La0.7Sr0.3MnO3 hybride nanowire networks prepared by electrospinning

Accepted Manuscript Properties of La1.85Sr0.15CuO4 / La0.7Sr0.3MnO3 hybride nanowire networks prepared by electrospinning

M.R. Koblischka, X.L. Zeng, T. Karwoth, U. Hartmann PII: DOI: Reference:

S0304-8853(18)32792-6 https://doi.org/10.1016/j.jmmm.2018.11.128 MAGMA 64687

To appear in:

Journal of Magnetism and Magnetic Materials

Please cite this article as: M.R. Koblischka, X.L. Zeng, T. Karwoth, U. Hartmann, Properties of La1.85Sr0.15CuO4 / La0.7Sr0.3MnO3 hybride nanowire networks prepared by electrospinning, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.11.128

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Properties of La1.85 Sr0.15 CuO4 /La0.7 Sr0.3 MnO3 hybride nanowire networks prepared by electrospinning M. R. Koblischka, X. L. Zeng, T. Karwoth, and U. Hartmann Institute of Experimental Physics, Saarland University, Campus C 6 3, 66123 Saarbr¨ucken, Germany.

Abstract We prepared La1.85 Sr0.15 CuO4 /La0.7 Sr0.3 MnO3 nanowire network hybrid systems by means of electrospinning. From obervation by scanning electron microscopy, the average diameter of the nanowires is around 220 nm and the average length can reach over 50 µm. The randomly aligned La1.85 Sr0.15 CuO4 and La0.7 Sr0.3 MnO3 nanowires exhibit numerous interconnects among each other and form a complicated, non woven hybrid network system. The nanowires are polycrystalline with a grain size at around 30 nm as confirmed by transmission electron microscopy. According to four-probe electrical transportation and magnetization measurements, superconductivity of the sample is suppressed and a positive magnetoresistance effect is observed. Measurements of M(T ) and M(H) were carried out as well, revealing the soft magnetic character of the hybrid nanowire network samples. Keywords: LSMO; LSCO; electrospinning; nanowires; hybrid network; magnetoresistance; magnetization PACS: 75.47.Gk,75.75.Cd,75.50.Tt 1. Introduction The two ceramic perovskite compounds, La0.7 Sr0.3 MnO3 (LSMO) – a ferromagnetic material exhibiting the colossal magnetoresistance (CMR) effect [1, 2, 3] – and La1.85 Sr0.15 CuO4 (LSCO), a high-T c superconducting material, are characterized by the same crystal structure and similar transport properties. Therefore, these two materials are ideally suited to build up hybrid multilayer structures, which are described in the literature either as laser-ablated thin films [4, 5] or powder compacts [6, 7]. These are prepared for different purposes: (i) to study the effects of interfacial electronic reconstruction on the magnetoresistance [5, 6] or (ii) the modelling of bulk, ferromagnetic superconductors [8, 9]. Considering the magnetoresistance (MR) effect, the cuprate in the normal state may modify the magnetotransport properties in the manganite as described by Hwang [10]. In the low-temperature case, the ferromagnetism in LSMO can prevent the Cooper pair formation in the LSCO material, and the quasiparticle injection into each component may generate additional resistance which increases with applied field, so a positive MR results [11]. In this contribution, we have prepared such a hybrid system consisting of LSMO and LSCO by means of electrospinning, a technique which was shown recently to produce nanowire network fabrics consisting of nanowires with a diameter in the 100-nm range and wire lengths in the micrometer range. These nanowires form a non woven fabric, and after the necessary heat treatment, there are numerous interconnects between the individual nanowires, which enable the flow of transport current through such a sample [12]. To increase the effect of the interfaces, it is interesting to see what will happen when the CMR nanowires and supercondcuting nanowires form a hybrid Preprint submitted to J.Magn.Magn.Mat.

structure together. This is achieved by electrospinning the two types of as-prepared fibers simultaneously from two individual orifices. Here we tried to prepare a pseudo-layered structure: Via optimizing the electrospinning periods of the two precursors, the two different types of nanowires stick together when occupying the collection regime and form a layer. The microstructure and the physical properties (magnetization, MR effect) of such hybrid LSMO/LSCO nanowire network fabrics are described in detail.

2. Experimental procedures Figure 1 presents a schematic drawing of the electrospinning instrument. The precursor is pushed by the microspeed boost pump from the syringe to the bottom of the needle. The precursor is dragged by the electric field between the needle and the grounding area, transforming the original droplets to nanofibers. This method is commonly used in the literature to prepare polymer nanofibers [13, 14], and recently, also ceramic materials were demonstrated to be prepared using this technique [15, 16, 17, 18]. The blow-up at the bottom of Fig. 1 gives details about the use of two orifices for the spinning process of two component. Controlling the distance between two orifices as shown in the blue frame, it is possible to fabricate different materials simultaneously without interference with each other. The electrospinning precursor is prepared by dissolving La, Sr, and Mn acetates in PVA (high molecular weight polyvinyl alcohol). The same process is carried out for LSCO using La, Sr, and Cu acetates. The PVA is slowly added to the acetate solution with a mass ratio of 2.5:1.5. This solution is stirred at 80 November 25, 2018

3. Results and discussion 3.1. Microstructure Figure 2 presents the XRD result of the hybrid sample. Both La1.85 Sr0.15 CuO4 and La0.7 Sr0.3 MnO3 phases are shown. All measured peaks can be properly indexed, and no additional phases appears. Combining these results with the EDX mapping shown in Fig. 5 belowt indicates that the hybrid nanowire network sample consists of well-separated LSCO and LSMO nanowires. There is no mixed phase present which would indicate that the Cu and Mn elements coexist in the same single fiber. The grain size values estimated from the half the maximum intensity are 47.3 nm at peak (012), 44.5 nm at peak (104), 41.5 nm at peak (202) for the La0.7 Sr0.3 MnO3 phase, and 58.4 nm at peak (101), 55.6 nm at peak (004), 46.6 nm at peak (103) for La1.85 Sr0.15 CuO4 phase. Gernerally, the grain sizes estimated from the XRD patterns of both phases are around 50 nm. In Fig. 3, a TEM image of the electrospun nanowires is presented. Note that the nanowire pieces are shown in the asgrown state without any additional surface treatment. Both compounds (LSMO and LSCO) exhibit a similar microstructure and grain growth. All grains visible have with similar characteristics (shape, size). Each individual nanowire is built up from many, randomly oriented LSMO or LSCO grains. As a result, we cannot distinguish LSMO and LSCO nanowires in the TEM images. 3 0 0 0 (1 0 4 )

L a (2 1 4 )

1 .8 5

S r S r

M n O

0 .3 0 0 .1 5

C u O 4

3

P D F # 5 1 -0 4 0 9 P D F # 4 6 -0 5 9 0

1 0 0 0 5 0 0 0

2 0

4 0

6 0

8 0

(3 1 6 ) (2 2 8 ) (3 2 3 ) (2 ,1 ,1 1 ) (3 1 8 ) (4 0 0 ) (3 2 7 ) (4 1 3 )

(1 0 3 ) (1 1 0 )

1 5 0 0

( 1 ( 0 2 5 0 ) 2 ( 1) 1 4 ) (2 0 0 ) (0 2 4 ) (1 1 6 ) ( 2 0 4 )( 1 0 7 )

SEM/EDX imaging was carried out using a JEOL 7000 F SEM microscope operating at 15 kV. The TEM analysis was performed by JEOL JSM-7000 F transmission electron microscope operating at 200 kV with a LaB6 cathode. For TEM investigations, several nanowire pieces separated from the asgrown nanowire fabric by ultrasound treatment and were placed on a carbon-coated TEM aluminium grid. This enabled to search nanowire regions which were thin enough to enable the transmission mode. No additional treatment of the sample surface was carried out.

0 .7 0

2 0 0 0

(0 1 2 ) (1 0 1 ) (0 0 4 )

In te n s ity ( a .u .)

2 5 0 0

L a

(3 1 (3 1

C for 2 h, and then spun into cohering nanofibers by electrospinning. To remove the organic compounds and to form the desired ceramic phases, the sample is subsequently heat treated in a lab furnace. An additional oxygenation process is required to obtain the correct phase composition. Further details about the electrospinning process of these nanowires are given elsewhere [19, 20, 21]. The constituent phases of the hybrid nanowire network samples were determined by means of a high-resolution automated RINT2200 X-ray powder diffractometer, using CuKα radiation generated at 40 kV and a current of 40 mA.

(1 (2

◦

(2 0 6 ) 1 8 ) (2 0 8 ) 2 0 ) ( 2 1 7 )( 2 0 8 ) 0 () 1 2 8 ) 4 ) (2 2 6 )

Figure 1: Schematic drawing of the electrospinning apparatus.The blow-up shows the situation when using two orifices to produce hybrid materials from two different sources. The distance between the two orifices must be set properly to avoid interference effects.

1 0 0

1 2 0

o

2 th e ta ( ) Figure 2: X-ray analysis of the nanowire network sample, with the peaks defined by the standard PDF card #50-0308 (upper) and the EDAX spectrum of the same sample on Aluminium holder (lower)

The magnetization of the nanowire networks were measured using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS3) with ±7 T magnetic field applied perpendicular to the sample surface, using a piece of the nanowire network with a size of ∼15 mm2 . The magnetoresistance was measured in a 10/12 T bath cryostat (Oxford Instruments Teslatron) with a Keithley source meter (model 2400) as a current source, and the voltage was recorded using a Keithley 2001 voltmeter.

From the TEM images, we extracted the grain size using image processing software. According to the statistics of the grain size (see Fig. 4) observed from the TEM images, the average grain size of the fibers determined by Gauss fitting is 33.2 nm, which is smaller than the estimated value from the XRD result (∼ 50 nm). This clearly indicates that there are multiply connected domains within the individual nanowires. This result is further corrobated by an EBSD analysis performed on the pure LSMO nanowires as described in Ref. [22]. 2

35

Size distribution Gauss fitting Avg. grain size = 33.2 nm

Frequency (%)

28 21 14 7 0

20

30 40 50 Grain size (nm)

Figure 4: Graph of the grain size analysis. The Gauss fitting (red line) of the data yields an average grain size of 33.2 nm.

nanowires are found to be homogeneously consisting of only one phase.

Figure 3: (a) SEM observation of the nanowires network at magnification 5000 ×; (b) TEM image of several individual nanowires revealing the polycrystalline, randomly-oriented grain structure. Figure 5: SEM image of the LSCO/LSMO nanowire network hybrid fabric and the corresponding EDX mapping for the elements O, Mn and Cu. The Mn and Cu mappings clearly demonstrate the opposed behavior, so one single nanowire consists only of LSMO or LSCO.

Figure 5 presents an SEM image of the nanowire network hybrid sample together with EDX elemental mapping. The averaged diameter of the nanowires is about 220 nm. The length of the nanowires reaches over 50 m. Both parameters are equal to the pure materials [20, 21]. In the Mn mapping, dark fibershape regimes can be found, which are the Cu rich regimes in the Cu mapping. The opposite case appears in Cu mapping image, whereas the element O is distributed equally in the image as expected. This observation demonstrates that the LSMO and LSCO nanowires are homogeneous: There is no mixture phase containing both Cu and Mn. This result can be expected from the growth of the nanowires by electrospinning, as the as-grown nanowires are polymer nanowires containing the raw precursor materials. In the subsequent heat treatment, a diffusion of Cu or Mn towards a different nanowire is unlikely, except at an interconnection between two nanowires. Therefore, the resulting

3.2. Magnetic and MR properties Figure 6 shows the M(T ) ZFC curves for pure LSMO (red) and the hybrid LSMO/LSCO sample. As expected, the resulting magnetization of the hybrid sample is smaller than that of the pure sample, corresponding to the smaller volume content of the LSMO phase as compared to the pure sample. The Curie temperature, TC , of both samples is, however, quite similar (333.2 K and 333.7 K). The superconducting diamagnetic behaviour does not display at the low temperature as other LSCO sample. It can be ascribed to the high detection field 1000 Oe for the measurement, as the pure LSCO nanofibers just present the superconductivity with low detection field around 20 Oe [20]. 3

45

21

R (MΩ)

27 18 La0.7Sr0.3MnO3-δ

9 TC=333.7 K

La0.7Sr0.3MnO3-δ O2/La1.85Sr0.15CuO4-δ TC=333.2 K 0 ZFC curves

0

50

100

150

200

T (K)

250

300

350

60 40

8 6 4

0

0

1200

0 -10

800

0 0

-20 -30 50 100 150 200 250 300 0 T (K)

50 100 150 200 250 300

Figure 8: Resistance-temperature R(T ) diagram and the MR ratio for the LSMO/LSCO hybrid sample (left) and the pure LSMO sample (right) as function of temperature.

10 K 300 K

of the metal-insulator (MI) transition at low temperature, the slop change at around 100 K certifies the residual of the MI transition. Such suppression can be attributed to the Coulomb blockage effect from the small grain size [21, 23, 24]. It is valid as the grain size of the pure La0.7 Sr0.3 MnO3 nanowires is around 20 nm. In the LSMO/LSCO hybrid nanowires, the MI transition is completely suppressed, even though the average grain size (∼30 nm) is slightly larger than the pure La0.7 Sr0.3 MnO3 nanowires (∼20 nm). The lower graphs in Fig. 8 give the MR ratio for both types of samples. Magnetoresistance (MR) plots are calculated from the resistance data obtained by sweep measurements at different field strengths using the relation MR[%] = (RH (T ) − R0 (T ))/R0 (T ). Unlike the pure La0.7 Sr0.3 MnO3 nanowires, the resistance of the hybrid sample increase with rising field, therefore, it shows positive MR effect. Overall, it seems like the hybrid sample shows no superconductivity in the electric measurement. Actually the it is result of the competition between the magnetoresistive component from La0.7 Sr0.3 MnO3 and the superconducting component from La1.85 Sr0.15 CuO4 : 1. The field increasing resistance stems from the La1.85 Sr0.15 CuO4 component as the resistance of the superconductor increases with rising magnetic field; 2. Different from the pure LSMO sample, the shape of the R − T curve varies with field. At 5 T, a bending appears at around 50 K, at 7.5 T, the bending becomes more obvious and appears at around 20 K. Such bending results from the competition of between the field increasing resistance effect of the La1.85 Sr0.15 CuO4 component and the negative MR effect of the La0.7 Sr0.3 MnO3 component. At the temperature below 150 K, the MR curves intent to merge together, as the this temperature range, the MR effect is stronger than the one at high temperature, it is sufficient to compensate the resistance increase from the superconducting component.

20 0

-20 -40 -60 -2000

La0.7Sr0.3MnO3

2

400

Figure 6: M(T ) measured in ZFC mode for pur LSMO (red) and the LSMO/LSCO hybrid sample in order to determine the Curie temperatures.

M (emu/g)

14

0.0 T 2.5 T 5.0 T 7.5 T

7

MR (%)

M (emu/g)

36

La0.7Sr0.3MnO3/La1.85Sr0.15CuO4

-1000

0

H (Oe)

1000

2000

Figure 7: Magnetization hysteresis M(H) for the LSMO/LSCO hybrid sample, measured at 10 K (black) and at 300 K (red).

Figure 7 presents the magnetization hysteresis loops (M(H)) for the LSMO/LSCO hybrid sample (H ⊥ sample surface) measured at 10 K and at 300 K. The magnetization loops clearly demonstrate the soft magnetic character of the LSMO/LSCO nanowire network samples. At 10 K, it is interesting that the initial curve deviates from the rest of the magnetization loop. Furthermore, at higher external field, where the ferromagnetic samples should reach their saturation, a weak field irreversible behaviour is discovered. It is rational to attribute these two features to the superconductivity contribution of the LSCO component. In Fig. 8, we finally present the resistance versus temperature curves measured in various applied magnetic fields (0 T, 2.5 T, 5.0 T and 7.5 T, applied perpendicular to the sample surface) in the temperature range 2 K ≤ T ≤ 300 K for the LSMO/LSCO hybrid sample (left column) and the pure LSMO sample (right column). Different from the bulk LSMO sample, the pure La0.7 Sr0.3 MnO3 nanowires present a suppression 4

4. Conclusion

[13] Z. M. Huang, Y. Z. Zhang, M. Kotaki, S. Ramakrishna, ”A review on polymer nanofibers by electrospinning and their applications”, Composites Science Technol. 63, 2223 (2003). [14] D. Li, Y. N. Xia, ”Electrospinning of nanofibers: Reinventing the wheel?”, Adv. Mater. 16, 1151(2004). [15] D. Li, J. T. McCann and Y. N. Xia, ”Electrospinning: A simple and versatile technique for producing ceramic nanofibers and nanotubes”, J. Am. Ceram. Soc., 89, 1861 (2006). [16] H. Wu, W. Pan, D. Lin and H. Li, ”Electrospinning of ceramic nanofibers: Fabrication, assembly and applications”, J. Adv. Ceramics, 1, 2 (2012). [17] D. Li, T. Herricks and Y. N. Xia, ”Magnetic nanofibers of nickel ferrite prepared by electrospinning”, Appl. Phys. Lett., 83, 4586 (2003). [18] R. Yensano, S. Pinitsoontorn, V. Amornkitbamrung and S. Maensiri, ”Fabrication and magnetic properties of electrospun La0.7 Sr0.3 MnO3 nanostructures”, J. Supercond. Novel Mag., 27, 1553 (2014). [19] J.-M. Li, X. L. Zeng, A.-D. Moa, and Z.-A. Xu, CrystEngComm, 13, 6964 (2011). [20] X. L. Zeng, M. R. Koblischka, U. Hartmann, Mat. Res. Express, 2, 095022 (2015). [21] T. Karwoth, X. L. Zeng, M. R. Koblischka, U. Hartmann, C. Chang, T. Hauet and J. M. Li, ’Magnetoresistance and structural characterization of electrospun La1−x Sr x MnO3 nanowire networks’, Solid State Commun., submitted for publication. [22] A. Koblischka-Veneva, X. L. Zeng, M. R. Koblischka, J. Schmauch, U. Hartmann, ”EBSD analysis of electrospun La1x Sr x MnO3 nanowires”, submitted to J. Magn. Magn. Mater. [23] S. Kar, J. Sarkar, B. Ghosh and A. K. Raychaudhuri, ”Effect of grain boundaries on the local electronic transport in nanostructured films of colossal magnetoresistive manganites”, J. Nanosci. Nanotech., 7, 2051 (2007). [24] Ll. Balcells, J. Fontcuberta, B. Martinez and X. Obradors, ”High field magnetoresistance at interfaces in manganese perovskites”, Phys. Rev. B, 58, R14697 (1998).

To summarize, we have fabricated LSMO/LSCO hybrid nanowire network fabric samples by means of electrospinning. TEM, SEM and EDX analysis showed that the individual nanowires consist of only one component, but all nanowires have many interconnects within the given sample. This large number of interfaces enable current flow through the entire sample perimeter. Even though the sample shows no obvious superconductivity, however, the LSCO component does provide the influence no matter in the magnetic properties or in the electric properties. In other words, the presenting magnetic properties and the electric properties of the hybrid nanowire sample is the result of the competition of the ferromagnetic magnetoresistive LSMO component and the superconducting LSCO component. Acknowledgment We thank J. Schmauch (Saarland University, group Prof. Birringer) for technical assistance and Prof. V. Presser for the use of the electrospinning apparatus. This work is supported by DFG grant Ko2323/8, which is gratefully acknowledged. References [1] S. Jin, T. H. Tiefel, M. McCormack, R. A. Fastnacht, R. Ramesh, L. H. Chen, ”Thousandfold change in resistivity in magnetoresistive La-CaMn-O films”, Science 264, 413(1994). [2] L. M. Rodriguez, J. P. Attfield, ”Cation disorder and size effects in magnetoresistive manganese oxide perovskites”, Phys. Rev. B 54, R15622(1996). [3] A. P. Ramirez, ”Colossal magnetoresistance”. J. Phys.: Condens. Matter 9, 8171(1997). [4] J.-M. Liu, Q. Huang, J. Li, C. K. Ong, Z. G. Lin, ”Pulseld laser deposition of perovskite La0.7 Sr0.3 MnO3 /La0.7 Sr0.3 CoO3 multilayers and their magnetotransport properties”, Appl. Phys. A 69(suppl.), S663 (1999). [5] B. Li, R. V. Chopdekar, A. T. N’Diaye, A. Mehta, J. Paige Byers, N. D. Browning, E. Arenholz, Y. Takamura, ”Tuning interfacial exchange interactions via electronic reconstruction in transition-metal oxide heterostructures”, Appl. Phys. Lett. 109, 152401 (2016). [6] Ch.-H. Yan, Zh.-G. Xu, T. Zhu, Zh.-M. Wang, F.-X. Cheng, Y.-H. Huang, Ch.-Sh. Liao, ”A large low field colossal magnetoresistance in the La0.7 Sr0.3 MnO3 and CoFe2 O4 combined system”, J. Appl. Phys. 87, 5588 (2000). [7] X.-H. Li, Y.-H. Huang, Z.-M. Wang, Ch.-H. Yan, ”Tuning between negative and positive magnetoresistance in (La0.7 Sr0.3 MnO3 )1−x (La1.85 Sr0.15 CuO4 ) x composites”, Appl. Phys. Lett. 81, 307 (2002). [8] H.-U. Habermeier, G. Cristiani, R. K. Kremer, O. Lebedev, G. van Tendeloo, ”Cuprate/manganite superlattices: A model system for a bulk ferromagnetic superconductor”, Physica C 364-365, 298 (2001). [9] X. Yao, Y. Jin, M. Li, Zh. Li, G. Cao, Sh. Cao, J. Zhang, ”Coexistence of superconductivity and ferromagnetism in La1.85 Sr0.15 CuO4 La2/3 Sr1/3 MnO3 matrix composites”, J. Alloy Compounds 509, 5472 (2011). [10] H. Y. Hwang, S.-W. Cheong, N. P. Ong, P. Batlogg, ”Spin-polarized intergrain tunneling in La2/3 Sr1/3 MnO3 ”, Phys. Rev. Lett., 77, 2041 (1996). [11] J. H. Lohr, C. A. Lopez, M. E. Saleta, R. D. Sanchez, ”Ferromagnetic and multiferroic interfaces in granular perovskite composite xLa(0.5)Sr(0.5)CoO(3)-(1-x)BiFeO3 ”, J. Appl. Phys. 120, 074103 (2016). [12] M. R. Koblischka, X. L. Zeng, T. Karwoth, T. Hauet, U. Hartmann, ”Transport and magnetic measurements on Bi-2212 nanowire networks prepared via electrospinning”, IEEE Trans. Appl. Supercond. 26, 1800605 (2016).

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