Charge Ordered Manganites

Most of the experimental studies were done on manganites showing an insulator-to-metal transition without field, but it was also shown that substrate-induced strain can affect the properties of charge-ordered (CO) compounds. CO is a phenomenon observed wherein electrons become localized due to the ordering of heterovalent cations in two different sublattices (Mn3+ and Mn4+). The material becomes insulating below the CO transition temperature, but it is possible to destroy this state and render the material metallic by, for example, the application of a magnetic field [136], but an electric field can also induce insulator-metal transitions in thin films of CO manganites [39]. Pr05Ca05MnO3 is an example of such a compound. In this case, a tensile stress can decrease the melting magnetic field [41] whereas a compressive strain induces a locking of the structure [120] at low temperature (i.e., under cooling, when the in-plane parameters of the film are equal to the parameters of the substrate, they are kept at this value). This idea of locking was confirmed in Pra5Sra5MnO3 where the structural and physical transitions are suppressed under cooling [137], as compared to the bulk (note that even if the compound Pr05Sr05MnO3 is not a typical CO, it has some similarities in the physical properties). In this material, the A-type antiferromagnetic phase with the Fmmm structure, which is obtained at low temperature (below 135 K) in the corresponding bulk compound, is not observed in the thin film. The consequence of the absence of structural transitions is that magnetotransport properties are affected. There is no antiferromagnetism (i.e., the A-phase) at low temperature. The material only becomes ferromagnetic insulating. This is one of the very few examples of substrate-induced strain upon the film structure. These results show that the strain effect can destabilized the charge-ordered state for CO materials but, surprisingly, it seems also possible to induce a CO state when the film composition is not a CO type (i.e., if the film has an insulator-to-metal transition without the presence of a magnetic field). This has been shown in a normally metallic Laa7Caa3MnO3 compound where the lattice-mismatch strain effects lead to a strain-induced insulating state [138]. This insulating behavior is related to the coexistence of a metallic state with a possibly charge-ordered insulating state [138, 139].

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