Browsing by Author "Trappen, Robbyn"

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Depth-dependent atomic valence determination by synchrotron techniques
(2018-11-01)
Trappen, Robbyn
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Zhou, Jinling
;
Vu Thanh Tra
;
Huang, Chih-Yeh
;
Dong, Shuai
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. A non-destructive method for the determination of layer-by-layer atomic valence as a function of material thickness is presented for La0.7Sr0.3MnO3 (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.
Electrostatic potential and valence modulation in La0.7Sr0.3MnO3 thin films
(2018-09-25)
Trappen, Robbyn
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Garcia-Castro, A. C.
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Vu Thanh Tra
;
Huang, Chih-Yeh
;
Ibarra-Hernandez, Wilfredo
The Mn valence in thin film La0.7Sr0.3MnO3 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 Mn3.7+, whereas the interface valence reduces down to Mn2.5+. 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.
Thickness dependence of La0.7Sr0.3MnO3/PbZr0.2Ti0.8O3 magnetoelectric interfaces
(2015-10-05)
Zhou, Jinling
;
Vu Thanh Tra
;
Dong, Shuai
;
Trappen, Robbyn
;
Marcus, Matthew A.
Magnetoelectric materials have great potential to revolutionize electronic devices due to the coupling of their electric and magnetic properties. Thickness varying La0.7Sr0.3MnO3 (LSMO)/PbZro(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. We found that Mn valence increases with both LSMO and PZT thickness. Piezoresponse force microscopy revealed a transition from monodomain to polydotnain 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 intetfacial 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. A smaller interfacial Mn valence than the bulk was found by globally fitting the experimental results. (C) 2015 AIP Publishing LLC.