The structures of antiferromagnetic Cr 2 O 3 (0001) thin films with perpendicular exchange bias were investigated using reflection high-energy electron diffraction, X-ray reflectivity, and synchrotron X-ray diffraction. We mainly investigated the specific ion positions of Cr 3þ and O 22 in the corundum structure and discussed their relationship to the magnetic anisotropy of Cr 2 O 3 . The Cr 2 O 3 (0001) thin film grown on a Pt(111) buffer layer exhibited a perpendicular exchange anisotropy density of 0.42 mJ/m 2 , in which the Cr 3þ position is the primary factor in the enhancement of magnetic anisotropy due to dipolar-interaction. In contrast, the single-crystalline Cr 2 O 3 (0001) film grown on a a-Al 2 O 3 (0001) substrate featured a low exchange magnetic anisotropy of 0.098 mJ/m 2 . In this film, the Cr 3þ position parameter is an insignificant factor, leading to low magnetic anisotropy. The O 22 ion position also differs between the two types of films, which can affect both the magnetic anisotropy energy originating from fine structures and the magneto-electric properties of Cr 2 O 3 .
The odd-even effects are renowned as a mysterious phenomenon in broad fields of science but have never been established as an effective approach for materials engineering. We demonstrate that the parity of alkyl carbon number n can cause alternating emergence of polar/antipolar organic semiconductor crystals. This is achieved by the development of a series of polar rod-like molecules, composed of a linkage between extended π-core (head) and alkyl chains (tail), exhibiting both high layered crystallinity and well-balanced end-to-end affinity. The molecules are unidirectionally aligned to form two-dimensional array, and the eventual polar monomolecular layers present two distinct types of interlayer stacking depending on the parity of n: alternating head-to-head and tail-to-tail (antipolar) alignment in odd-n crystals, and uniform head-to-tail (polar) alignment in even-n crystals. The latter allows to obtain polar semiconductor films that considerably improve interfacial carrier transport characteristics. The findings are key for creating polarity-controlled optoelectronic materials and devices.
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