Abstract
La-substituted Sr hexaferrite with different La concentrations $x$ were prepared and investigated in order to observe changes in magnetic and structural properties with the substitution. With diffraction methods a change in crystal structure below approximately $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ from hexagonal with spacegroup $P{6}_{3}∕mmc$ to orthorhombic with spacegroup $Cmcm$ was observed for the fully substituted La hexaferrite and attributed to a lattice distortion. A large increase in magnetocrystalline anisotropy of ${\mathrm{LaFe}}_{12}{\mathrm{O}}_{19}$ at low temperatures was confirmed by measurements and associated with the formation of an orbital momentum of ${\mathrm{Fe}}^{2+}$ together with the lattice distortion. The localization of the ${\mathrm{Fe}}^{2+}$ ion at the $2a$ site was determined from the M\"ossbauer spectrum, showing also an indication for an orbital momentum in the hyperfine field. In contrast, neither a structural phase transition nor the formation of ${\mathrm{Fe}}^{2+}$ was observed at $x<1$ in x-ray diffraction or NMR spectroscopy.
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