Microstructure, Magnetic, and Magnetoresistance Properties of La0

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Microstructure, magnetic and magnetoresistance properties of La0.7Sr0.3MnO3: CuO nanocomposite thin films Meng Fan, Han Wang, Shikhar Misra, Bruce Zhang, Zhimin Qi, Xing Sun, Jijie Huang, and Haiyan Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17398 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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ACS Applied Materials & Interfaces

Microstructure, magnetic and magnetoresistance properties of La0.7Sr0.3MnO3: CuO nanocomposite thin films Meng Fan1, Han Wang1, Shikhar Misra1, Bruce Zhang2, Zhimin Qi1, Xing Sun1, Jijie Huang1, Haiyan Wang1,2* 1

School of Materials Engineering, Purdue University, West Lafayette, IN, 47907, USA.

2

Department of Electrical and Computer Engineering, Purdue University, West Lafayette,

IN, 47907, USA. *

Contact Author: [email protected]

Abstract (La0.7Sr0.3MnO3)0.67: (CuO)0.33 (LSMO: CuO) nanocomposite thin films were deposited on SrTiO3 (001), LaAlO3 (001) and MgO (001) substrates by pulsed laser deposition and their microstructure, magnetic and magnetoresistance properties were investigated. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that LSMO: CuO films grow as highly textured self-assembled vertically aligned nanocomposite (VAN), with a systematic domain structure and strain tuning effect based on the substrate type and laser deposition frequency. A record high low-field magnetoresistance (LFMR) value of ~80% has been achieved in LSMO: CuO grown on LaAlO3 (001) substrate under high frequency. Detailed analysis indicates that both the strain state and the phase boundary effect play a significant role in governing the overall LFMR behavior. Key words: La0.7Sr0.3MnO3: CuO, vertically aligned nanocomposite (VAN), low field magnetoresistance (LFMR), epitaxy, thin films

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Introduction Metallic ferromagnetic materials have received much interest because of the technological importance in the emerging field of spintronics. Among those, manganite with the chemical formula of A1-xRxMnO3, in which A-cations are typically La, Pr, and Nd and R-cations are Sr, Ca, Ba, is especially an important family of such compounds. The colossal magnetoresistance (CMR) induced by “double-exchange” mechanism in manganites also draws intensive research.1-3 In addition to the intrinsic CMR phenomenon, extensive efforts are being devoted to explore their extrinsic low field magnetoresistance (LFMR) properties for practical applications recently.4-7 It has been demonstrated that the MR behavior in La1-xSrxMnO3 (LSMO) at much lower magnetic fields could be obtained by natural or artificially introduced grain/phase boundaries, where spin polarized tunneling/scattering might occur. Incorporating with a secondary phase has shown to be an effective way to improve the LFMR in both composite bulk and thin films.1,4,8,9 Enhanced LFMR have been obtained in CMR/insulator nanocomposites of LSMO: CeO2,10-12 LSMO: ZnO,13,14 LSMO: Mn3O4,15 LSMO: NiO,4 LSMO: MgO,5 etc., in different temperature ranges and magnitude, benefitted from a secondary phase and phase boundary effect. Meanwhile, for manipulating the magnetic and transport properties of the LSMO perovskite materials, strain has been proven as an alternative effective approach, with the capability of modifying the Mn-O-Mn bonds angle and length. Reports have shown that the Curie temperature Tc and electrical transport properties of La0.7Sr0.3MnO3 (LSMO) thin films (thickness of 22 nm) strongly depend on the substrate induced biaxial strain16. Recently, two phase vertically aligned nanocomposites (VAN) have been developed,


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providing an effective way to induce high density of vertical phase boundaries, and simultaneously couple the strains in the 2 phases that are epitaxially grown on a substrate together.17-18 Various VAN morphologies can be achieved, including nanopillars-inmatrix,10,19,20 nanocheckerboard structures,21 and other domain structures,13,23 depending on the materials selection and composition. Enhanced LFMR has been observed in highly strained VAN systems such as LSMO:CeO2,10,11 LSMO: MgO5,8 and LSMO: ZnO.13,14 The enhanced LFMR properties were attributed to the strong vertical interface strain coupling built in the vertical coupled interfaces in VAN as well as the phase boundary effects induced by secondary insulating phases (e.g., CeO2, MgO and ZnO, for the above systems). In this work, a new secondary phase of CuO has been explored in the LSMObased nanocomposite system with the goal to enhance the LFMR property further. CuO is a narrow bandgap semiconductor with Eg of ~1.2 eV,24 and an antiferromagnet with Neel temperature (TN) of ~230 K.25 With a monoclinic crystal structure with the lattice parameters of a = 4.6837 Å, b = 3.4226Å, c = 5.1288Å, with β = 99.54°,26 CuO was reported to be able to grow highly textured on various oxide substrates.27,28 CuO has not been used as secondary insulating phase for MR property enhancement of manganite nanocomposite thin films. Furthermore, the phase immiscibility between LSMO and CuO, as well as the chemical compatibility allow the possibility of LSMO: CuO vertically aligned nanocomposite structure. This leads to a strong vertical interface coupling between the two phases which leads to enhanced properties. In this article, the epitaxial growth and magnetic transport properties of La0.7Sr0.3MnO3: CuO films were investigated with phase boundary density and strain states tuned by modifying the growth frequency 3 ACS Paragon Plus Environment


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and substrates selected. Enhanced and systematically tunable LFMR have been achieved and correlated with the unique microstructure as well as the overall phase strain states. Experimental Thin Film deposition: All the thin films were prepared by pulsed laser deposition (PLD) with KrF excimer laser (λ = 248 nm). Before the deposition, the chamber was pumped to < 10-6 Pa. During the deposition, adjustable deposition frequency (1 to 10Hz), oxygen pressure, were used for growth optimization and frequency dependent investigation. The composite targets used were prepared using a conventional solid state sintering process from high purity stoichiometric powders, i.e, La2O3, SrCO3, MnO2, CuO. The powders were mixed well and pressed into pellets first. Then the pellets were annealed at 1300 °C, forming condense targets. Microstructure and Physical Properties Characterization: The crystallinity and microstructures of the films were investigated by X-ray diffraction (XRD) (PANalytical Empyrean XRD), and transmission electron microscopy (TEM) (FEI Tecnai G2 F20 operated at 200kV). The samples used for TEM analysis were prepared by a standard manual grinding and thinning process followed by an ion milling procedure in a precision ion polishing system (PIPS 691, Gatan). Magneto-transport properties were examined using a Physical Property Measurement System (PPMS Model 6000, Quantum Design) in a four-point probe configuration (in Van der Pauw geometry) with magnetic field applied out-of-plane. 200-nm-thick gold electrodes as the contacts were deposited with shadow mask by magnetron sputtering. The MR is evaluated by the following equation: MR (%) = [(ρ0-ρH)/ρ0] × 100%, where ρH, ρ0 are the electrical resistivity with


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and without applied magnetic field, respectively. The magnetizations of the films were measured with Vibrating Sample Magnetometer (VSM) in the PPMS system. Results and Discussion Figure 1a shows XRD θ-2θ plots of the LSMO: CuO nanocomposite films on MgO, STO and LAO substrates, with a deposition frequency of 10 Hz. It is clear that, for all the nanocomposite films, both LSMO and CuO grow highly textured on the substrates without any obvious impurity peak, i.e., LSMO grown epitaxially along the (00l) of the substrates, and CuO grown along (110) and (111) directions. Note that LSMO peaks show very strong intensity comparable to the substrate, indicating high epitaxial film growth, while the lower intensity of the CuO peaks is possibly due to the relative low CuO concentration (33% here) as well as the small size of separated CuO pillars, which could be seen in the TEM images. In addition, the large lattice mismatch and different crystal structures between CuO and the substrates could also lead to relative lower epitaxial quality, thus weaker XRD intensities. Figure 1b presents the corresponding calculated out-of-plane lattice parameter of LSMO phase (

) for the nanocomposite

films. With respect of the substrates used, the LSMO sustained distinguished different strain states. More specifically, the film on LAO shows up with the largest out-of-plane tensile strain of ~2.6%, followed by that of 0.5% and -0.25% for MgO and STO, respectively. Meanwhile, the laser frequency also plays an important role on determining the strain states. The frequency dependent XRD θ-2θ results and the calculated LSMO out-of-plane lattice parameter (

) for LSMO: CuO films on STO substrate are

plotted in Figure S1a and S1b. The LSMO experiences a compressive strain for all the samples, while with increasing growth frequency,

is getting closer to the bulk 5

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value of 3.87 Å, which is correlated with the substrate induced strain and the lattice mismatch between LSMO and CuO considering the d-spacing of LSMO (002) (1.935 Å), CuO (111) (~2.3 Å) and CuO (110) of (~2.7 Å). The derived out-of-plane d-spacing and corresponding strain states for LSMO phase of the films are summarized in Table S1. The microstructure of the LSMO: CuO nanocomposite films was explored using TEM. Clearly columnar VAN structure can be observed with a systematical dimension tuning effect according to different growth frequency. Figure 2a and c display the crosssectional TEM images of LSMO: CuO on STO (001) grown under 1Hz and 10Hz respectively. The corresponding selective area electron diffraction pattern (SAED) shown in inset of Figure 2a with distinguished diffraction dots indicate high quality epitaxial growth of the nanocomposite films. The domain size is greatly reduced with increasing deposition frequency. Clearly large clusters with the dimension in the range of 100-200 nm and separation spacing of ~200 nm can be observed in the LSMO matrix for the sample deposited under 1 Hz. In addition, significant outer growth can be seen for the clusters. However, for the film grown under high frequency of 10 Hz, shown in Figure 2c, the surface is much smoother and the column size is in the range of ~10-20 nm with a much smaller spacing. The systematic domain size tuning effect as a function of laser deposition frequency can be attributed to the frequency dependent adatoms diffusion length variation, which is consistent with the reports on other self-assembled systems.11,29 As the laser frequency decreases, the ad-atoms of the two phases could diffuse longer time to form larger domain under the slower growth rate. It is interesting to note that both vertical and tilted CuO domains exist in the films while LSMO domains are mostly vertically aligned. High resolution TEM image of Figure 2b and 2d gives an out-of-plane


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d spacing of 2.3 Å and 2.7 Å for the tilted and vertical CuO pillars, corresponding to CuO (111) and CuO (110), respectively. The TEM results are consistent well with the above XRD results. To examine the chemical distribution in the LSMO: CuO VAN system, X-ray energy-dispersive spectroscopy (EDS) analyses were performed with scanning transmission electron microscopy (STEM) imaging. Figure 2e shows a typical EDS mapping of the LSMO: CuO film on LAO substrate grown under 10 Hz. The features of vertical and tilted CuO nanopillars embedded in the LSMO matrix are clearly seen in the elemental mapping for each element. No obvious inter-diffusion exists in between the CuO pillars and LSMO matrix. Both the TEM, STEM and the EDS results suggest that the LSMO: CuO has been grown as well separated 2-phase VAN with both vertical and tilted CuO nanopillars. The magnetic transport properties of nanocomposite films were first investigated as a function of deposition frequency, with the temperature dependent resistance (R-T) curves for the films on STO substrates plotted in Figure 3a. It can be seen that, with increasing laser deposition frequency, the resistance of the nanocomposite increases, and the metal-insulator transition temperature shifts to lower temperatures. This transport property tunability is mainly correlated with the enhancement of the phase boundary and grain boundary effect due to reduced domain size and increased disorder at the vertical phase boundaries,10,11,13 as observed in the TEM images. The corresponding MR results measured under 1T are plotted in Figure 3b. With increasing deposition frequency, the MR peak value increases and the peak position shifts to lower temperature, which couples with TMI. The MR peak value is estimated to be 8 % at 275K, 9 % at 235K and



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14 % at 195K, for 1Hz, 5Hz and 10Hz sample, respectively. The increase of MR% and decrease of peak temperature is correlated to the increased decoupling of LSMO domains and enhanced boundary scattering/tunneling effect, corresponding to the reduced domain size and increased phase boundary densities. Next the magnetic and transport performance of the LSMO: CuO films on various substrates LAO (001), MgO (001), STO (001) were compared. Figure 4 plotted the temperature dependent magnetization (M-T) behavior of the LSMO: CuO films grown under 10 Hz on different substrates. The Curie temperature (Tc) was determined from the M-T curves where magnetization starts to increase dramatically, which is also the magnetic phase transition temperature. Tc can be derived to be ~350K, ~250K, ~220K for the film on STO, LAO and MgO substrates, respectively. The Tc modulation could be easily correlated with the phase boundary effect together with the distinguished strain state. First, the disordered phase boundaries could suppress the double exchange coupling in LSMO matrix, leading to the decrease of Curie temperature.10 Second, the biaxial distortion which is induced from substrate strain effect, will increase the Jahn-Teller splitting of the eg electron levels and will also cause a decrease in Tc.30 Second, the biaxial distortion which is induced from substrate strain effect, will increase the JahnTeller splitting of the eg electron levels and will also cause a decrease in Tc.30 Third, the defects could serve as energy barriers, which can suppress short-range and long range double-exchange between neighboring Mn ions and FM domains, decreasing the systems’ macroscopical Tc. Here the larger strain in the samples on LAO and MgO can contribute to the lower Tc, which is consistent with previous report of pure LSMO films.31 Although the strain is 0.5% for the film on MgO, considering the large lattice


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mismatch (9%) between LSMO and MgO, it is expected that a large amount of defects, i.e, grain boundaries and dislocations exist in the film, which partially relax the large substrate induced biaxial strain. Therefore, the epitaxial quality of the film plays a critical role in determining the Tc modulation which is maximum for the film grown on STO. In addition, interestingly, bifurcations between FC and ZFC curves (which determines the irreversible temperature Tirr) were observed in the samples on MgO and LAO substrates, which indicates the lacking of long-range FM ordering in LSMO phase and becomes spin-glass state under such temperature. However, such bifurcation is not observed for the film on STO substrate. Also, it is noted that the neighboring magnetic spins in CuO, being an antiferromagnetic material, point in opposite directions. This causes a zero magnetization in the absence of an external magnetic field and a small net magnetization in the presence of the magnetic field. This type of ordering vanishes above Neel temperature and CuO becomes paramagnetic. However, since the effect is relatively small, the overall magnetization in the film is dominated by the ferromagnetic LSMO phase. The magneto-transport behavior of both LSMO: CuO nanocomposite films as a function of the substrates was investigated and compared with pure LSMO films grown under same condition. Shown in Figure 5 are the R-T and the magnetoresistance behavior of the films. The data clearly shows that both the strain states and the phase boundaries density play an important role to determine the magnetotransport properties of the films. In agreement with previous work,16,32 the biaxial strain induced by the substrate has a significant impact on the metal insulating transition behavior of pure LSMO films and larger strain could lower metal-insulator transition temperature (TMI). Here the TMI for



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pure LSMO films are derived from Figure 5c to be 310K, 220K for STO (є ~-0.4%) and MgO (є ~0.5%) substrate, respectively. The pure LSMO film on LAO (є ~2.71%) substrate shows an insulating behavior in all the temperature region investigated. In comparison, for LSMO: CuO films, the TMI of ~250K and ~200K, ~50K can be calculated for STO, MgO and LAO substrates, respectively. This is attributed to both the phase boundary effect and the strain state for the LSMO phase. On one hand, CuO pillars embedded in LSMO matrix serve as energy barriers among LSMO domains and introduce magnetic disorder at phase boundaries, suppressing double-exchange effects, contributing to lower TMI.10 On the other hand, the strain state of the LSMO phase impacts simultaneously on the transport behavior.16 The lower TMI of the nanocomposite films than pure LSMO on STO and MgO substrates are mostly correlated with the induced phase boundary effect considering smaller or comparable strain, i.e, є ~-0.26% and є ~0.53592 for LSMO: CuO on STO and MgO. A lower strain of є ~2.56% for LSMO: CuO on LAO substrate might be responsible for the lower resistance and the observed metal-insulating transition behavior. The MR-T behavior measured under 1T are plotted in Figure 5b and 5d. For LSMO: CuO nanocomposite films, each film has its specific MR peak, which couples with TMI. The MR behavior around the TMI can be understood similar as CMR.10,13 Here all the LSMO: CuO nanocomposite films show enhanced MR value than their pure LSMO counterpart, which could be ascribed to the phase boundary induced spin fluctuation, considering the similar strain states.4 It is interesting to note that the MR peak value of LSMO: CuO on STO substrate is 13% at 200K, while the highly strained LSMO: CuO nanocomposite film on LAO substrate exhibit the highest LFMR value of


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~80% at ~50K. The significantly enhanced LFMR for LSMO: CuO film on LAO indicates an important role of the film strain in determining the MR behavior. While the LSMO: CuO film on LAO grown under 1Hz with a tensile strain of 1.8% only shows the MR of 15% at 100K (shown in Figure S3). Note that for LSMO grown on MgO, due to the large lattice mismatch between LSMO and MgO (~9%), substrate induced strain and defect are able to enhance the LFMR effect, compared to the films on other substrates, such as LAO and STO.33 In addition, no obvious MR for pure LSMO film on LAO are observed at the temperatures investigated. The remarkable enhanced LFMR of 80% in this case implies that both the high strain (over 2%) and the phase boundary effect are essential for enhancing the overall MR behavior. Moreover, this enhancement is also attributed to the presence of greater interfacial area between LSMO and CuO in LAO. The incorporation of a secondary phase (CuO in this case) can generate additional artificial grain boundaries/ barriers. In addition, the local structure, chemical and spin disorders raise the tunneling barrier, which further improves the LFMR effect. Therefore, this morphology on LAO case is more effective in coupling the strain between the two phases. Conclusion In summary, the growth of the (LSMO)2: (CuO)1 nanocomposite thin films with various laser deposition frequency and selected substrates has been demonstrated. Detailed microstructure analysis with XRD and TEM indicates that the films were grown as VAN structures with CuO pillars embedded in LSMO matrix. Systematic magnetic and transport property tuning effects have been observed, and correlated with the phase dimension, density and strain states in the VAN films. A record high MR value of 80%



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has been achieved for the nanocomposite film on LAO substrate, indicating large strain combined with phase boundary effect as an efficient method for enhancing MR performance.

Supporting Information Derived out-of-plane d spacing and strain states for LSMO phase for the films under 10Hz; XRD θ-2θ plots of LSMO: CuO nanocomposite films on STO (001) substrate grown under different laser frequencies and their corresponding out-of-plane lattice parameter of LSMO phase in LSMO: CuO films as respect of laser frequency; MR-T (1T) for the films grown on LAO substrate.

Acknowledgement The work is supported by the U.S. National Science Foundation (Ceramic program, DMR-1643911 for VAN thin film growth and DMR-1565822 for TEM study).


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[26] Siemons, W.; Koster, G.; Blank, D. H. A.; Hammond, R. H.; Geballe, T. H.; Beasley, M. R. Tetragonal CuO: End member of the 3d transition metal monoxides. Phys. Rev. B - Condens. Matter Mater. Phys., 2009, 79, 1–7. [27] Itoh, T.; Maki, K. Growth process of CuO(111) and Cu2O(001) thin films on MgO(001) substrate under metal-mode condition by reactive dc-magnetron sputtering. Vacuum, 2007, 81, 1068–1076. [28] Kawwam, M.; Alharbi, F.; Aldwayyan, A.; Lebbou, K. Morphological study of PLD grown CuO films on SrTiO3, sapphire, quartz and MgO substrates. Appl. Surf. Sci., 2012, 258, 9949–9953. [29] Okada, K.; Sakamoto, T.; Fujiwara, K.; Hattori, A. N.; Kanki, T.; Tanaka, H. Three dimensional nano-seeding assembly of ferromagnetic Fe/LaSrFeO4 nanohetero dot array. J. Appl. Phys., 2012, 112, 0–7. [30]

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[33] Majumdar, S.; Huhtinena, H., Majumdar, H. S.; Paturi, P.; Journal of Alloys and Compounds, 2012, 512, 332-339


Page 19 of 24

b

20

40

2θ (°)

LAO (003)

LSMO (003)

60

10Hz

LSMO (003)

STO (003) LSMO (003)

LSMO (002) LAO (002)

MgO (002)

LSMO (002)

STO (002) LSMO (002)

CuO (111) CuO (111) CuO (111)

STO (001)

LAO

CuO (110)

LSMO (001)

STO

CuO (110)

MgO

LAO (001)

a

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

Figure 1. a) XRD θ-2θ plots of LSMO: CuO nanocomposite films on MgO (001), STO

(001) and LAO (001) substrates deposited under 10Hz. b) Calculated out-of-plane strain states of LSMO phase in LSMO: CuO films on different substrates.



Page 20 of 24

Figure 2. Microstructures of LSMO: CuO nanocomposite films, showing well-separated

two-phase VAN growth. a) and b) Cross-sectional TEM image of LSMO: CuO on STO (001) grown under 1Hz. Inset of a) is the corresponding SAED pattern. C and d) Crosssectional TEM images of LSMO: CuO on STO (001) grown under 10Hz. e) STEM and EDS mapping results of LSMO: CuO on LAO (001) substrate grown under 10Hz.


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a

350 LSMO:CuO on STO

Resistance (Ω Ω)

300

1Hz 5Hz 10Hz

250 200 150 100 50 0 0

100

200

300

400

Temperature (K)

b

20 LSMO:CuO on STO 10 Hz 5 Hz 1 Hz

15

MR (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

5

0

0

100

200

300

400

Temperature (K) Figure 3. Frequency dependence of a) R-T and b) MR-T plots for LSMO: CuO films on STO. Measurement is under 1T.



0.0020 Moment (emu/cc2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 24

STO FC STO ZFC MGO FC MGO ZFC LAO FC LAO ZFC

0.0015 0.0010 0.0005 0.0000 0

100

200 T (K)

300

Figure 4. Field cooling (FC) and zero field cooling (ZFC) temperature dependenct magnetization (M-T) of the LSMO: CuO nanocomposite films on different substrates. Measurement was conducted under H=1000 Oe.


Page 23 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Substrates dependence of magnetic and transport properties for LSMO: CuO thin films in comparison with pure LSMO counterpart films. a) R-T and b) MR-T (1T) for LSMO: CuO nanocomposite films. c) and d) are R-T and MR-T (1T) behavior of pure LSMO films.



Abstract graphics

100 LSMO: CuO STO MGO LAO

80

MR (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 24

60 40 20 0 0

100

200

300

T(K)


Microstructure, Magnetic, and Magnetoresistance Properties of La0

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