Mr. Derek Lau Profile

Derek Lau

at National Energy Technology Lab.

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Publications (2)

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Proceedings Article | 14 May 2019 Presentation + Paper

Fabrication and evaluation of sapphire fiber cladding via magnesium aluminate spinel sol-gel based approaches

S. Bera, B. Liu, J. Wuenschell, J. Baltrus, D. Lau, B. Howard, M. Buric, B. Chorpening, P. Ohodnicki

Proceedings Volume 11000, 110000J (2019) https://doi.org/10.1117/12.2518506

KEYWORDS: Sapphire, Spinel, Fiber coatings, Cladding, Crystals, Sol-gels, Coating

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The absence of an effective and stable cladding has been a major hurdle in utilizing single crystal fibers for harsh environment sensing applications despite the promise of sapphire for temperatures as high as 1800°C. This work discusses the development of a high temperature cladding for sapphire fibers using wet chemical methods. Magnesium aluminate spinel has been chosen as the material for the cladding as it has a lower refractive index compared to sapphire and does not undergo significant interdiffusion with sapphire at temperatures below approximately 1200°C. Different sol-gel based approaches have been pursued to develop polycrystalline cladding layers with thicknesses greater than a micron, as required to ensure adequate confinement of the guided electromagnetic radiation within the fiber core. For sapphire fibers, high temperature stability of the cladded fibers as well as the effect of the cladding layer on optical characteristics under different application relevant gas environments at elevated temperatures has been investigated.

Proceedings Article | 18 September 2018 Presentation

Magnetic imaging of Dzyaloshinskii domain walls in Co/Ni based asymmetric superlattices (Conference Presentation)

Vincent Sokalski, Maxwell Li, Derek Lau, Marc De Graef

Proceedings Volume 10732, 107323E (2018) https://doi.org/10.1117/12.2322495

KEYWORDS: Magnetism, Superlattices, Microscopy, Velocity measurements, Image resolution, Transmission electron microscopy

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Topologically protected magnetic features including Skyrmions have drawn a great deal of attention recently for future computing applications due to the unprecedented efficiency with which they can be manipulated by spin current. These features are stabilized by the Dzyaloshinskii-Moriya Interaction (DMI), which favors a chiral winding of neighboring electron spins. In this talk, we examine the impact of an interface-induced DMI on the structure of magnetic domain walls (DWs) in asymmetric [Pt/Co/Ni/Ir]xN based superlattices where structural inversion symmetry is broken. The interfacial DMI vector is measured by examining the asymmetric growth of magnetic bubble domains via Kerr microscopy where both in-plane and perpendicular magnetic fields are applied. Experimental growth velocity measurements are fit to the dispersive stiffness model[1] based on highly anisotropic energy of Dzyaloshinskii DWs coupled with an attempt frequency that depends on the DW’s internal magnetization akin to chiral damping.[2] DWs were directly imaged using high resolution Lorentz mode transmission electron microscopy on thicker asymmetric superlattices which display a remnant labyrinth domain pattern characteristic of bubble materials. Striking features of reconstructed Lorentz induction maps associated with Dzyaloshinskii DWs are presented and discussed theoretically. We confirm a preferred chirality of Néel domain walls in our system and demonstrate the ability to tune the DMI constant through variations in thickness and composition. [1] J. P. Pellegren, D. Lau, and V. M. Sokalski, "Dispersive Stiffness of Dzyaloshinskii Domain Walls," Physical Review Letters vol. 119, p. 027203, 2017. [2] E. Jue, C. K. Safeer, M. Drouard, A. Lopez, P. Balint, L. Buda-Prejbeanu, et al., "Chiral damping of magnetic domain walls," Nat Mater, vol. 15, pp. 272-277. 2016.

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