Cation uniformity and magnetic properties of La0.7Sr0.3Mn0.5Fe0.5O3 thin films

Journal of Magnetism and Magnetic Materials 325 (2013) 69–74

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Cation uniformity and magnetic properties of La0.7Sr0.3Mn0.5Fe0.5O3 thin films Meng Gu a,1, Fan Yang a, Elke Arenholz b, Nigel D. Browning a,c,1, Yayoi Takamura a,n a

Department of Chemical Engineering and Materials Science, University of California-Davis, One Shields Avenue, Davis, CA 95616, USA Advanced Light Source, Lawrence Berkeley National Laboratory, One Cyclotron Road, MS 6R2100, Berkeley, CA 94720, USA c Department of Molecular and Cellular Biology, University of California-Davis, Davis, CA, 95616, USA b

a r t i c l e i n f o

abstract

Article history: Received 14 May 2012 Received in revised form 11 July 2012 Available online 15 August 2012

In this paper, we report on the effect of the target–substrate distance during pulsed laser deposition on the uniformity of the cation distribution in La0.7Sr0.3Mn0.5Fe0.5O3 (LSMFO) thin films. Through a combination of x-ray diffraction, atomic scale scanning transmission electron microscopy, and soft x-ray magnetic spectroscopy, the microstructure and magnetic properties were characterized in detail and correlated to one another. With a large target–substrate distance, cation segregation occurred such that the Mn and Fe ions were found in mixed valence states and coordination environments. As a consequence, an unexpected ferrimagnetic ordering in the LSMFO films was observed. Uniformly cation-distributed thin films could be deposited with smaller target–substrate distance, and no such magnetic ordering was observed. This difference is discussed in the context of the kinetic energy of the plume considering the interaction of the plume species with the background O2 gas molecules. & 2012 Elsevier B.V. All rights reserved.

Keywords: PLD XMCD STEM EELS Cation segregation

1. Introduction Superlattices consisting of perovskite oxide materials have attracted significant interest in recent years as a potential means of combining the distinct functional properties of the individual sublayers. Furthermore, the interfaces between these oxide sublayers can possess unexpected functional properties not found in the constituent materials [1]. These interfacial properties result from a number of different phenomena, including structural discontinuities, atomic intermixing, electronic reconstruction, and magnetic interactions. A particularly noteworthy example involves the formation of a 2D electron gas, ferromagnetism, and superconductivity at the interfaces between two nonmagnetic insulators, SrTiO3 and LaAlO3 [2,3]. Other studies have investigated the exchange interactions that occur in all perovskite oxide systems, such as the ferromagnetic (FM) metal La0.7Sr0.3MnO3 (LSMO) and the antiferromagnetic (AFM) insulator La0.7Sr0.3FeO3 (LSFO), which have potential applications in next generation data storage and logic devices [4–7]. These studies have shown that the electrical and magnetic properties of the superlattices change dramatically as the sublayer thickness varies from 3 to 18 unit cells and with changes in the density of the interfaces. For example, the resistivity of the superlattices varies n

Corresponding author. Tel.: þ1 5307547124; fax: þ1 5307521031. E-mail address: [email protected] (Y. Takamura). 1 Present address: Pacific Northwest National Laboratory, Richland, WA, 99352, USA. 0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.08.005

by about four orders of magnitude at 200 K over this sublayer thickness range, even though the overall chemical composition remains unchanged [8]. In order to determine the impact of an intermixed interface layer on the properties of these LSMO/LSFO superlattices, a detailed study of the solid solution between LSMO and LSFO, i.e. La0.7Sr0.3Mn0.5Fe0.5O3 (LSMFO) has been performed. The conductivity and magnetic properties of the La1  xAxMnO3 (A ¼Sr, Ca) manganites is determined by the double exchange mechanism involving electrons hopping along Mn3 þ –O2  –Mn4 þ chains, and is characterized by coincident FM/paramagnetic and metal/insulator transitions [9]. The incorporation of Fe ions in La0.7Ca0.3Mn1  xFexO3 (LCMFO) has been shown to break up the Mn3 þ –O2  –Mn4 þ double exchange network, thus weakening the ferromagnetism and increasing the resistivity [10]. With increasing Fe doping, x, from 0 to 0.7, LCMFO changes from a spin glass state at x ¼0.1 to an AFM state with a Ne´el temperature, TN below 50 K at x ¼0.4 [10]. Similarly for La0.7Sr0.3Mn1  xFexO3, Fe doping causes the depletion of hopping electrons and hopping sites and causes a transition from a metallic to insulating phase for x40.2 [11]. For Fe doping in LSMO, the prevalent Fe valence state is 3þ [12], while the average Mn valence state increases due to the Fe doping [10]. Fe3 þ has an identical ionic radius as Mn3 þ but with a stable half-filled 3d state, it does not participate in the double exchange mechanism and encourages an AFM superexchange interaction between the Fe3 þ ions and the neighboring Mn3 þ , Mn4 þ or Fe3 þ ions [10,13]. These previous studies used bulk materials synthesized using conventional solid state methods under equilibrium conditions. In this work, we show that using

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a non-equilibrium growth technique such as pulsed laser deposition (PLD), the cation distribution in LSMFO films can be controlled through the deposition conditions, and that it has a direct impact on the magnetic properties of the films. The quality of thin films deposited by PLD depends closely on the growth conditions, such as substrate temperature, laser energy, background gas pressure, and target–substrate distance [14,15]. Target–substrate distance has been shown to influence the texture, orientation, and crystallite size of thin films such as the superconducting oxide YBa2Cu3O7 [16–20]. Each value of O2 background pressure possesses a corresponding optimal target– substrate distance. According to Christou et al. [16] the electron temperature of the plume drops quickly with distance away from the target due to the interaction with the background gas. In addition, the quantity of species in the laser plume arriving at the substrate surface increases with decreasing target–substrate distance. The mobility of adatoms on the substrate surface is influenced by their total energy, including both the kinetic energy given by the plume and the thermal energy acquired from the heated substrate. In this study, a fixed O2 pressure of 0.3 Torr was used during the deposition and the target–substrate distance was varied, while other important growth parameters such as laser energy density, substrate temperature, and laser frequency were kept the same. Plasma dynamics were found to play an important role on the uniformity of the cation distribution during LSFMO thin film deposition. Using a combination of high-resolution x-ray diffraction (XRD), soft x-ray magnetic spectroscopy, and scanning transmission electron microscopy (STEM), we report novel magnetic properties related to cation segregation in the film which persisted after prolonged annealing in O2 ambient. In contrast, a homogeneous solid solution LSMFO film with no FM ordering was deposited by decreasing the target–substrate distance.

2. Materials and methods The PLD target with a composition of La0.7Sr0.3Mn0.5Fe0.5O3 was synthesized through solid state reaction by mixing of La2O3, SrCO3, Fe2O3, and Mn2O3 powders in the proper stoichiometric ratios and sintering overnight at 1500 1C. LSMFO thin films were grown by PLD on (001)-oriented single crystal (LaAlO3)0.3  (Sr2AlTaO6)0.7 (LSAT), SrTiO3 (STO), and LaAlO3 (LAO), and (110)-oriented GdScO3 (GSO) substrates using a substrate temperature of 700 1C and an oxygen pressure of 0.3 Torr. A KrF laser (248 nm) was used with an energy density 0.8 J/cm2 and a repetition rate of 5 Hz. The films were cooled in 300 Torr of O2 after the deposition. For sample (1), the target–substrate distance was 8.5 cm, while the target–substrate distance was decreased to 5.5 cm for sample (2). For the 0.3 Torr O2 pressure used, the outer ring of the plume was  1–2 cm away from the substrate at a target–substrate distance of 5.5 cm while it was 4– 5 cm away from the substrate at a target–substrate distance of 8.5 cm. The thickness of sample (1) was around 3771 nm (27,000 laser pulses) and the thickness for sample (2) was around 7571 nm (30,000 laser pulses) as determined by x-ray reflectivity (XRR) fitting. After the growth, sample (1) was annealed at 700 1C for 16 h with 100 cc/min of flowing O2 through a glass furnace using heating/cooling rate of 5.6 1C/min. XRD/XRR measurements were performed using a Bruker D8 Discover four circle diffractometer. Z-contrast images were obtained using a CEOS aberration corrected TEM/STEM JEM2100F operating at 200 kV (Fig. 4(a)) and the TEAM 0.5 microscope at the National Center for Electron Microscopy (NCEM) at the Lawrence Berkeley National Laboratory (LBNL) operating at 80 kV (all other Z-contrast images and electron energy loss spectroscopy (EELS)). To investigate the FM properties of the LSMFO thin films, soft x-ray magnetic

spectroscopy measurements were performed at beamline 6.3.1 at the Advanced Light Source. X-ray absorption (XA) spectra were obtained in total electron yield mode with the x-rays incident upon the sample at a 301 angle relative to the sample surface. X-ray magnetic circular dichroism (XMCD) was calculated as the difference between two XA spectra taken with right-circularly polarized x-rays and an applied magnetic field, Ha ¼1 T, parallel/ antiparallel to the x-ray helicity. X-ray magnetic linear dichroism (XMLD) was calculated as the difference between two XA spectra taken with linearly polarized x-rays with vertical and horizontal polarization, corresponding to the E-vector lying in-plane and canted out-of-plane at a 301 angle relative to the sample surface.

3. Results and discussions 3.1. Structural characterization by X-ray diffraction The average structural properties of the LSMFO films were determined using XRD and XRR measurements. For the samples on LSAT substrates, the o–2y scans around the 002 reflection of sample (1) before and after O2 annealing and sample (2) are compared in Fig. 1(a). These curves display distinct thickness fringes, indicating smooth surfaces and interfaces with no additional peaks from other phases. XRR fitting confirmed the film thicknesses and indicated that both types of samples show a surface roughness in a similar scale ( o1 nm). Reciprocal space maps around the 303 reflection for the samples on LSAT substrates are shown in Fig. 1(b–d). In each case, a fully-strained relationship between the film and the LSAT substrate was ˚ For observed with an in-plane lattice parameter of a ¼3.874 A. sample (1), the out-of-plane lattice parameter was calculated as c¼3.900 A˚ corresponding to a slight tetragonal distortion under compressive strain characterized by a c/a ratio¼ 1.007. After O2 annealing, the out-of-plane lattice parameter decreased to 3.885 A˚ (c/a ratio ¼1.003), possibly due to a decreased oxygen vacancy concentration in the film [15,21–23]. For sample (2) grown with a target–substrate distance¼5.5 cm, the out-ofplane lattice constant was calculated as 3.916 A˚ (c/a ratio¼1.011), which corresponds to an increase in the unit cell volume compared to sample (1). The XRD results (o–2y scans and RSMs) for the LSMFO films grown on LAO, STO, and GSO with a target–substrate distance¼8.5 cm are shown in Fig. S1 and the calculated lattice parameters and c/a ratios for the films are listed in Table 1. The RSMs showed that the choice of the substrate could determine the strain state of the LSMFO film with the tetragonal distortion ranging from 0.989 to 1.007. On STO substrates, the film was nearly lattice matched with a c/a ratio¼0.997; on LAO substrates, the film was fully relaxed with a c/a ratio ¼1.007; and on GSO substrates, the film was partially relaxed with a c/a ratio ¼0.989. 3.2. Soft x-ray magnetic spectroscopy Soft x-ray magnetic spectroscopy was performed in order to probe the element-specific electronic and magnetic properties of the top 5–10 nm of the LSMFO films. Fig. 2 compares the Mn XA and XMCD spectra acquired at 300 K from the LSMFO sample (1) before and after annealing with a LSMO film and sample (2). Little difference is observed in the spectra taken at 150 K. For comparison, the fundamental spectra for Mn2 þ (MnFe2O4), Mn3 þ (Mn2O3), and Mn4 þ (MnO2) powders are also shown. The only change in the XA spectra for sample (1) after annealing was that the small shoulder 1.9 eV below the main L3 peak became slightly less distinct after annealing. Judging from the L2,3 ratio, the average Mn valence state in sample (1) before and after

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Fig. 1. (a) o–2y scans taken around the 002 reflection for sample (1) as-grown and annealed, as well as sample (2); RSMs around the 303 reflection for (b) sample (1) asgrown, (c) sample (1) annealed, and (d) sample (2). ‘s’ and ‘f’ denote the locations of the LSAT substrate and film peaks, respectively. In the RSMs, colors black to red indicate increasing intensity from low to high. (For interpretation of the reference to color in this figure legend, the reader is referred to the web version of this article.)

Table 1 Calculated in-plane and out-of-plane lattice constants for LSMFO films grown on different substrates using a target–substrate distance of 8.5 cm. LSAT

LAO

STO

GSO

˚ a (A) ˚ c (A)

3.874

3.872

3.905

3.919

3.900

3.898

3.895

3.878

c/a

1.007

1.007

0.997

0.989

annealing is around 3.3 þ as in the LSMO film. In comparison, the L3 peak for sample (2) was narrower and shifted to higher energy with a more pronounced shoulder feature. Both of these features can be ascribed to a slightly larger Mn4 þ ion concentration in sample (2) than the LSMO film and sample (1) and corresponds well to the Mn4 þ spectra for MnO2. In contrast, prominent differences were observed in the XMCD spectra for the samples studied as shown in Fig. 2(c). The LSMO film with Mn3 þ /Mn4 þ ions in octahedral coordination displayed its characteristic spectrum with a strong stair step negative feature B followed by a weaker positive feature C. In contrast, the as-grown sample (1) showed two narrow peaks, A and B with nearly equal positive/negative values. The position of peak A aligned well with the position of the peak for Mn2 þ ions in an

tetrahedral coordination in the XA and XMCD spectra for MnFe2O4 [24]. This strong Mn2 þ XMCD signal was observed in as-grown sample (1) despite the lack of any prominent feature from Mn2 þ ions in the XA spectra. After O2 annealing, the XMCD spectrum of sample (1) became more LSMO-like: the magnitude of peak A decreased substantially, the magnitude of peak B increased and it moved to slightly higher photon energy, and peak C appeared. The spectral features of the XMCD curves for sample (1) before and after annealing could be reasonably reproduced using a linear combination of the LSMO and MnFe2O4 spectra. Finally, sample (2) showed no features in the XMCD spectrum above the 0.02 (arbitrary units) noise level, indicating no magnetic moment resides on the Mn ions. The sign of the features in XMCD spectra can arise due to the parallel/antiparallel alignment of the Mn moments with respect to the applied magnetic field, as well as a change in the crystal field. For example, the XMCD spectra for Mn2 þ ions in Mn-doped GaAs shows the opposite sign compared to the spectra for MnFe2O4 [25]. In sample (1), the small concentration of Mn2 þ ions are likely found in a defect state where the tetrahedral coordination is created by the formation of oxygen vacancies and lattice distortion, while a majority of the Mn ions have the 3þ or 4þ valence state. The concentration of this defect state decreases but does not completely disappear after a prolonged anneal in an O2 ambient. For sample (2), both the Mn XAS and XMCD results

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XA (a.u.)

T = 300K

MnFe2O4

a

Mn2O3

T = 300K

Fe L3 Fe L2

XA (a.u.)

MnO2

(1) as-grown (1) annealed MnFe2O4 (2) as-grown LSFO

Mn L2

b

A

(1) as-grown C

(1) as-grown (1) annealed (2) as-grown LSMO A

XMCD (a.u.)

÷4

(1) annealed

C B

640

LSMO (2) as-grown

645 650 Photon Energy (eV)

MnFe2O4 ÷2 (2) as-grown

MnFe2O4 (1) as-grown (1) annealed

635

B

XMCD (a.u.)

XA (a.u.)

Mn L3

655

660

Fig. 2. Mn XA spectra acquired at 300 K for (a) MnFe2O4 (Mn2 þ ), Mn2O3 (Mn3 þ ), and MnO2 (Mn4 þ ); (b) sample (1) as-grown and annealed, sample (2) as-grown, and an LSMO film; and (c) XMCD spectra for sample (1) before and after annealing, sample (2) as-grown, an LSMO film, and MnFe2O4 (Mn2 þ ).

are in agreement with previous reports on antiferromagnetic La0.7Ca0.3Mn0.5Fe0.5O3 with TN  50 K [10]. In Fig. 3, the room temperature Fe XA and XMCD spectra for sample (1) before and after annealing, MnFe2O4, as-grown sample (2), and a LSFO film are plotted together for comparison. The fine structure of the Fe XA spectra is sensitive to several factors, such as changes of the lattice parameters and oxygen coordination. Therefore the Fe valence states can be determined more reliably using the Fe L2,3 peak ratio rather than the spectral features [26–29]. For all the samples investigated, the Fe L2,3 peak ratios are similar, indicating a similar Fe valence state near 3þ. After annealing, only a slight change in the spectra could be observed as the shoulder of the L3 peak became a little less distinct. The XMCD spectra sample (1) before and after annealing showed distinct features that resemble the spectra of the ferrimagnetic spinel oxides MxFe3  xO4 (M¼Mn, Zn) [30]. Experimental and multiplet calculations have ascribed the origin of the peaks in the Fe XMCD spectra as peak A from octahedral coordinated Fe2 þ ions, peak B from tetrahedral coordinated Fe3 þ ions, and peak C from octahedral coordinated Fe3 þ ions [31]. After annealing, the relative intensity of peaks A and C did not change, while the relative intensity of peak B was greater than the as-grown sample (1). This result signified that after annealing, the Fe ions entered into the tetrahedral sites while the Mn ions took octahedral sits, as indicated by the Mn XMCD spectra where the peak for the Mn2 þ in the tetrahedral sites decreased in intensity after annealing. No XMCD signal above the 0.02 (arbitrary units) noise level was detected for sample (2) or the LSFO film, as expected for an AFM material with no uncompensated spins. These results indicate that while the bulk of the film had perovskite structure with the Fe and Mn ions in octahedral

LSFO 705

710

715 720 725 Photon Energy (eV)

730

735

Fig. 3. Fe (a) XA and (b) XMCD spectra acquired at 300 K for sample (1) before and after annealing, sample (2) as-grown, an LSFO film on LSAT substrate and MnFe2O4 (Fe3 þ ) (The spectra for ‘sample (1) annealed’ was normalized to peak c of the spectra of ‘(1) as-grown’ in order to analyze the relative change of the A–C peaks).

coordination, when grown under certain deposition conditions, the LSMFO films can possess Fe and Mn ions in tetrahedral coordination with an unexpected magnetic ordering. This magnetic ordering remains after O2 annealing, suggesting that either oxygen vacancies are not the cause of the magnetic ordering or that the oxygen vacancies are not fully satisfied by the O2 anneal. The magnitude of the Mn and Fe XMCD spectra for sample (1) remain nearly unchanged at 150 K and 300 K, indicating that the Curie temperature of this spinel-like phase lies well above 300 K, consistent with reported values for Fe3O4 and MnFe2O4. In XMLD, dichroism results when the E-vector of the linearly polarized x-rays lies parallel/perpendicular to the AFM spin axis of an AFM material. While the Mn XMLD spectra were not conclusive about the AFM nature of the Mn ions, a robust Fe XMLD signal was observed at 150 K for sample (1), which disappeared by 300 K. For sample (2), no clear Mn or Fe XMLD signal was observed at 150 K or 300 K. This result is consistent with previous reports for LSMFO (TN o150 K) [7] and La0.7Ca0.3Mn0.5Fe0.5O3 (TN 50 K) [10]. In order to identify the relationship between the observed magnetic ordering and the strain state of the films, LSMFO films were grown on various single crystal oxide substrates with a target–substrate distance of 8.5 cm. Despite the variation in the tetragonal distortion from 0.987 and 1.007, the XMCD results for the LSMFO films (Fig. S2) showed that irrespective of the strain state, the Mn and Fe XA and XMCD spectra were identical. In addition, the changes to the XMCD spectra after the annealing were nearly identical for the films grown on LSAT and STO substrates. Therefore, the magnetic ordering was not related to the strain state of the LSMFO film. 3.3. Z-contrast imaging and EELS To further investigate the local defect structures and the origin of the unexpected magnetic ordering in the LSMFO thin films grown with the larger target–substrate distance, chemically-sensitive

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The composition analysis using Z-contrast imaging and EELS results suggest that the cations have a non-uniform distribution in sample (1) while they are uniformly distributed in sample (2). This non-uniform distribution could result from the large target– substrate distance (8.5 cm) used during the PLD growth. Under these conditions, the adatoms lose their kinetic energy through interactions with the background gas molecules before arriving at the substrate surface. The low kinetic energy leads to a nonuniform cation distribution because the adatoms with low mobility cannot reach the low energy sites on the substrate surface. A consequence of the non-uniform distribution is the unexpected magnetic ordering more typical of a ferrimagnetic spinel oxide such as MnxFe3  xO4 with Mn and Fe ions in various valence states (Mn2 þ , Fe2 þ /Fe3 þ ) and coordination environments (tetrahedral and octahedral). Annealing in an O2 environment may reduce the concentration of oxygen vacancies in the film, but it cannot transform the film from a cation-segregated state to a homogenously cation-distributed state. Therefore, the ferrimagnetic ordering was observed to persist after the annealing. With a decreased target–substrate distance, the adatoms arrive at the substrate surface with sufficient kinetic energy to reach the low energy sites of a uniform perovskite structure. In this case, no magnetic signal was observed for the Mn or Fe ions, in agreement with an AFM material where the Fe ions have broken up the Mn3 þ –O2  –Mn4 þ double exchange network. Cation segregation effects were also observed after annealing in the LSMFO film grown on STO with the same growth condition as sample (1) as shown by the integrated EELS line scan in Fig. S3.

Z contrast images were obtained for sample (1) after annealing and sample (2) utilizing an aberration-corrected STEM. As shown in Fig. 4(a), the Z-contrast image showed non-uniform contrast, indicating possible stoichiometric variations throughout the film. However, no spinel phase was detected in these images. EELS line scans were taken at various locations of the sample (either near the film surface, in the middle of the film, or near the film–substrate interface) to analyze the composition variations of the film. Fig. 4(b) plots the integrated La M edge, Mn L edge, and Fe L edge intensities in one of the EELS line scans taken near the film surface. A minimum in the La concentration corresponds to the maximum in the Fe concentration, while the Mn concentration did not change substantially with either the Fe or La concentration. Similar results were obtained from other regions of the sample. In comparison, the Z-contrast image for sample (2) in Fig. 4(c) showed uniform contrast, indicating the absence of cation segregation. Furthermore, Fig. 4(d) plots the integrated La M edge, Mn L edge, and Fe L edge intensities in one of the EELS line scans taken along the direction from the film–substrate interface to the film surface. The intensities of these edges vary coherently (increase or decrease together), indicating that these fluctuations result from local thickness variations of the STEM sample. Therefore, a homogenous cation distribution was achieved using a smaller target–substrate distance (5.5 cm) during PLD. Detailed comparison of the Mn and Fe, L2,3 ratios revealed that the Mn ions have a higher valence in sample (2) while the average Fe valence states are the same within measurement error in samples (1) and (2). This result is consistent with the XA spectroscopy results.

1.0

La

Intensity (a.u.)

0.8 0.6 Mn 0.4 Fe

0.2 0.0 0

2

6 4 8 Scan distance (nm)

10

12

1.0

Intensity (a.u.)

La 0.8 0.6 0.4

Mn

0.2

Fe

0.0 0

2

4 6 8 Scan distance (nm)

10

Fig. 4. (a) HAADF Z-contrast image and (b) integrated intensity of the La M edge, Fe L edge, and Mn L edge for sample (1) showing non-uniform concentration profiles due to cation segregation; (c) Z-contrast image and (d) integrated intensity of La M edge, Fe L edge and Mn L edge for sample (2) showing that the cations are homogenously distributed in the film.

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4. Conclusions In conclusion, target–substrate distance and plume dynamics were found to be important tuning parameters in the growth of complex oxides such as La0.7Sr0.3Mn0.5Fe0.5O3 by pulsed laser deposition. With a large target–substrate distance, the kinetic energy of the plume species as well as the quantity of plume species arriving at the substrate surface was reduced through interactions with the background gas molecules, leading to a nonuniform cation distribution. This non-uniform distribution caused variations in the cation valence states and coordination environments, leading to unexpected magnetic ordering. This magnetic ordering was shown to be unrelated to the strain states of the film. A decrease in the target–substrate distance led to a homogeneous cation distribution with no Fe or Mn magnetic signal, in agreement with an antiferromagnetic material. In addition, samples (1) and (2) mimic the situation of non-uniform mixing and close to complete mixing of the interfaces of LSMO/LSFO superlattices, respectively. The differing properties reflect the possible interfacial magnetic ordering of LSMO/LSFO superlattices due to cation intermixing.

Acknowledgments The film growth, XRD characterization, and electron microscopy was supported by the National Science Foundation (Contract no. DMR-747896) and by the Office of Science, Office of Basic Energy Sciences of the US Department of Energy (DOE) (Contract no. DE-FG0203ER46057). The ALS and NCEM are supported by the Office of Science, Office of Basic Energy Sciences of the US DOE under Contract no. DE-AC02-05CH11231.

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jmmm.2012. 08.005.

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