose whose thickness thickness is is around around 28 28 nm. nm. Open Open circles circles and dashed line represent the magnetoelastic anisotropy contribution to KꞱ (see text). and dashed line represent the magnetoelastic anisotropy contribution to K⊥ (see text).

4.4. Conclusions Conclusions LSCOLSCO films films were were grown grown with variousvarious thicknessesthicknesses on on different different tensile tensile and and compressive compres- sivestrain strain substrates substrates to analyzeto analyze their their effect effect on theon the structural structural and and magnetic magnetic properties properties of theof thesystem. system. Both Both lattice lattice parameter parameter and and crystal crystal sizesize valuesvalues depend on the the substrate substrate strain strain character,character, indicating indicating that tensile strainstrain is is maintained maintained up up to to larger larger thicknesses thicknesses of theof the film. film. The Theferromagnetic ferromagnetic order order was was observed observed in all in samples, all samples, with with a Curie a Curie temperature temperature approaching, approach- in ing,general, in general, the bulk the value bulk whenvaluefilm when thickness film thickness increases. increases. The strain The imposed strain imposed by the substrate by the substratewas also was responsible also responsible for a soft for magnetic a soft magnetic contribution, contribution, which which stands stands out mainly out mainly in the inthinner the thinner samples. samples. The The magnetism magnetism in LSCO in LSCO films films depends depends strongly strongly on on the the competition competi- tionbetween between both both shape shape and and perpendicular perpendicular anisotropies. anisotropies. The The in-plane in-plane tensile tensile strain strain keeps keeps the themagnetization magnetization in in the the film film plane, plane, whereas whereas the the compressive compressive strain strain forces forces the the easy easy axis axis to topoint point out out of of the the film. film.

AuthorAuthor Contributions: Contributions: Conceptualization,Conceptualization, O.J.D.; O.J.D.; methodology, methodology, M.S.-P.; M.S.-P.; software, software, J.A.G.; J.A.G.; valida- valida- tion,tion, J.A.G., J.A.G., J.P.A. J.P.A. and and R.L.A.; R.L.A.; formal formal analysis, analysis, M.S.-P. M.S.-P. and and O.J.D.; O.J.D.; investigation, investigation, M.S.-P.; M.S.-P.; resources, resources, M.A.L.d.l.T.,M.A.L.d.l.T., J.A.G. J.A.G. and and J.P.A.; J.P.A.; data data curation, curation, R. R.L.A.;L.A.; writing—original writing—original dr draftaft preparation, preparation, M.S.-P.; M.S.-P.; writing—reviewwriting—review and and editing, editing, O.J.D. O.J.D. and and J.A.G.; J.A.G.; visu visualization,alization, M.S.-P.; M.S.-P.; supe supervision,rvision, O.J.D.; O.J.D.; project project administration,administration, M.A.L.d.l.T. M.A.L.d.l.T. and and J.P.A.; J.P.A.; funding funding acquisition, acquisition, M.A.L.d.l.T. M.A.L.d.l.T. and and J.P.A. J.P.A. All All authors authors have have readread and and agreed agreed to to the the publis publishedhed version version of of the the manuscript. manuscript. Funding:Funding: ThisThis research research was was funded funded by by the the Spanish Spanish “Ministerio “Ministerio de de Economia Economia y yCompetitividad” Competitividad” (MINECO),(MINECO), grant grant number number 2014-58034-R. 2014-58034-R. Acknowledgments:Acknowledgments: TheThe authors authors thank thank fruitful fruitful discussions discussions and and relevant relevant advices advices given given by by A. A. Rivera, Rivera, C.C. León León and and J. J.Santamaría Santamaría and and technical technical support support provided provided by by M. M. Rivera Rivera and and E. E. Pardo. Pardo. ConflictsConﬂicts of of Interest: Interest: TheThe authors authors declare declare no no conflict conﬂict of of interest. interest.

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