ReX 2 (X = S, Se) remains a copious source of controversies and unanswered questions due to its widely contrasting experimental and theoretical results. With the help of comparative first-principles electronic structure and phonon calculations, the correct structures for both systems are established, which minimize the apparent divergence of different experimental results. It is demonstrated that ReS 2 and ReSe 2 are neither iso-structural nor iso-electronic. The contributions of the in-plane and out-of-plane orbitals at the band-edges of the bulk and monolayers are coordinated with their anisotropic optical response. Under moderately high pressure, both of these systems are observed to undergo a semiconductor to metal transition. With the help of a combined full-potential density functional theory and multiplet ligand field theory (DFT+MLFT) approach, the X-ray spectral properties of these two systems are analyzed in the light of their intricate differences of optimized structures and electronic correlations.
Engineering of interfacial magnetic properties provides an extra edge in designing heterostructures with desired properties for spintronics and spincaloritronics, without drastically changing the structure of the neighboring nonmagnetic material. Here, we report on the surface termination-enhanced magnetic properties of the ferrimagnetic insulator (FMI) nickel ferrite (NFO) with the inclusion of graphene (Gr) and monolayer hexagonal boron nitride (hBN). Depth-dependent X-ray photoelectron spectroscopy (XPS) measurements reveal the presence of a layer of adsorbed oxygen at the NFO/Gr and NFO/hBN interfaces. Magnetometry and transverse susceptibility measurements indicate that the inclusion of monolayer Gr increases the saturation magnetization (M s ) by 40% and decreases the effective magnetic anisotropy by 50% across 5 K ≤ T ≤ 300 K. A similar but less pronounced effect is observed for the inclusion of hBN. Density functional theory calculations further indicate that the increase in M S due to the inclusion of Gr or hBN arises on oxygen-terminated NFO, as observed in XPS measurements. These results present ways for engineering strong interfacial magnetic effects in FMI/2D nanomaterial systems, controlling magnetism by surface termination, and developing advanced spinterfaces for applications in spincaloritronics and spin insulatronics.
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