Substrate-dependent post-annealing effects on the strain state and electrical transport of epitaxial La_5/8-yPr_yCa_3/8MnO₃ films

In perovskite manganite films RE_1−xAE_xMnO₃ (where RE is a trivalent rare earth element and AE is a divalent alkaline earth element), the complex interaction between charge, lattice, spin, and orbital degrees of freedom results in a variety of fascinating physical properties, such as insulator-to-metal transition, negative differential resistance and memory effect.^1–5 Among them, electric phase separation (EPS), i.e. the coexistence of ferromagnetic (FM) metallic domains and antiferromagnetic (AFM) charge-ordering (CO) insulating domains, is recognized as the intrinsic feature and origin for percolation-type metal-insulator transition in several strongly correlated electron systems.^6,7 Besides electric and magnetic fields, epitaxial strain is found to be an effective way to tune the EPS and MIT in these materials.^8–12 The strain-induced transport changes are usually interpreted with the enhanced or suppressed double-exchange effect, super-exchange interaction and Jahn-Teller effect.^13,14 As a general tendency, the in-plane compressive strain prefers FM metallic phase while tensile strain favors AFM insulating phase.^15,16 The strain of film is usually tuned by lattice mismatch,^17–19 thickness,^20,21 or piezoelectric effect from substrates.^22–24 

La_5/8−yPr_yCa_3/8MnO₃ (LPCMO) possesses a high Curie temperature and is often regarded as a model system for EPS studies.^25–28 M. Uehara et al. studied the prototypical (La,Pr,Ca)MnO₃ system using electron microscopy and found that it electronically phase-separates into a sub-micrometre-scale mixture of charge-ordering insulating regions and ferromagnetic metallic domains and this phenomenon can be used to explain the percolative transport through the ferromagnetic domains.²⁵ T. Z. Ward et al. proposed a phenomenological model and constructed an H-T phase diagram in which the system evolves through a hierarchy of energy barriers which separates the coexisting phases.^26,27 In this work, we investigated the effects of post-annealing on the strain state and electrical transport properties of LPCMO films epitaxially grown on (001)_pc SrTiO₃, LaAlO₃ and NdGaO₃ substrates. We found that lattice-mismatch combined with post-annealing is an effective approach to tune the strain in epitaxial LPCMO films, and thus to control the EPS and MIT in the films.

28 nm-thick La_5/8-0.3Pr_0.3Ca_3/8MnO₃ epitaxial films were RF-magnetron sputtered on (001)_pc STO, LAO, and NGO substrates at 790 °C under a pressure of 4.5 Pa (Ar:O₂ = 20:40), in one batch. The stoichiometric LPCMO target was synthesized by solid-state reaction. After deposition, the films were kept in situ at 600 °C in 1 atm O₂ for 8 hours to eliminate oxygen vacancy²⁹ before cooling down. To determine the thermal treatment effect on the strain state and electrical transport of the films, an additional post-annealing was performed at 800 °C / 4 hours. The crystal structure and strain state were characterized by x-ray diffraction on both conventional diffractometer (Rigaku SmartLab Film Version, Cu-Kα radiation) and synchrotron radiation (BL14B1 station at Shanghai Synchrotron Radiation Facility). The temperature- and magnetic-field- dependent electrical transport properties in the temperature range of 10∼300 K and magnetic field up to 7 T were measured by a PPMS (Quantum Design). Before the electric measurement, Ag electrodes with 0.5 mm width and 50 nm thickness were deposited on the top of the film by magnetron sputtering to form ohmic contact.

Figure 1 presents a typical x-ray diffraction pattern of LPCMO film grown on NGO substrate. Except the LPCMO/NGO (00l) and electrode Ag (111), no other reflection from different orientations or phases could be observed, indicating that the film is phase pure and highly oriented. As shown in the small angle x-ray reflectivity curve in the inset, the presence of interference peaks confirms the uniformity of the film, and the thickness is measured to be about 28 nm.

Figure 2(a) shows the typical θ-2θ XRD patterns around the (002) reflection of LPCMO films deposited on LAO, NGO and STO, respectively. Black circle curves stand for as-grown films and red triangle lines stand for post-annealed films. In all these patterns, the film (002) peak shifts closer to the substrate's peak after post-annealing, indicating that the strain in the films is partially released by the post-annealing. To quantitatively analyze the effect of thermal treatment on the strain state of the films, XRD reciprocal space mapping were performed around non-specular |$({\bar 103})$|$(1¯03)$ reflection and the results are presented in Figure 2(b). As indicates by the ‘+’ marks, all the as-grown films retain the same in-plane lattice constant (a = b = λ/Q_X) as the substrates, revealing the coherent epitaxial growth of the films on the substrates. The out-of-plane lattice constants of the films (c = 3λ/Q_Z) are in agreement with the θ-2θ scans in Figure 2(a). After post-annealing, the film's peaks shift to the nominal one of bulk LPCMO, suggesting the partial release of strain.

The lattice constants along the out-of-plane and the in-plane directions, which reflect the relevant strain state, are summarized in Figure 3. The lattice constants of bulk La_5/8-0.3Pr_0.3Ca_3/8MnO₃ obtained from powder XRD, as indicated by the dotted lines, were used as the reference to evaluate the strain in the films.²³ Comparing the lattice constant of bulk LPCMO (a = 5.465 Å, b = 5.480 Å, c = 7.723 Å) with that of the substrates, STO (cubic structure with a = 3.905 Å), LAO (rhombohedral structure with a = 3.79 Å) and NGO (orthorhombic with a = 5.43 Å, b = 5.50 Å, and c = 7.72 Å) will cause different strain in the LPCMO films epitaxially grown above them. After thermal treatment, the in-plane strain in LPCMO/LAO film varies from −1.5% to −0.1%, while that in LPCMO/ STO changes from 1.6% to 1.3%. The strain in LPCMO/NGO film remains the same due to the near-zero lattice-mismatch between LPCMO and NGO.

Figure 4 present the results of the electrical transport measurement. For the as-grown films with in situ 600 °C heat-treatment, at H = 0 T, all the three films, LPCMO/LAO, LPCMO/NGO and LPCMO/STO, are observed to be insulating in the temperature range of 90∼300 K, as shown in Figure 4(a). Under 7 T magnetic field, LPCMO/LAO and LPCMO/NGO films show metal-insulator transition at temperature (T_MI) 260 K and 182 K, respectively, while LPCMO/STO film keeps insulating even at 7 T field. This result can be explained as following: an in-plane compression tends to decrease the Mn-O bond length and increase the Mn-O-Mn bond angle. Consequently, it will reduce the electron-phonon interaction and increase the electronic hopping amplitude, which promotes the formation of FM metallic phase. The induced MIT also demonstrate that external magnetic field could effectively melt the CO, enhance the EPS.^30–35 On the other hand, as the in-plane tensile strain tends to decrease the Mn-O-Mn bond angle and reduce the double exchange interaction, in-plane tensile strain will localize electrons and increase the robustness of CO. This is why the CO in the LPCMO/STO film (+1.6% tensile strain) is so stable and robust^17,33 that 7 T magnetic field is not enough to melt the CO and induce the MIT, as shown in Figure 4(b).

Since the oxygen vacancy in the as-grown films has been compensated during the in situ 600 °C heat-treatment,²⁹ the high-temperature post-annealing effect on the electrical transport is attributed to strain engineering. For the 800 °C post-annealed films, the LPCMO/LAO film exhibits a MIT at 144 K and 0 T (Figure 4(c)), suggesting that the striking change of the strain (from −1.5% to −0.1%) may trigger the MIT. This result is consistent with that reported by Rao et al.,²⁰ where relief of the compressive strain in La_0.8Ca_0.2MnO₃/LAO cause monotonically increase in T_MI. Although decrease of the tensile strain (from 1.6% to 1.3%) in LPCMO/STO film could weaken the charge ordering (CO),¹⁵ MIT is not observed in this film. For the LPCMO/NGO film, since the tiny mismatch between film and substrate, post-annealing is hard to change the strain, and therefore, the R-T curve remains almost the same. For the post-annealed films at 7 T (Figure 4(d)), all of these films present MIT, revealing the co-action effect of strain and magnetic field on the EPS. Comparing the curves at 7 T before and after annealing, the T_MI of LPCMO/LAO shifts from 182 K to 252 K and the T_MI of LPCMO/NGO shifts from 256 K to 260 K, indicating that strain could tune the T_MI. In the case of LPCMO/STO film, when the in-plane tensile strain is partially released after thermal treatment, the CO insulating background is no longer robust and effectively melted by 7 T magnetic field. The above distinctive electrical transport behavior observed in the LPCMO films indicates that the formation of EPS depends sensitively on the strain, i.e., proportion of EPS domains and the T_MI can be guided by the interfacial stress as well as conventional magnetic field.

Excluded the non-stoichiometry effect caused by oxygen defects, the substrate-dependent high-temperature post-annealing effect on the strain states and electrical transport of the epitaxial LPCMO films have been investigated. Before the 800 °C post-annealing, all the films grown on (001)_pc STO, LAO, and NGO were found to be coherent-epitaxial, and insulating without external magnetic field. After the high-temperature post-annealing, in-plain strain in LPCMO/LAO varies from −1.5% to −0.1% while that in the LPCMO/STO changes from 1.6% to 1.3%. Consequently, MIT appears in the LPCMO/LAO film at T_MI = 144 K with H = 0 T and in the LPCMO/STO film at T_MI = 214 K with H = 7 T. These results indicate that lattice-mismatch combined with post-annealing could be used to tune the strain in epitaxial films, and thus to manipulate the functional properties of films.

This work was supported by the National Basic Research Program of China (2010CB934501, 2012CB922004) and the Natural Science Foundation of China. The authors thank beamline BL14B1 of SSRF for providing the beam time. YJ Yang acknowledge the Fundamental Research Funds for the Central Universities.