Date of Award
Doctor of Philosophy
Materials Science and Engineering
David G Mandrus
Zheng Gai, David C Joy, Bin Hu
Various metal silicide and germanide magnetic nanowires were synthesized using a home-built CVD [chemical vapor deposition] system. The morphology, composition, and magnetic properties of the nanowires were studied and correlated with growth parameters such as temperature, pressure, time, and source-substrate distance.
One of the compositions targeted for synthesis was MnSi [manganese silicide]. In bulk, this material orders helimagnetically at Tc [curie temperature] = 30K, with a helical pitch of about 20 nm. After extensive study, we learned that the thickness of the silicon dioxide layer on the substrate is a critical parameter for the growth of MnSi nanowires. An oxide assisted one-dimensional growth mechanism was proposed. A growth diagram was created that shows the effect of various growth parameters on the products. The nanowires were characterized magnetically in a SQUID [superconducting quantum interference device] magnetometer using AC [alternating current] susceptibility. These measurements confirmed helimagnetic ordering and skyrmion lattice formation in the nanowires.
Another composition targeted for synthesis was FeGe2 [iron digermanide]. In bulk, FeGe2 undergoes a spin density wave transition at 289 K. Nanowires of FeGe2 have not been previously reported. We were able to grow FeGe2 nanowires on a Ge substrate. The FeGe2 nanowires were identified by selected area diffraction using a transmission electron microscope. The growth direction was determined to be . FeGe2 nanowires were integrated into four-probe nanodevices, which were fabricated through an e-beam lithography system via beam dose correction and a double-layer geometry. The combined magnetic and electronic transport measurements show that while the antiferromagnetic spin density wave is still present along the FeGe2 nanowire long axis, ferromagnetism was identified in other directions.
Tang, Siwei, "Synthesis and Characterization of Magnetic Nanowires Prepared by Chemical Vapor Deposition. " PhD diss., University of Tennessee, 2014.