Enhancement of Low-field Magnetoresistance in Self-Assembled Epitaxial La0.67Ca0.33MnO3:NiO and La0.67Ca0.33MnO3:Co3O4 Composite Films via Polymer-Assisted Deposition

Polymer-assisted deposition method has been used to fabricate self-assembled epitaxial La0.67Ca0.33MnO3:NiO and La0.67Ca0.33MnO3:Co3O4 films on LaAlO3 substrates. Compared to pulsed-laser deposition method, polymer-assisted deposition provides a simpler and lower-cost approach to self-assembled composite films with enhanced low-field magnetoresistance effect. After the addition of NiO or Co3O4, triangular NiO and tetrahedral Co3O4 nanoparticles remain on the surface of La0.67Ca0.33MnO3 films. This results in a dramatic increase in resistivity of the films from 0.0061 Ω•cm to 0.59 Ω•cm and 1.07 Ω•cm, and a decrease in metal-insulator transition temperature from 270 K to 180 K and 172 K by the addition of 10%-NiO and 10%-Co3O4, respectively. Accordingly, the maximum absolute magnetoresistance value is improved from −44.6% to −59.1% and −52.7% by the addition of 10%-NiO and 10%-Co3O4, respectively. The enhanced low-field magnetoresistance property is ascribed to the introduced insulating phase at the grain boundaries. The magnetism is found to be more suppressed for the La0.67Ca0.33MnO3:Co3O4 composite films than the La0.67Ca0.33MnO3:NiO films, which can be attributed to the antiferromagnetic properties of the Co3O4 phase. The solution-processed composite films show enhanced low-field magnetoresistance effect which are crucial in practical applications. We expect our polymer-assisted deposited films paving the pathway in the field of hole-doped perovskites with their intrinsic colossal magnetoresistance.

Solution deposition method has advantages of low cost, easy setup, and coating of large area as it does not require vacuum-based expensive capital equipment 23 . Here we report self-assembled LCMO:NiO and LCMO:Co 3 O 4 composite films epitaxially grown by polymer-assisted deposition (PAD) technique which demonstrate enhanced LFMR. Facile and cost-effective PAD method utilizes metal salts dissolved in an aqueous polymer solution [24][25][26][27][28][29] . Amount of metal cations bound to the polymer and the solution viscosity are directly related to amount of the polymer used. Thus, desired stoichiometry ratio could easily be controlled by mixing different metal-polymer precursor solutions with corresponding metal molar ratios. Thickness of the film could also be controlled by adjusting concentration of the solution or spin-coating rate. The properties of NiO or Co 3 O 4 added LCMO composite films were investigated in both electronic and magnetic perspectives to assess LFMR effects. Under a magnetic field of 3 T, MR values of − 44.6% at 255 K for LCMO, − 59.1% at 180 K for LCMO:10%-NiO, and − 52.7% at 172 K for LCMO:10%-Co 3 O 4 were achieved, respectively. This revealed that the composite films possess high LFMR and their effect was enhanced even further with the addition of 10%-NiO or 10%-Co 3 O 4 insulating phases of the grain boundaries.

Results
Lattice Parameters of Self-epixatially Grown LCMO. Lattice parameters and epitaxial growth were analyzed by in-plane and out-of-plane X-ray diffraction (XRD). XRD data of LCMO:Co 3 O 4 and LCMO:NiO films grown on LaAlO 3 (LAO) substrate are shown in Fig. 1. θ -2θ scans in Fig. 1a confirm the introduction of NiO (002) and Co 3 O 4 (004) along with the (002) peaks for LCMO and LAO. The appearance of (00l) only series peaks of the films indicates that the films are highly textured along the c-axis, which is perpendicular to the substrate surface. It is noted that additional peaks appear from the single crystal substrates, apparently the substrates contain impurities. The similar peaks from LAO substrates were found in other reference as well 30 . The out-of-plane lattice parameters of LCMO:10%-and 30%-Co 3 O 4 were calculated from the LCMO (002) peak to be 3.898 Å and 3.909 Å, respectively, while the out-of-plane lattice parameters of LCMO:10%-and 30%-NiO film were 3.897 Å and 3.891 Å, respectively. As expected, all the values were larger than that of the bulk LCMO (a = 3.858 Å for a pseudocubic perovskite unit cell). This elongation along the c-axis can be attributed to relatively larger lattice parameters of NiO (cubic structure with a = 4.177 Å) and Co 3 O 4 (inverse spinel with a = 8.084 Å). According to the Poisson relationship, the in-plane lattice parameters are compressed when the out-of-plane parameters are expanded. Therefore, we know that the magnitude of the in-plane lattice parameter is in the following order, a (30%-Co 3 O 4 ) < a (10%-Co 3 O 4 ) ≈ a (10%-NiO) < a (30%-NiO). Figure 1b  The microstructures of these composite films were also studied by cross-sectional TEM and high resolution TEM (HRTEM). From Fig. 3a,b, in addition to the substrates, two phases are clearly observed in the films as triangular NiO nanoparticles (50-100 nm) and tetrahedral Co 3  Electrical Properties of the LCMO Composites. By studying temperature dependent resistivity (ρ) over temperature at different applied magnetic fields, different metal-insulator transition temperatures (T P s) were recorded from the composite films. In Fig. 4(a), a well-defined metal-insulator transition feature is displayed from the LCMO film, with the temperature of maximum resistivity at 270 K for 0 T and 295 K for 3 T. Nevertheless, unlike the single phase LCMO films, the composite films do not show metallic behaviors at lower temperatures. For the LCMO:10%-NiO and LCMO:10%-Co 3 O 4 composite films in Fig. 4b,c, respectively, the   transition peaks are much broader and less obvious. The T P s under zero field are 180 K for LCMO:10%-NiO and 172 K for LCMO:10%-Co 3 O 4 . Moreover, the peak resistivity of these two composite films (0.59 Ω• cm for LCMO:10%-NiO and 1.07 Ω• cm for LCMO:10%-Co 3 O 4 at zero magnetic field) are much higher than that of the single phase LCMO film (0.0061 Ω• cm at zero magnetic field). These two observations can be attributed to the presence of a large number of the grain boundaries, increased disorder, as well as insulating phase-induced barriers to the electrical transport 14,[31][32][33] . The introduced insulating phase at the grain boundary has been known to obstruct the magnetic spin alignment near the grain boundary region. Therefore, it increased the tunneling barrier height between the neighboring magnetic grains 13,34,35 .
As for the LCMO:30%-NiO and LCMO:30%-Co 3 O 4 composite films shown in Fig. 4d,e, respectively, within the measured temperature range, LCMO:30%-NiO shows a semiconductor behavior without metal-insulator transition, whereas for LCMO:30%-Co 3 O 4 , the film manifests an interesting behavior. There is a clear metal-insulator transition at the T P of 265 K in comparison to the other composite films; the resistivity increases sharply with the further temperature decrease. Also, the resistivity increases for the LCMO:30%-Co 3 O 4 composite film in comparison to that of the LCMO:10%-Co 3 O 4 composite film. This is because the metal-insulator transition was suppressed by the expansion of in-plane lattice parameter in LCMO 14,22 .
As mentioned above, the LCMO:30%-NiO composite film has the largest in-plane lattice constant and this led to the disappearance of metal-insulator transition. On the other hand, for the LCMO:30%-Co 3 O 4 composite film, it has the smallest in-plane lattice constant. Therefore, even though the secondary phase of Co 3 O 4 suppressed the metal-insulator transition, the compressed in-plane lattice constant enhanced the transition, leading to an enhanced T P as shown in Fig. 4(e). The sharp increase in resistivity at low temperatures can be attributed to the possible localization originating from the large amount of Co 3 O 4 secondary phase 36,37 . Several previous studies have shown that the behavior of paramagnetic phase in LCMO can be well described by the small-polaron transport 37,38 . For the charge transport by polarons, the resistivity is given by the equation 1: where ρ a is resistivity coefficient, k B is Boltzmann's constant, and E a is activation energy that is related to the polaron binding energy 13 . Therefore, we can have the expression 2: a a B Figure 4f shows the plots of ln (ρ /T) vs 1/T and their linear fittings for the LCMO, LCMO:10%-NiO, and LCMO:10%-Co 3 O 4 films at temperatures higher than T P with zero magnetic field. The slope of the linear fitting line is proportional to the activation energy E a . This indicates the energy barrier height for the spin-dependent electron hopping at the grain boundaries 17 . Thus, we can patently see that the energy barrier height of the LCMO film increases with the addition of the second phase i.e., NiO and Co 3 O 4 . In addition, the ln (ρ /T) vs 1/T curves for the LCMO:10%-NiO, and LCMO:10%-Co 3 O 4 films could not show good linear relation. It might be from the variable range hopping mechanism, low-temperature conduction in strongly disordered systems with localized charge-carrier states 39 .

Magnetic Properties of the LCMO Composites and enhanced LFMR. Enhancement in LFMR is
a unique feature and important advantage of the PAD-produced composite films. Figure 5 compares the temperature dependent MR of single phase LCMO and the LCMO composite films with different percentages of NiO and Co 3 O 4 , which were calculated from the resistivity at magnetic fields of 0 T and 3 T using the equation, MR (%) = (ρ H − ρ 0 )/ρ 0 × 100%. For the composite films, MR curves have several small maxima at different temperatures, while the single phase LCMO film shows a more simplistic curve. This phenomenon may be due to phase inhomogeneity of the composite films 29 . The maximum MR values increased for the films with the 10% addition of either NiO or Co 3 O 4 : − 44.6% at 255 K for LCMO, − 59.1% at 180 K for LCMO:10%-NiO, and − 52.7% at 172 K for LCMO:10%-Co 3 O 4 . These MR values are similar to those of other LCMO-based composite films prepared by PLD technique, but the temperature dependent resistivity behavior is different 37,[40][41][42] . For the LCMO:30%-NiO film, the MR value was slightly lower than that of the single phase LCMO film, due to its larger expansion of the in-plane lattice parameter. Whereas, the LCMO:30%-Co 3 O 4 film delivered a comparable MR value to that of the single phase LCMO film. It is worth noting that the T P of LCMO:30%-Co 3 O 4 film was the highest among the four composite samples.
The nature of the magnetically ordered state was investigated by classical zero-field cooled (ZFC) and field cooled (FC) cycles. Figure 6 presents the temperature-dependent magnetization curves measured under an applied field of 100 Oe for the four composite films. All the curves show a similar trend and the FC curves coincides with the ZFC curves at the high temperatures, while the FC curves differ from the ZFC curves in the lower temperature region 37,43 . It should be mentioned that Curie temperature (T C ) of the LCMO:Co 3 O 4 films increases with the addition of Co 3 O 4 , whereas for the LCMO:NiO films, T C decreases with the addition of NiO. The enhanced T C for the LCMO:Co 3 O 4 films can be attributed to the decreased in-plane lattice parameter with the addition of Co 3 O 4 . On the contrary, the decreased T C for the LCMO:NiO films should be attributed to the increased in-plane lattice parameter with the addition of NiO. Moreover, the similar T C s for the LCMO:10%-NiO and LCMO:10%-Co 3 O 4 owing to the similar in-plane lattice parameter further suggests that the magnetic properties as well as transport properties are dominantly controlled by the in-plane lattice parameter. In addition, the magnetism at low temperatures is clearly suppressed for the LCMO:Co 3 O 4 composite films compared to that of the LCMO:NiO films, which can be explained by the antiferromagnetic properties of Co 3 O 4 phase.
For a facile comparison, out-of-plane lattice parameter, T P , ρ , MR, and T C are summarized in Table 1. Difference between the LCMO:30%-Co 3 O 4 and LCMO:30%-NiO films, and similarity between the LCMO:10%-NiO and LCMO:10%-Co 3 O 4 films in out-of-plane lattice parameter can be seen. In addition, T P and T C show an opposite tendency to the in-plane lattice parameter. LCMO:10%-NiO and LCMO:10%-Co 3 O 4 films show similar T P and T C because of the similar lattice parameters. Meanwhile, the resistivities are different due to distinct conducting properties of Co 3 O 4 and NiO.

Conclusion
In summary, we successfully demonstrated PAD method to fabricate LCMO:NiO and LCMO:Co 3 O 4 epitaxial self-assembled composite films on LAO substrates. PAD method, which provides an alternative yet simpler approach to the growth of self-assembled composite films, is more cost effective compared to the conventional PLD methods. Moreover, the films produced by the PAD method showed enhancement in the LFMR effect which is crucial in the practical application, as demonstrated by the temperature dependent MR analysis. The maximum absolute MR values were further improved by the addition of NiO or Co 3 O 4 due to introduced insulating phase at the grain boundaries. This is entails dramatic increase in resistivity and decrease in T P with the addition of NiO and Co 3 O 4 . The temperature-dependent magnetization curve also revealed that T C showed the same trend as T P . More suppressed magnetism was observed from the LCMO:Co 3 O 4 composite films than the LCMO:NiO films on account of the antiferromagnetic properties of the Co 3 O 4 phase. The solution-processed epitaxial films with the enhanced LFMR effect presented here will bring a breakthrough in the field of hole-doped perovskites and their growth.   metal-polymer solutions were then filtered in an Amicon unit, which is designed to pass materials with molecular weight of less than 30,000 g/mol, to remove the unbound ions and concentrate the solutions. Sample Characterization. The crystal structure of the films was characterized by X-ray diffraction (XRD).

Methods
The surface morphology and microstructures were analyzed by atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). The temperature dependence of resistivity was measured by a Quantum Design Physical Property Measurement System (PPMS) along the film surface using a standard four-probe method, with the magnetic field applied normal to the film surface. Temperature-dependent magnetization was measured by a superconducting quantum interference device (SQUID) magnetometer.