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Tuning of Transport and Magnetic Properties in Epitaxial LaMnO3+δ Thin Films
The effect of compressive strain on the transport and magnetic properties of epitaxial LaMnO3+δ thin films has been investigated. It is found that the transport and magnetic properties of the LaMnO3+δ thin films grown on the LaAlO3 substrates can be tuned by the compressive strain through varying film thickness. And the insulator-metal transition, charge/orbital ordering transition, and paramagnetic-ferromagnetic transition are suppressed by the compressive strain. Consequently, the related electronic and magnetic transition temperatures decrease with an increase in the compressive strain. The present results can be explained by the strain-controlled lattice deformation and the consequent orbital occupation. It indicates that the lattice degree of freedom is crucial for understanding the transport and magnetic properties of the strongly correlated LaMnO3+δ.
Perovskite manganese oxides La1−xAxMnO3 (A = alkaline earth) have attracted a great deal of attention due to their interesting properties such as colossal magnetoresistance (CMR), charge/orbital ordering (COO), and insulator-metal transition (IMT) [1–3]. The interesting properties originate from the strong correlation of charge, orbital, spin, and lattice degrees of freedom. In this family, the substitution of divalent A for La introduces hole carriers in the Mn 3D band and oxidizes Mn3+ to Mn4+, which results in the ferromagnetic metal (FMM) state in terms of double exchange (DE) interaction . In particular, even if without chemical substitution, the LaMnO3 compound also exhibits a wide range of oxygen nonstoichiometry, which involves the oxidation of some Mn3+ to Mn4+ in samples of global composition LaMnO3+δ. However, the insertion of excess oxygen is impossible in the perovskite structure as there is no straightforward way of accommodating any extra oxygen in the close-packed structure. The oxygen nonstoichiometry in LaMnO3+δ is incorporated via cation vacancies on both A and B sites . Therefore, the actual crystallographic formula of such compositions should be written as La1−xMn1−xO3, with .
The structure, transport, and magnetic properties of LaMnO3+δ highly depend on the value of δ [7–9]. As , the crystallographic structure of LaMnO3+δ is orthorhombic structure (Pbmn, ) and is strongly Jahn-Teller (JT) distorted; with the δ increasing, the structure changes to the slightly JT distorted orthorhombic structure (Pbmn, ); when the , it falls into the rhombohedral structure. Ritter et al. reported that a small fraction of FMM phase appears in LaMnO3.07 due to the local Mn3+–O–Mn4+ DE interaction . Furthermore, the COO phase emerges at low temperatures below ~110 K, which is far below the FM transition temperature (~150 K) for the and 0.1 samples. The magnetization measurement results of the samples () show a step-like jump at and . Meanwhile Choi et al. reported that a giant softening by 30 cm−1 of the 490 and 620 cm−1 JT and breathing optical phonon modes had been observed by Raman spectroscopy below in the LaMnO3+δ () compounds . The results indicate the importance of the electron-phonon coupling in the appearance of COO phase. For these LaMnO3+δ () compounds, with decreasing temperature these samples exhibit transition from a paramagnetic insulator (PMI) to FMM at , where the resistivity starts to decrease. At low temperatures, the samples undergo transition from FMM state to COO state at , while the resistivity shows an upturn. This COO phase coexists with the isotropic three-dimensional FM state in spite of the insulating behavior. The overall behaviors of LaMnO3+δ (0.085 ≤ δ ≤ 0.125) compounds are quite similar to those of the lightly doped La1−xSrxMnO3 (0.11 < x < 0.15) [13, 14].
The study on the lightly doped La1−xSrxMnO3, the counterpart of the LaMnO3+δ, has verified the essential role of lattice deformation in the formation of COO phase. Chen et al. succeeded in realizing the COO phase in the La7/8Sr1/8MnO3 thin films induced by the anisotropic strains on (011) SrTiO3 substrates . Wang et al. uncovered that the in-plane tensile strain in the La7/8Sr1/8MnO3 thin film grown on the 0.7Pb (Mg1/3Nb2/3)O3–0.3PbTiO3 substrate can induce COO . However, few works have been reported on the strain effect on the properties of nonstoichiometric LaMnO3+δ thin films. Zheng et al. found that the JT distortion of MnO6 reduces the charge coupling of the LaMnO3+δ thin film under an in-plane tensile strain on 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 substrate [17, 18]. In this letter, the epitaxial LaMnO3+δ thin films have been fabricated on (001)-oriented LaAlO3 substrate, and in-plane compressive strains have been modified by varying the thickness of thin film. The effect of compressive strain on COO transition and FM transition of the films has been systematically investigated.
The LaMnO3+δ thin films were grown on single crystal substrates by dc magnetron sputtering. We chose (001)-oriented LaAlO3 (LAO) and (001)-oriented SrTiO3 (STO) as substrates, and the lattice parameters of the substrates are aLAO = 3.792 Å and aSTO = 3.905 Å. The deposition temperature is 700°C and there are 10 Pa oxygen-argon mixed gases flowing during deposition. After deposition, the films were in situ cooled to room temperature in the deposition atmosphere. The thickness of the films was controlled by the deposition time. The degree of orientation and crystallographic characterization of the LaMnO3+δ thin films were measured on X-Pert-PRO system using Cu Kα radiation. The superconducting quantum interference device (SQUID) magnetometer was used to measure the magnetic properties of the LaMnO3+δ thin films and electrical properties were performed by four-electrode method in a physical property measurement (PPMS) system.
3. Results and Discussion
Figure 1(a) shows the XRD θ–2θ scan for the LaMnO3+δ/LAO thin film of thickness ~50 nm. The LaMnO3+δ thin films are c-axis oriented and there are no secondary phases. The inset shows the XRD scans on the LaMnO3+δ (101) and LAO (101) reflections. Fourfold symmetry is clearly seen for both LaMnO3+δ thin film and LAO substrate, which is an indication of cubic-on-cubic epitaxial growth of the LaMnO3+δ thin films on the LAO substrates. The out-of-plane lattice parameter of 50 nm LaMnO3+δ/LAO was calculated to be 3.9445 Å. As mentioned above, the lattice constants of the LaMnO3+δ bulk materials vary with δ. By comparing the and of the LaMnO3+δ thin films with those of LaMnO3+δ bulk materials, it is estimated that the δ of the LaMnO3+δ thin films is ~0.09. For δ0.09, the lattice constant of the LaMnO3+δ bulk materials is 3.903 Å [10, 12]. This value is smaller than that of the 50 nm LaMnO3+δ/LAO, indicating that the films are subjected to in-plane compressive and out-of-plane tensile strain. Figure 1(b) shows the out-of-plane lattice parameters of the LaMnO3+δ/LAO depending on the film thickness. Clearly, for the 30 nm LaMnO3+δ/LAO thin film, the out-of-plane lattice parameter is larger than that of the bulk material, which indicates that there exists in-plane compressive strain in the LaMnO3+δ/LAO thin film. With the increasing of film thickness, the out-of-plane lattice parameter decreases and approaches the value of the bulk material. The results show that the in-plane compressive strain of the LaMnO3+δ/LAO exhibits a relaxation with the increase of film thickness.
Figure 2(a) shows the temperature-dependent resistivity of the LaMnO3+δ/LAO. The LaMnO3+δ/LAO presents a large variation of electrical transport properties with the increase of film thickness. For the 30 nm LaMnO3+δ/LAO, it exhibits insulating behavior in the whole temperature range. For the 50 nm, 75 nm, and 100 nm LaMnO3+δ/LAO, with lowering temperature these films exhibit IMT at , where the resistivity starts to decrease. The FMM state undergoes a transition to COO state at , while the resistivity shows an upturn. The () of the 50 nm, 75 nm, and 100 nm LaMnO3+δ/LAO are 182 (94) K, 186 (139) K, and 194 (141) K, respectively. Furthermore, both transition temperatures and of the LaMnO3+δ/LAO can be enhanced by an external magnetic field. As shown by the dashed line in Figure 2(a), the magnetic field of 5 T increases the and of 50 nm LaMnO3+δ/LAO to 208 K and 110 K, respectively. The results indicate that the magnetic field could stabilize the COO phase in LaMnO3+δ thin films, which is similar to the reported in the lightly doped La1−xSrxMnO3 () . Figure 2(b) presents the temperature dependence of resistivity of the LaMnO3+δ/STO thin film. The electrical transport properties of LaMnO3+δ/STO are almost independent of the film thickness, in contrast to those of LaMnO3+δ/LAO. The 30 and 100 nm LaMnO3+δ/STO exhibit two successive transitions upon cooling: high-temperature PMI phase to intermediate FMM phase and then to low-temperature COO phase at . The () of the 30 nm and 100 nm LaMnO3+δ/LAO are 125 (206) K and 134 (205) K, respectively.
Figure 3 shows the temperature dependence of the field-cooled (FC) magnetization under a field of 0.1 T for the LaMnO3+δ/LAO. The FM transition temperature is defined as the inflection point in the M–T curves. Similar to the transport properties, the magnetic properties of LaMnO3+δ/LAO are strongly dependent on the film thickness as well. The 30, 50, 75, and 100 nm LaMnO3+δ/LAO exhibit the of 203, 205, 210, and 220 K. The values of the LaMnO3+δ/LAO increase with the increase of the film thickness.
The variation of the transport and magnetic properties of the LaMnO3+δ/LAO with the film thickness suggests the essential role of the lattice degree of freedom. In LaMnO3+δ/LAO under large compressive strain, the MnO6 octahedra are stretched at the out-of-plane direction and compressed in the in-plane. The lattice distortion can make electrons tend to occupy the orbital and the in-plane transfer integral for the FM-DE decreases, leading to a lower and the insulating transport behavior . With the decrease of the compressive strain, the orbital occupation is weakened and the is increased with the film thickness. Another important feature is the evolution of the COO transition of LaMnO3+δ/LAO with the compressive strain. For the 30 nm LaMnO3+δ/LAO, the COO transition is quenched and the film exhibits insulating behavior in the whole temperature range. Furthermore, the increases with the decrease (increase) of in-plane compressive strain (film thickness). It is well known that the occurrence of COO transition in bulk material of LaMnO3+δ is intimately correlated with structural anomalies that -axis increases and a-, b-axes decrease . The lattice deformation associated with the structural anomalies would prefer an occupation of special orbital arrangements that are characteristic of COO [19, 21, 22]. For the LaMnO3+δ thin films, the epitaxial strain provided by the substrate would clamp the in-plane lattice of the film. As a result, the lattice deformation associated with structural anomalies of LaMnO3+δ thin films would be largely diminished by the epitaxial strain. For the thinnest 30 nm LaMnO3+δ/LAO, the in-plane lattice constants are fixed to that of LAO substrate and the out-of-plane lattice parameter is elongated due to the large compressive strain. Consequently, no COO transition is observed. As shown in Figure 1(b), the compressive strain and the clamp effect decrease with the increase of the film thickness. Then the presence of COO phase at low temperatures is expected to appear in thicker films and increases with the increase of film thickness. For the LaMnO3+δ/STO with negligible compressive strain, the suppression of lattice deformation and the associated structural anomalies by the STO substrate can be neglected. Consequently, the LaMnO3+δ/STO films with different thickness show bulk-like behavior and the COO transitions are independent of the film thickness, as shown in Figure 2(b). These results verify that the transport and magnetic properties of the LaMnO3+δ thin film can be tuned by the epitaxial compressive strain and confirm the importance of the lattice degree of freedom in the LaMnO3+δ compound.
In summary, the effect of compressive strain induced by substrate on the transport and magnetic properties of LaMnO3+δ thin films has been investigated. It was found that the transport and magnetic properties of LaMnO3+δ thin films could be tuned by the compressive strain. We demonstrated that the , , and increase with the decrease of the compressive strain. These results indicate the vital role of lattice degree of freedom and the importance of coupling among charge, orbital, spin, and lattice in LaMnO3+δ.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the National Nature Science Foundation of China under Contract nos. 11274311, 10904147, and 11104273, the Joint Funds of the National Natural Science Foundation of China and the Chinese Academy of Sciences’ Large-scale Scientific Facility (Grand no. U1232210), and the Knowledge Innovation Program of the Chinese Academy of Sciences under Contract no. Y04N371121.
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