Engineering topics

Trappen, Robbyn

Publications and source records attributed to Trappen, Robbyn.

6 records

DOE OSTI2019

Effect of oxygen stoichiometry on the magnetization profiles and negative magnetization in LSMO thin films

The depth-dependent magnetization in thin film oxygen stoichiometric and oxygen-deficient La 0.7 Sr 0.3 MnO 3 is investigated by using polarized neutron reflectivity and DC bulk magnetometry. The polarized neutron reflectivity results reveal that the stoichiometric sample shows enhanced interfacial magnetization relative to the rest of the film. The oxygen-deficient sample exhibits a reduced average magnetization from the optimized recipe. Both films show regions of suppressed magnetization at the surface regardless of the growth pressure. The oxygen stoichiometric film does not show an interfacial dead layer, whereas the oxygen-deficient film exhibits a dead layer whose thickness changes with temperature. At a low applied field, we observe striking differences in the depth dependence of the magnetic ordering, with the oxygen-deficient film exhibiting exchange spring behavior, while the stoichiometric film shows a constant magnetization direction across the film. Furthermore, these results suggest that the incorporation of oxygen vacancies during growth leads to an accumulation of vacancies at the interface, which is enhanced at higher temperature due to increased oxygen mobility, creating a region of reduced magnetism that couples to the rest of the film. These results offer insight into the complex behavior and role of oxygen vacancies in the magnetism of these systems. Additionally, the study reveals further details of the negative magnetization in La 0.7 Sr 0.3 MnO 3 reported in previous studies, which are discussed here.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

Depth-dependent atomic valence determination by synchrotron techniques

The properties of many materials can be strongly affected by the atomic valence of the contained individual elements, which may vary at surfaces and other interfaces. These variations can have a critical impact on material performance in applications. In this work, a non-destructive method for the determination of layer-by-layer atomic valence as a function of material thickness is presented for La 0.7 Sr 0.3 MnO 3 (LSMO) thin films. The method utilizes a combination of bulk- and surface-sensitive X-ray absorption spectroscopy (XAS) detection modes; here, the modes are fluorescence yield and surface-sensitive total electron yield. The weighted-average Mn atomic valence as measured from the two modes are simultaneously fitted using a model for the layer-by-layer variation of valence based on theoretical model Hamiltonian calculations. Using this model, the Mn valence profile in LSMO thin film is extracted and the valence within each layer is determined to within an uncertainty of a few percent. The approach presented here could be used to study the layer-dependent valence in other systems or extended to different properties of materials such as magnetism.

Hamiltonian calculations↗

DOE OSTI2018

Electrostatic potential and valence modulation in La 0.7 Sr 0.3 MnO 3 thin films

The Mn valence in thin film La 0.7 Sr 0.3 MnO 3 was studied as a function of film thickness in the range of 1–16 unit cells with a combination of non-destructive bulk and surface sensitive X-ray absorption spectroscopy techniques. Using a layer-by-layer valence model, it was found that while the bulk averaged valence hovers around its expected value of 3.3, a significant deviation occurs within several unit cells of the surface and interface. These results were supported by first principles calculations. The surface valence increases to up to Mn 3.7+ , whereas the interface valence reduces down to Mn 2.5+ . The change in valence from the expected bulk value is consistent with charge redistribution due to the polar discontinuity at the film-substrate interface. The comparison with theory employed here illustrates how this layer-by-layer valence evolves with film thickness and allows for a deeper understanding of the microscopic mechanisms at play in this effect. These results offer insight on how the two-dimensional electron gas is created in thin film oxide alloys and how the magnetic ordering is reduced with dimensionality.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Thickness dependence of La 0.7 Sr 0.3 MnO 3 /PbZr 0.2 Ti 0.8 O 3 magnetoelectric interfaces

Magnetoelectric materials have great potential to revolutionize electronic devices due to the coupling of their electric and magnetic properties. Thickness varying La 0.7 Sr 0.3 MnO 3 (LSMO)/PbZr 0.2 Ti 0.8 O 3 (PZT) heterostructures were built and measured in this article by valence sensitive x-ray absorption spectroscopy. The sizing effects of the heterostructures on the LSMO/PZT magnetoelectric interfaces were investigated through the behavior of Mn valence, a property associated with the LSMO magnetization. Here, we found that Mn valence increases with both LSMO and PZT thickness. Piezoresponse force microscopy revealed a transition from monodomain to polydomain structure along the PZT thickness gradient. The ferroelectric surface charge may change with domain structure and its effects on Mn valence were simulated using a two-orbital double-exchange model. The screening of ferroelectric surface charge increases the electron charges in the interface region, and greatly changes the interfacial Mn valence, which likely plays a leading role in the interfacial magnetoelectric coupling. The LSMO thickness dependence was examined through the combination of two detection modes with drastically different attenuation depths. The different length scales of these techniques' sensitivity to the atomic valence were used to estimate the depth dependence Mn valence. Finally, a smaller interfacial Mn valence than the bulk was found by globally fitting the experimental results.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Phase Diagram of a Three-Dimensional Antiferromagnet with Random Magnetic Anisotropy

Three-dimensional (3D) antiferromagnets with random magnetic anisotropy (RMA) that were experimentally studied to date have competing two-dimensional and three-dimensional exchange interactions which can obscure the authentic effects of RMA. The magnetic phase diagram of Fe x Ni 1-x F 2 epitaxial thin films with true random single-ion anisotropy was deduced from magnetometry and neutron scattering measurements and analyzed using mean field theory. Regions with uniaxial, oblique and easy plane anisotropies were identified. A RMA-induced glass region was discovered where a Griffiths-like breakdown of long-range spin order occurs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2015

Structural and magnetic properties of epitaxial delafossite CuFeO{sub 2} thin films grown by pulsed laser deposition

Growth of pure phase delafossite CuFeO{sub 2} thin films on Al{sub 2}O{sub 3} (00.1) substrates by pulsed laser deposition was systematically investigated as a function of growth temperature and oxygen pressure. X-ray diffraction, transmission electron microscopy, Raman scattering, and x-ray absorption spectroscopy confirmed the existence of the delafossite phase. Infrared reflectivity spectra determined a band edge at 1.15 eV, in agreement with the bulk delafossite data. Magnetization measurements on CuFeO{sub 2} films demonstrated a phase transition at T{sub C} ≈ 15 ± 1 K, which agrees with the first antiferromagnetic transition at 14 K in the bulk CuFeO{sub 2}. Low temperature magnetic phase is best described by commensurate, weak ferromagnetic spin ordering along the c-axis.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗