Abstract
We present simple but quantitative theoretical evidence suggesting that the magnetoelastic (MEL) stress, which breaks the hexagonal symmetry of the basal plane in (${\mathrm{Ho}}_{n}/$${\mathrm{Lu}}_{15}$)\ifmmode\times\else\texttimes\fi{}50 (0001) superlattices ($n$=8--85 atomic planes, separated by $c/2),$ has a single-ion crystal-electric-field (CEF) origin. The MEL stress ${M}^{\ensuremath{\gamma}}$ has both a volume component ${M}_{\mathrm{vo}}^{\ensuremath{\gamma}}$ as well as an interface component ${M}_{s}^{\ensuremath{\gamma}}.$ We show within the Hartree-Fock approximation that screening of the magnetostrictively distorted CEF by conduction-band electrons and the introduction of a spatial structure for the ${\mathrm{Ho}}^{3+}$ ionic charge are both needed to explain quantitatively the experimental values of ${M}_{\mathrm{vo}}^{\mathrm{\ensuremath{\gamma}}}$=+0.275 GPa and ${M}_{s}^{\ensuremath{\gamma}}/(c/2)=\ensuremath{-}7$ GPa; these effects are needed in particular to account for the different orders of magnitude and opposite signs. The simple point-charge model for bare ions in the magnetostrictively distorted CEF, as well as the simple free-electron screening approximation, both fail.
Published Version
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