Abstract
Spatially resolved analysis of magnetic properties on the nanoscale remains challenging, yet strain and defects on this length-scale can profoundly affect a material's bulk performance. We present a detailed investigation of the magnetic properties of La0.67Sr0.33MnO3 thin films in both free-standing and nanowire form and assess the role of strain and local defects in modifying the films' magnetic properties. Lorentz transmission electron microscopy is used to measure the magnetocrystalline anisotropy and to map the Curie temperature and saturation magnetization with nanometric spatial resolution. Atomic-scale defects are identified as pinning sites for magnetic domain wall propagation. Measurement of domain wall widths and crystalline strain are used to identify a strong magnetoelastic contribution to the magnetic anisotropy. Together, these results provide unique insight into the relationship between the nanostructure and magnetic functionality of a ferromagnetic complex oxide film.
Highlights
The multiplicity of functionalities available in heteroepitaxial complex oxides has stimulated intense research in recent years and suggests a variety of future microelectronic technologies.[1−6] La1−xSrxMnO3 (LSMO) is one of the most studied perovskite oxides because at an optimal Sr doping of x = 0.33 it exhibits colossal magnetoresistance, high spinpolarization, and a relatively high Curie temperature of 370 K
Previous reports indicate that the magnetic and electronic properties of LSMO are sensitive to structural defects such as grain boundaries[27] or the type of ion-induced damage that might arise during patterning,[28] and there has been much discussion of the origin and extent of magnetic “dead layers” at the surface of LSMO thin films.[29,30]
Unusually narrow domain wall (DW) were observed compared with those typically found in the ferromagnetic manganites, which was attributed to mild strain in the film
Summary
A 120 nm thick La0.67Sr0.33MnO3 film was deposited onto a SrTiO3(111) substrate by metalorganic aerosol deposition.[47]. (The orthogonal components used to generate the DPC image are presented in the Supporting Information.) The DW structure observed here arises as a result of the shape anisotropy and two-dimensional behavior induced by the constrained “nanowire” geometry and is comparable to that typically found in soft ferromagnetic nanowires such as permalloy.[70] It is similar in appearance to an asymmetric transverse DW71 and has a width of 85 nm, which is in agreement with the expectation of a linear scaling of the extent of the wall with the nanowire width.[72] Vortex DW structures have previously been observed in a similar TEM lamella, grown on a (100) oriented substrate and prepared in cross-section to have similar dimensions and shape anisotropy to those here.[73] We confirmed the vortex state to be the lowest energy configuration using OOMMF74 micromagnetic simulations (see Supporting Information for details). A comparison with the DPC image of the DW suggests that there are greater fluctuations near defects, which has implications for device applications and emphasizes the importance of atomic scale magnetic characterization in improving our understanding of defects in nanoscale systems
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