JOURNAL OF APPLIED PHYSICS 111, 013911 (2012)
The effect of interfacial charge transfer on ferromagnetism in perovskite oxide superlattices F. Yang,1 M. Gu,1 E. Arenholz,2 N. D. Browning,1,3 and Y. Takamura1,a) 1
Department of Chemical Engineering and Materials Science, University of California, Davis, Davis, California 95616, USA 2 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 3 Department of Molecular and Cellular Biology, University of California, Davis, Davis, California 95616, USA
(Received 5 August 2011; accepted 2 December 2011; published online 5 January 2012) The structural, magnetic, and electrical properties of superlattices composed of the ferromagnetic/ metal La0.7Sr0.3MnO3 and non-magnetic/metal La0.5Sr0.5TiO3 grown on (001)-oriented SrTiO3 substrates have been investigated. Using a combination of bulk magnetometry, soft x-ray magnetic spectroscopy, and scanning transmission electron microscopy, we demonstrate that robust ferromagnetic properties can be maintained in this superlattice system where charge transfer at the interfaces is minimized. Therefore, ferromagnetism can be controlled effectively through the C 2012 American Institute chemical identity and the thickness of the individual superlattice layers. V of Physics. [doi:10.1063/1.3674325]
INTRODUCTION
Perovskite oxides possess intriguing and technologically important physical properties such as ferromagnetism, superconductivity, and ferroelectricity.1 Extensive efforts have been dedicated to exploit the interactions which occur at and across interfaces in perovskite oxide superlattices composed of alternating sublayers.2–4 These superlattices can possess enhanced or completely different properties compared to the individual constituent materials resulting from interfacial effects, including octahedral distortions, tilts, and rotations, electronic reconstruction, atomic intermixing, exchange coupling, and finite size effects.2,5–8 La0.7Sr0.3MnO3 (LSMO) is an attractive candidate for spintronic devices because it displays colossal magnetoresistance as well as half-metallicity, and possesses a Curie temperature, TC, above room temperature.9,10 In this material, the TC marks the transition between the ferromagnetic (FM)/metallic and the paramagnetic (PM)/ insulating states, as well as the maximum in magnetoresistance (MR).9 This correlation between the electrical and magnetic properties is explained by the double-exchange mechanism, which involves the hopping of electrons along Mn3þ – O2 Mn4þ chains and Jahn-Teller distortions.11 Due to the strong interaction between the charge, orbital, lattice, and spin degrees of freedom, these properties are extremely sensitive to interfacial effects which occur readily in superlattice structures. Previous studies on manganite (La1x(Sr/Ca)xMnO3) superlattices have shown that a charge transfer occurs over a ˚ ) at the distance of approximately three unit cells (12 A manganite interfaces. This charge transfer has been reported in a number of different superlattice systems, including LSMO/SrTiO3 (STO),7,12,13 LSMO/La0.7Sr0.3 FeO3,14,15 and La0.7Ca0.3MnO3/YBa2Cu3O7,16 and it occurs due to differena)
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ces in the valence states of the B site elements of the perovskite (ABO3) structure within each layer and/or affinity of these elements for a particular valence state. These valence state differences, canted spins at the interfaces, and magnetic phase separation in ultrathin manganite layers have been postulated as possible sources of a surface or interface layer with suppressed magnetization and Curie temperature.4,12,17,18 In addition, Bruno et al.13 reported that in LSMO/STO superlattices with STO layer thicknesses below 1 nm, an interfacial charge transfer occurs such that the Ti valence state is reduced from 4.0þ to 3.7þ, leading to a small Ti moment with an orientation antiparallel to the neighboring Mn ions. In this work, superlattices consisting of LSMO and the non-magnetic metal, La0.5Sr0.5TiO3 (LSTO), were investigated. This system serves as a model system where the charge transfer effect is minimized by a nearly uniform Sr doping across both sublayers and the relative affinity of the Mn and Ti ions for the 3þ and 4 þ valence states. The resulting effect on the magnetic and magnetotransport properties is discussed. EXPERIMENT
Epitaxial superlattices composed of La0.7Sr0.3MnO3 (LSMO, rhombohedral with pseudocubic lattice parameter ˚ ) and La0.5Sr0.5TiO3 (LSTO, orthorhombic with a ¼ 3.873 A ˚ )19 were grown on (001)-oriented pseudocubic a ¼ 3.93 A STO substrates by pulsed laser deposition (PLD). A KrF laser (248 nm) operated at 3 Hz was used with a laser energy density of 0.7 J/cm2 while the substrate temperature was held at 700 C. The LSTO sublayers were grown in vacuum with a pressure of 106 Torr. A buffer layer of LSMO with a thickness of one unit cell was grown in a vacuum before the growth of the LSMO sublayers in an oxygen pressure of 500 mTorr. After deposition, the superlattices were cooled slowly to room temperature in an oxygen pressure of 300 Torr to ensure the proper oxygen stoichiometry of the
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LSMO sublayers. A nearly constant Sr concentration throughout the superlattice minimizes the amount of Sr/La interdiffusion at the interfaces. The notation for the superlattices consists of the following: [(LSMO)n/(LSTO)m]r where n and m are the number of unit cells in each sublayer and r is the number of repeats. The LSTO sublayer was grown first, so that the LSMO sublayer lies at the surface of the superlattice. The total thickness for the LSMO/LSTO superlattices is approximately 57.4 nm and the number of unit cells of the LSMO sublayer is twice as that of the LSTO sublayer. RESULTS AND DISCUSSION
Structural characterization was performed using a combination of x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). X-ray reflectivity and high-resolution XRD were performed using a Bruker D8 Discover four-circle diffractometer and beamlines 2-1 and 7-2 at the Stanford Synchrotron Radiation Lightsource (SSRL). STEM and electron energy loss (EEL) spectroscopy were collected using the TEAM0.5 microscope at the National Center for Electron Microscopy (NCEM) operating at 80 kV to avoid beam damage. The samples were prepared by Multiprep wedge polishing, i.e. two samples were attached to one another and polished to around 1 lm thickness, then ion milled (5 kV/5 mA/10 incidence angle) to electron transparency. A final ion mill step (1 kV/5 mA/10 incidence angle) was used to clean the samples. The bulk magnetic properties were characterized using a Quantum Design Magnetic Property Measurement System (MPMS) superconducting quantum interference device magnetometer, with the magnetic field, H, applied along the in-plane h100i and h110i substrate direction. The magnetotransport measurements were performed in the MPMS using a two-probe configuration with H ¼ 0.00 T and 0.75 T applied along the in-plane [100] substrate direction. Additional magnetic characterization of the top 5-10 nm (1-2 repeat units) of the superlattices was carried out using x-ray magnetic circular dichroism (XMCD) at beamline 6.3.1 of the Advanced Light Source (ALS). The x-ray incident angle was 30 relative to the sample surface. The XMCD signal was calculated as the difference between two x-ray absorption (XA) spectra taken with Ha ¼ 1.0 T parallel/anti-parallel to the x-ray helicity. In Fig. 1, the XRD L scans around the out-of-plane 002 reflection show the presence of satellite peaks and thickness fringes attesting to the smooth interfaces and the periodic nature of the superlattice structures. The 0th order superlattice peak reflects the average value of the out-of-plane lattice parameters for the LSMO and LSTO sublayers, cave. This peak overlaps with the 002 reflection from the STO substrate ˚ . Reciprocal space maps (not indicating that cave ¼ 3.905 A shown) show that the superlattices are fully strained to the ˚ ). underlying STO substrate (i.e. aLSMO ¼ aLSTO ¼ 3.905 A Therefore, the LSMO sublayers exist under tensile strain, while the LSTO sublayers are under compressive strain. Furthermore, high angle annular dark field (HAADF) STEM images (Figs. 2(a) and 2(b)) confirm the sublayer thicknesses and show that the superlattices have sharp interfaces and are free of structural defects. EEL spectroscopy maps (insert in
FIG. 1. (Color online) XRD L scans around the out-of-plane 002 reflection for the LSMO/LSTO superlattices.
Fig. 2(b)) and integrated Ti and Mn L edge spectra (Fig. 2(d)) show that the interfaces are characterized by an interdiffused region with less than 2 unit cell thickness. Figure 3(a) plots the temperature dependence of the bulk magnetization for the LSMO/LSTO superlattices. The data for a 40 nm thick LSMO thin film are plotted for comparison. The magnetization has been normalized to the total thickness of the LSMO sublayers assuming that the LSTO sublayers remain non-magnetic as in bulk material. The LSMO thin film exhibits a bulk-like TC of 340 K and saturation magnetization, MS of 605 emu/cm3 at 10 K. Decreasing TC and MS is observed with decreasing sublayer thickness as listed in Table I. Qualitatively, this observation for our LSMO/LSTO superlattices agrees with literature reports for ultrathin LSMO films, as well as LSMO/STO and LSFO/LSMO superlattices (also listed in Table I).7,12–14,17,20–24 It has been reported that for optimal growth conditions, the magnetic and magnetotransport properties deviate from bulk-like behavior for thicknesses below 10-13 unit cells with a complete loss of magnetization below 3 unit cells. While the TC values for all systems grown by PLD14,21 are comparable for a given LSMO sublayer thickness (e.g. 110 -150 K for LSMO sublayer thickness of 5-6 unit cells), the LSMO/LSTO system presented here retains higher MS values at the lowest measurement temperatures (e.g. 320 emu/cm3 versus 90 emu/cm3 for the LSFO/LSMO system14 versus 160 emu/cm3 for a 5 unit cell LSMO film).21 The samples grown by high pressure pure oxygen sputtering13 show slightly higher MS and TC values, suggesting a strong sensitivity of the functional properties to the details of the growth process in addition to the chemical identity of each sublayer. Figure 3(b) shows the hysteresis loops taken at 45 K with the magnetic field applied along the in-plane h100i substrate direction. The bulk magnetic measurements show that the easy magnetization direction for all superlattices lies along the in-plane h110i direction, in agreement with published results for LSMO films grown on (001)-oriented STO substrates as a consequence of the 0.64% lattice mismatch.25 In the LSMO/LSTO system, the coercive field, HC, values are extremely small (e.g. 1.8 mT for the
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FIG. 2. (Color online) (a) HAADF image showing the overview of the [(LSMO)12/(LSTO)6]8 superlattice. (b) Atomic resolution HAADF image; the upper panel shows the original image and the bottom panel is after noise filtering. The inset shows the local Ti and Mn EELS maps. The Ti EELS map was obtained by integrated Ti L edge with 16 eV energy window while the Mn EELS map was obtained by integrating the Mn L edge with 20 eV energy window. (c) Atomic resolution image showing the region where the EELS line scans were acquired. (d) Integrated Ti and Mn L edge EELS line scans after background subtraction. (e) Ti L edge EEL spectra collected from spots a - c and the STO substrate. (f) Mn L edge EEL spectra collected from spots 1, 2, and 3.
[(LSMO)12/(LSTO)6]8 superlattice at 45 K) and they are only weakly dependent on sublayer thickness, similar to reported values for ultrathin LSMO films and LSMO/STO superlatti-
FIG. 3. (Color online) (a) Magnetization as a function of temperature for the LSMO/LSTO superlattices and a 40 nm thick LSMO thin film. A magnetic field of 0.01 T is applied along the in-plane substrate direction. (b) Hysteresis loops of the LSMO/LSTO superlattices taken at 45 K.
ces.12,21 The [(LSMO)6/(LSTO)3]16 superlattice has a small remnant magnetization with little hysteresis. In contrast, previous results for [(LSFO)6/(LSMO)6]10 superlattices displayed much larger HC values (170 mT at 55 K) due to the exchange interactions in the form of spin-flop coupling between the LSFO and LSMO sublayers which occur in this range of sublayer thickness.26 Therefore, the magnetic properties of ultrathin LSMO sublayers in superlattice structures depend not only on the sublayer thickness, but also on the nature of the adjacent layers and the types of interactions that occur between them. The magnetotransport properties as a function of temperature for the LSMO/LSTO superlattices are shown in Fig. 4. The resistivity is calculated from the resistance using the simplified assumption that the current flows equally through the LSMO and LSTO sublayers, while the magnetoresistance is given by MR ¼ ½qðH ¼ 0:75T Þ qðH ¼ 0T Þ=qðH ¼ 0T Þ 100. The resistivity curves for 40 nm thick LSMO and LSTO thin films are also included for comparison. A well-defined metalinsulator transition is observed at TC for the [(LSMO)12/ (LSTO)6]8 superlattice and it is accompanied by a minimum in MR. Compared to the LSMO thin film, the critical temperature has dropped by 100 K and the resistivity has increased by nearly two orders of magnitude. The critical temperature from the magnetotransport measurements is in good agreement with the FM/PM transition obtained from bulk magnetometry measurements. In contrast, the [(LSMO)6/(LSTO)3]16 superlattice exhibits purely insulating behavior over the temperature range studied with a pronounced minimum in MR around 125 K, and a further increase in the resistivity by an order of magnitude over the [(LSMO)12/(LSTO)6]8 superlattice at room temperature. Similar magnetotransport properties for the LSFO/LSMO system indicates that the electrical resistivity is dominated by the properties of the ultrathin LSMO sublayers.14
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TABLE I. Magnetic properties of LSMO thin films with varying thickness, LSMO/STO, LSFO/LSMO, and LSMO/LSTO superlattices.
Sample LSMO thin film (5 u.c.) [a] LSMO thin film (8 u.c.) [a] LSMO thin film (40 nm) [(LSFO)6/(LSMO)6]10 [c] [(LSMO)6/(STO)7]8[d] [(LSMO)6/(LSTO)3]16 [(LSMO)12/(LSTO)6]8
MS [emu/cm3]
TC [K]
HC at 45 K [mT]
MR at TC [%]
160 390 605 90 400 320 380
110 270 340 150 200 125 240
32.5 16.0 7 170 10 0.6 1.8
-– –33 [b] 69 [b] –38.5 15
a
Ref. 21. MR measured for H ¼ 0 T and 5 T. c Ref 14. d Ref 13. b
This result also suggests a large deviation from the metallic behavior of bulk LSTO for the small sublayer thicknesses and particular deposition conditions used in these superlattices. The bulk magnetic properties of the superlattices were compared to the properties from the top 5-10 nm (1-2 repeat units) measured using soft x-ray magnetic spectroscopy. Figure 5 plots the XA and XMCD spectra taken at 45 K around the Mn L3,2 edges for the LSMO/LSTO superlattices as well as a bulk LSMO sample. The surface magnetic properties follow the same trend of decreasing XMCD signal with decreasing LSMO sublayer thickness. However, the XMCD signal from the [(LSMO)6/(LSTO)3]16 superlattice decreases more rapidly than the bulk magnetization. This
FIG. 4. (Color online) (a) Resistivity and (b) magnetoresistance, MR as a function of temperature for the LSMO/LSTO superlattices. 40 nm thick LSMO and LSTO thin films are also included for comparison.
FIG. 5. (Color online) (a) Mn XA and (b) XMCD spectra for LSMO/LSTO superlattices and a bulk LSMO sample.
result may suggest that at a thickness of 6 unit cells, the magnetic properties of the topmost LSMO sublayer is more sensitive to not being surrounded symmetrically by an LSTO sublayer on both sides. No detectible moment above the þ/ 0.03 (arb. units) noise level of the XMCD measurement was observed in the Ti XMCD spectra for either LSMO/ LSTO superlattice, indicating that the magnetization lies solely on the Mn ions. In contrast, for the manganite/STO systems,8,13 the Ti moments were shown to be oriented opposite to the magnetization of the Mn moments and resulted from the charge transfer at the interface. The XA spectra provide information concerning the Mn valence states of the superlattices. The spectra (Fig. 5(a)) for the LSMO/LSTO superlattices and LSMO single layer film are nearly identical regardless of repeat period and resemble the expected spectra for the 2:1 ratio of Mn3þ:Mn4þ due to the Sr doping level. This behavior differs from the LSFO/ LSMO system14 which showed the clear signature of an increased Mn4þ concentration (a shift of the main Mn L3 peak to higher photon energy and the appearance of an additional peak at 2 eV below the main peak) as the sublayer thickness decreased. These signatures indicate that despite the uniform Sr doping across the interface, an electron transfers from the Mn3þ to Fe3þ þ ligand hole state due to the greater affinity of the Mn ions for the 4þ valence states.14 The characterization of the valence states was obtained on a local scale using atomic resolution EEL spectra (Figs. 2(d)–2(f)). Figure 2(d) plots the integrated Ti and Mn L edge spectra after background subtraction, acquired in the region shown in Fig. 2(c). Individual Ti and Mn EEL spectra taken on an atomic column basis are shown in Fig. 2(e) and 2(f), respectively. Columns ‘a’ and ‘b’ lie within a LSTO sublayer, while column ‘c’ lies at the interface region. Similarly,
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columns ‘1’ and ‘3’ lie at the interface region, while column ‘2’ lies within a LSMO sublayer. The Ti EEL spectra were normalized to the L2 peak intensity while the Mn EEL spectra were normalized the L3 peak intensity. For the Ti EEL spectra, no evidence of peak shifts was observed for the spectra acquired within the superlattice curves a-c in Fig. 2(e). These spectra are shifted toward lower energy by 0.3 eV compared to the STO substrate with little spectral difference. Using the method previously demonstrated for the manganite/STO system,13,27 these results, together with the O K edge EEL spectra (not shown) were used to determine the Ti valence states. Based on the peak separation between the O K edge pre-peak and the second main peak, the Ti valence state was determined to be constant at 3.47þ (60.13) throughout the superlattice, consistent with the La0.5Sr0.5TiO3 stoichiometry. Similarly for the Mn EEL spectra, the L3/L2 ratio28 indicates that the average Mn valence state remains constant at 3.25þ (60.06) throughout the superlattice structure. Therefore, these results demonstrate that charge transfer does not occur at the LSTO/LSMO interfaces. As a result, no moment develops on the Ti ions and robust ferromagnetic properties are preserved in ultrathin LSMO sublayers. CONCLUSION
In conclusion, we have investigated the magnetic and magnetransport properties of LSMO/LSTO superlattices where charge transfer across the interfaces has been minimized. Due to the Sr doping in the individual layers, the Mn ions in the LSMO sublayers have an average valence of 3.25 (60.06) and the Ti valence state in the LSTO sublayers is 3.47þ (60.13). By minimizing the charge transfer at the interfaces, no magnetization is observed on the interfacial Ti ions and robust ferromagnetic properties (higher saturation magnetization and similar Curie temperature) are retained compared to other superlattice systems where charge transfer occurs readily. This work highlights the importance of the proper choice of the chemical composition of each sublayer in order to tune the functional properties of superlattices. ACKNOWLEDGMENTS
The authors thank Matt Bibee and Apurva Mehta (SSRL) for their assistance with acquiring the XRD data. The work performed at NCEM and ALS was supported by the Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy (DOE) under Contract No. DE-AC02-05CH11231. Portions of this research were carried out at SSRL, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. DOE by Stanford University. The growth and characterization work at U.C. Davis was funded by the National Science Foundation Award DMR-0747896 and the
J. Appl. Phys. 111, 013911 (2012)
electron microscopy by the DOE, Office of Basic Energy Sciences, Division of Materials Science and Engineering under Contract No. DE-FG0203ER46057. 1
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