ARTICLE pubs.acs.org/crystal
Tilted Aligned Epitaxial La0.7Sr0.3MnO3 Nanocolumnar Films with Enhanced Low-Field Magnetoresistance by Pulsed Laser Oblique-Angle Deposition Aiping Chen,† Zhenxing Bi,† Chen-Fong Tsai,‡ Li Chen,‡ Qing Su,‡ Xinghang Zhang,§ and Haiyan Wang*,†,‡ †
Department of Electrical and Computer Engineering, Texas A&M University (TAMU), College Station, Texas 77843, United States Materials Science and Engineering Program, Texas A&M University, College Station, Texas 77843, United States § Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843, United States ‡
bS Supporting Information ABSTRACT: Single-phase epitaxial La0.7Sr0.3MnO3 (LSMO) thin films with significantly enhanced low-field magnetoresistance (LFMR) properties (23% at 20 K) are demonstrated in this work. The LSMO films on SrTiO3 (001) substrates exhibit tilted and well-aligned nanocolumn structure achieved by pulsed laser oblique-angle deposition (PLOAD) followed by subsequent postannealing. The tilted aligned nanocolumnar (TAN) arrays have been achieved at relative high deposition angles (g30o) and low deposition temperatures (e450 °C). More attractively, the tilted grain boundaries (GBs) can be systematically manipulated by the postannealing process and so can the LFMR values of the LSMO TAN films. These results demonstrate that the tilted nanocolumnar films achieved by PLOAD and the GB tailoring by postannealing may provide a new approach to control and manipulate the magnetotransport properties of single-phase manganite perovskite films for device applications that require large LFMR effects, high epitaxial quality, and low resistivity.
’ INTRODUCTION The discovery of colossal magnetoresistance (CMR) in pervoskite manganites opens up a new avenue for memory device applications.1 Nevertheless, the practical utilization of the intrinsic CMR effect has been limited by the required high magnetic fields of several teslas.1 Both experimental and theoretical work has demonstrated that grain boundaries (GBs) could induce a large low-field magnetoresistance (LFMR) effect in the low magnetic field (H e 1 T) regime.2 Conventionally, single-phase epitaxial manganite films exhibit negligible LFMR effects, and the LFMR effects can be achieved by preparing polycrystalline bulk composites or two-phase nanocomposite films.3,4 However, doping a secondary phase, especially an insulating phase, at the GBs of the ferromagnetic (FM) grains, on one hand, could dramatically increase the resistivity of the composite by several orders of magnitude compared with the epitaxial counterpart;3 on the other hand, the incorporation of a nonmagnetic phase could reduce the magnetic properties of the manganite-based bulk composites and thin films, and thus the LFMR effect could decrease rapidly with the increase of temperature, resulting in a poor LFMR effect at room temperature.4 To overcome these drawbacks, one effective way is to explore new approaches to design GBs in single-phase pervoskite manganite thin films to achieve large LFMR with high epitaxial quality and low resistivity. GBs are regions of structural disorder, which are naturally generated during thin film deposition, and they play a significant role in determining the electrical and magnetic properties of thin films. Designing the GBs has been demonstrated by using r 2011 American Chemical Society
bicrystal substrates, step-edged substrates, and nanowireengineered substrates.5,6 However, such substrate treatments will significantly increase the resistivity of the films. Here, we propose to engineer the GBs by using oblique-angle deposition (OAD), which has been developed as a method to design nanostructured films.7 Arrays of unique nanostructures have been created including tilted nanopillars,8 arc-shaped nanowires,9 nanocolumns,10 and helices.11 Because of their large interfacial areas, these nanostructures could have extensive potential applications in solar cells,12 sensors,13 and water splitting.14 In this work, we implement tilted nanocolumns and GBs manipulation in La0.7Sr0.3MnO3 (LSMO), a typical manganite with Curie temperature (TC) above room temperature, using pulsed laser oblique-angle deposition (PLOAD) and postannealing. The PLOAD method has been used to grow nanostructured porous carbon and YBa2Cu3Ox films.15 Through adjustment of the deposition angles and temperatures as well as the annealing conditions, it is possible to control the nanocolumn tilting angle and GB density in the films and thus achieve enhanced and tunable LFMR. An illustration of the PLOAD configuration and the formation of tilted aligned nanocolumnar (TAN) films by PLOAD are shown in Figure 1, panels a and b, respectively. In these experiments, the plasma plume transports toward the tilted substrates with a deposition angle α, varied from zero (i.e., the plume Received: August 2, 2011 Revised: October 3, 2011 Published: October 17, 2011 5405
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Figure 2. Cross-sectional TEM images of as-deposited LSMO films: (a) TS = 450 °C and α = 70°, (b) TS = 450 °C and α = 85°, and (c) TS = 600 °C and α = 70°. The insets are the corresponding SAED patterns. Figure 1. (a) Schematic of the pulsed laser oblique-angle deposition configuration (PLOAD). (b) The formation of TAN films by PLOAD method.
is normal to the substrate) to 85 degree (the plume is almost parallel to the substrate). The deposition angle α is the angle between the substrate normal with the plume, and the nanocolumn tilting angle β is the angle between the substrate normal with the nanocolumn, as shown in Figure 1b. Under certain deposition configurations, the PLOAD technique could produce films that consist of separated and tilted nanocolumns resulting from strong self-shadowing and limited adatom surface diffusion effects.
’ EXPERIMENTAL SECTION A LSMO target was obtained by the conventional ceramic sintering process. TAN LSMO thin films were deposited on single-crystal STO (001) substrates by PLOAD with a KrF excimer laser (Lambda Physik, λ = 248 nm, laser deposition frequency 10 Hz). The laser beam was focused onto the target with an incidence angle of 45°, obtaining an energy density of ∼4.5 J/cm2. Before deposition, the chamber was pumped to a base pressure of 1.0 106 Torr or greater. A series of LSMO nanostructured films were deposited over a substrate temperature (TS) ranging from room temperature to 600 °C and an oxygen pressure of 100 mTorr was maintained during the deposition. The substrate configuration is shown in Figure 1 as above-mentioned. After deposition, the films were cooled to room temperature at 10 °C/min in an oxygen pressure of 200 Torr. In order to demonstrate the relationship between the GB and magnetotransport properties, the as-deposited films were ex-situ annealed at 550850 °C in air for 30 min to ensure good oxygen stoichiometry, increase the crystallinity, and manipulate the GBs. The microstructural characteristics of the as-deposited and postannealed films were investigated by XRD (BRUKER D8 powder X-ray diffractometer) and TEM (JEOL JEM-2010 analytical microscope). The high-resolution TEM images were taken with an FEI Tecnai G2 F20 operated at 200 kV. Fast Fourier transform patterns were obtained through GATAN Digital Micrograph software to reveal the edge dislocation along GBs. Cross-sectional samples for TEM analysis were prepared by a standard manual grinding and thinning procedure followed by a final polishing step in a precision ion polishing system (PIPS 691, Gatan). Au electrodes deposited by sputtering were used for the magnetotransport measurements. The magnetoresistance properties in
a temperature range of 20350 K and a magnetic field up to 1 T were measured by a commercial physical properties measurement system (PPMS 6000, Quantum Design) in the van der Pauw geometry. During the magnetotransport measurements, the magnetic field was applied perpendicular to the film plane and the electrical measurements were performed in-plane. Magnetic properties were recorded by the vibrating sample magnetometer (VSM) option in the PPMS. The magnetizations were measured out-of-plane by applying a magnetic field perpendicular to the film plane.
’ THEORETICAL BASIS The resistivity of the TAN LSMO films is determined by the LSMO grains and the GB regions. Therefore, the resistivity of this system in the whole temperature regime can be simulated by the parallel connection channel model,16 1 G 1 ¼ þ F Fgb ð1 f ÞFpm þ f Ffm
ð1Þ
where G stands for a factor that rates the relative contribution from the GB and nanocolumn, Fgb, Fpm, and Ffm are resistivity for grain boundary, paramagnetic (PM) domains, and ferromagnetic (FM) domains, respectively, and f is volume fraction of the FM domain in the LSMO nanocolumn. The first term on the right side describes the contribution of GBs, and the second term represents the contribution of nanocolumns, which consist of PM and FM domains, depending on temperature. Here Fgb and Fpm, following a thermal activation law, are given by Fgb = exp(ΔEgb/(kBT)) and Fpm = exp(ΔEpm/(kBT)), respectively. ΔEgb and ΔEpm are the effective activation energy for the GB and the PM domains in LSMO tilted nanocolumns, respectively. kB is Boltzmann’s constant, and T is the absolute temperature. FM domains show a metallic behavior, and FFM is given by FA + FBT2.5.17 The parameter FA corresponds to the residual resistivity arising from defect scatterings. The volume fraction of the FM domain in the LSMO nanocolumn is phenomenologically defined by FermiDirac distribution with f = (1 + exp(U(1 T/TC)/(kBT)))1.18
’ RESULTS AND DISCUSSION To reveal the microstructure of the TAN LSMO films by PLOAD method, a detailed cross-sectional TEM study was 5406
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Figure 3. Nanocolumnar film distribution under different deposition parameters. Region 1 (red) represents the TAN film structure, and region 2 (green) indicates the continuous film structure.
performed for all the samples deposited under different deposition angles. The low-magnification TEM images and the corresponding selected area electron diffraction (SAED) patterns are shown in Figure 2 for typical samples deposited at (a) TS = 450 °C and α = 70°, (b) TS = 450 °C and α = 85°, and (c) TS = 600 °C and α = 70°. It can be seen that well-ordered and tilted aligned nanocolumns are self-assembled on STO substrate over a large area for the samples deposited at 450 °C (Figure 2a,b). The nanocolumn tilting angle β increases from ∼45o to ∼65o when the deposition angle α changes from 70° to 85°. The column tilt angle β and vapor incident angle α in this case do not exactly follow the empirical tangent rule or the cosine rule probably because the bombardment- and thermal-induced mobility could favor the surface diffusion effect. Thus, the growth of TAN LSMO films is most likely located in zone T of the structure zone model.19 As seen in Figure 2a,b, there are obvious gaps existing between the neighboring columns because the shadowing effect and limited adatom surface diffusion at relatively low TS are still dominating factors in growth. It is interesting to note that, compared with the TAN films deposited at 450 °C, the LSMO films grown at 600 °C present a more continuous film structure similar to standard PLD epitaxial LSMO films. Figure 2c exhibits no or little tilted GBs and shows very distinguished diffraction dots in the SAED pattern (inset in Figure 2c). This suggests that higher deposition temperatures lead to surface diffusion dominated growth and thus limit the formation of the tilted nanocolumns. In addition, from the SAED and XRD results (Figure S1, Supporting Information) of all the above films, the orientation relations between the tilted films and the substrates are determined to be perfect cube-on-cube relations, that is, (001)LSMO//(001)STO and [100]LSMO//[100]STO. Based on the detailed TEM study, the distribution of film morphology as a function of substrate temperature, TS, and deposition angles, α, is summarized in Figure 3. It can be seen that the nanocolumnar films can be mainly obtained at relative low substrate temperatures (TS e 450 °C) and large deposition angles (α g 30°) (region 1). The ad-atoms propagate toward substrates with certain kinetic energy. When the substrate temperature is relative low (TS e 450 °C in this case), the selfshadowing and limited adatom surface diffusion effects could dominate the growth and result in the formation of TAN films. However, the ad-atoms will possess high surface mobility, which allows the ad-atoms to diffuse effectively if the TS is high enough (>500 °C). In this case, the high adatom mobility could eliminate the self-shadowing effect and lead to the absence of GBs in LSMO films despite the deposition angle (region 2). In addition, the self-shadowing effect is highly limited at low angle deposition (α < 30°). Thus, the effective TAN films are mainly obtained
Figure 4. (a) Temperature dependence of resistivity of TAN LSMO film measured at magnetic field of 0 T (black line) and 1 T (red line) for the 650 °C annealed sample, the open triangle (Δ) represents the MR measured at 1 T. [MR(%) = [(F0 FH)/F0] 100%, where F0 and FH are the resistivity under a zero field and magnetic field, respectively]. (b) Field dependence MR measurement at 20 K for the 650 °C annealed sample. This isothermal MR solid curve (red) shows a butterfly profile. The open circles (blue) represents the well-defined magnetic hysteresis loop for this sample measured at the same temperature.
under low deposition temperatures and large deposition angles, as shown in region 1 of Figure 3. To directly correlate the structureproperty relationship in the TAN LSMO films, we conducted detailed magnetotransport property measurements for all samples. The as-deposited LSMO nanocolumnar films usually exhibit semiconductor-like behavior with a very large resistivity and a very limited MR effect (Figure S2a, Supporting Information), mainly due to poor GB connection (Figure 2a). Therefore, GB manipulation by postannealing is necessary to improve the connection of the FM grains, lower the resistivity, and enhance the probability of electron tunneling through the GBs. A specific sample, that deposited at 450 °C (α = 70°), was selected to demonstrate the correlation between magnetotransport properties and annealing effects. The FT characteristics of 650 °C annealed TAN LSMO films measured at 0 and 1 T are shown in Figure 4a. A welldefined metalinsulator transition (MIT) can be observed in the annealed TAN LSMO films with the MIT temperature, TMI, around 260 K. More interestingly, the 650 °C annealed TAN film has a quite low resistivity (∼7 mΩ 3 cm at room temperature), which is 24 orders of magnitude lower than that of bulk composites, nanocrystalline films, and nanocomposite films.3 The annealed TAN LSMO film shows a MR value of 23% at 20 K and 5% at 300 K. The MR value was confirmed by isothermal MR measurement (solid curve in Figure 4b). Table 1 compares the resistivity and MR values for the TAN LSMO films with other composite systems.4 It suggests that the single-phase TAN LSMO films exhibit not only much lower resistivity but also better LFMR properties compared with other bulk composites and nanocomposite thin films. The larger LFMR in TAN LSMO films can be attributed to spin-polarized tunneling of the conduction electrons through the fine tilted GBs, created by the self-shadowing effect during the PLOAD. To the best of our knowledge, this is the first report of single-phase LSMO films with large LFMR effect, low resistivity, and high epitaxial quality through the tilted aligned nanocolumnar structure. 5407
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Table 1. Comparison of the MR and G in TAN LSMO Films with Other Composites at a Magnetic Field of 1 T max MR
MR
F (300 K)
(1 T) [%]
(300 K) [%]
[mΩ 3 cm]
LSMO/ZnO LSMO/CeO2
∼15 ∼21
∼3
