Mechanical and chemical strain induced anomalous properties in lanthanide perovskite thin films
Perovskite crystalline thin films, especially lanthanide cobaltites and manganites, are well known to exhibit changes in physical properties due to strained lattices. Most commonly, strain arises in thin film heterostructures due to a lattice parameter mismatch between the thin film and substrate, resulting in stretched or compressed lattices. This type of interfacial strain typically begins to relax after a few tens of nanometers as the film surpasses a critical thickness. While film-substrate interfacial strain has been the subject of intense study, this work will focus more on types of strain that can persist in the films independent of thickness and introduced by other means. To clarify, chemical strain refers to strain caused by local lattice distortions due to fractional stoichiometry or atomic vacancies. Mechanical strain refers to induced strain in the film by other structures such as horizontal multilayers, vertical nanopillars, or nanoparticles for example.
Changes in physical properties due to chemical strain are studied in PrBaCo2O5+delta double perovskite thin films. Electronic and magnetic properties in perovskite cobaltites are very sensitive to the oxygen content of the films and it is demonstrated that magnetization is dramatically enhanced while resistivity is drastically reduced by increasing the oxygen content in the films. Additionally, variations in cation stoichiometry are shown to tune La1-xMn1-yO3 films from paramagnetic insulators to ferromagnetic metallic behaviors. Mechanical strain is introduced into La0.7Sr0.3MnO3 thin films via embedded MgO nanoparticles. The overall strain in the LSMO matrix is tunable from approx. 0.2% - 0.9% according to nanoparticle size, molar concentration, and distribution. As a result, detailed tunability of metal-insulator transition temperature, magnetoresistance, and resistivity are demonstrated.