Epitaxial thin films of (0 0 0 1)-oriented M-type barium hexaferrite, BaFe12O19 (or BaM), have been grown on sapphire (0 0 0 1) substrates by plasma-assisted molecular beam epitaxy. In situ monitoring of the evolution of the reflection high-energy electron diffraction (RHEED) patterns showed that the growth of BaFe12O19 films began with an unintentional Fe2O3 (0 0 0 1) layer with an in-plane lattice parameter of 5.135 angstrom. Fortuitously, this unintentional Fe2O3 layer with a thickness of about 13 nm acts as an important "bridge" layer for the subsequent BaFe12O19 growth, and assists in accommodating the large lattice mismatch between BaFe12O19 and sapphire. Comparison of the RHEED patterns of the unintentional Fe2O3 layers and their corresponding intensity profiles to those of the BaFe12O19 films indicated rotation of the BaFe12O19 lattice by 30 degrees with respect to Fe2O3. The epitaxial relationships for BaFe12O19 growth are BaFe12O19 [1 1 0 0]//Fe2O3 [1 1 2 0]//sapphire [1 1 2 0] and BaFe12O19 [1 1 2 0]//Fe2O3 [1 1 0 0]//sapphire [1 1 0 0]. The lattice mismatch between the 30 degrees-rotated BaFe12O19 films and the Fe2O3 bridge layers is less than 1%. The in-plane lattice parameter of BaFe12O19 determined from RHEED patterns is about 5.88 angstrom at the growth temperature of 750 degrees C. (C) 2009 Elsevier B.V. All rights reserved.