X-ray magnetic circular dichroism (XMCD) of metallic nano-structures on epitaxial graphene

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Anderson, Nathaniel
Major Professor
David Vaknin
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Physics and Astronomy

X-ray magnetic circular dichroism (XMCD) reveals the properties of intercalated Eu/Dy/Gd and Fe/Dy islands on top of graphene on SiC(0001). Intercalated nano-clusters of Eu/Dy/Gd are formed by deposition on epitaxially grown graphene then annealment at high temperatures. STM images reveal that the graphene surface is defect free and that Eu/Dy/Gd have distinct nucleation patterns, with Eu forming distinct clusters on the Moir{\'e} pattern, while Dy and Gd are randomly distributed. XMCD measurements show the metals have a paramagnetic-like behavior with moments lower than predicted by the Brillouin function and that the intercalants are passivated beneath the graphene and stable under atmospheric exposure for months.

Fe and Dy nano-islands are grown on top of graphene then capped with Au for passivation. From XMCD, field and temperature dependence of the magnetic moment of Dy show paramagnetic behavior. The magnetic moment is not saturated at $H = 5$ T even at temperatures below which bulk-Dy is ferromagnetic. For the Fe, XMCD reveals that the gold was insufficient at capping and allowed for oxidation and spontaneous formation of magnetite nano-particles \ce{Fe3O4}. XMCD spectra show a thin sample fully oxidized and a thicker sample as part \ce{Fe3O4} and part metallic Fe. Field dependence shows a hysteresis similar to that of magnetite and temperature shows evidence of the Verwey transition at T = 125 K.

The magnetic properties for the combination of Fe islands and intercalated Gd is explored. Gold capping of the Fe was successful for a thin sample, but failed to prevent oxidation of the thicker sample. Field dependence XMCD of the Fe shows saturation at $H = 0.5$ T with values of $\sim$0.31$\mu_B$ and $\sim$0.16$\mu_B$ for the thin and thick samples, respectively. The oxidized thicker sample shows anisotropy with the respect to the incident angle while the metallic thinner sample has no anisotropy. For Gd, the two samples show nearly identical linear behavior with moments well below those of the Brillouin function.

The inverse spinel \ce{Co2VO4} is also investigated with XMCD. At 75K, the magnetic moment of the spinel flips directions; however, with XMCD we show that the octahedral \ce{V^{4+}} and tetrahedral \ce{Co^{2+}} flip while the octahedral \ce{Co^{2+}} remains unchanged.

Sat May 01 00:00:00 UTC 2021