Epitaxial hexagonal ferrites for millimeter wave devices

The objective of this research was to develop single crystal hexagonal ferrite films of superior quality. These films would be useful in millimeter wave devices such as tunable filters. They also would be useful in studying basic magnetic phenomena in these materials; for example, their high magnetocrystalline anisotropy. A novel two-step liquid phase epitaxy (LPE) process was developed which resulted in improved quality of barium hexaferrite films deposited on gallate spinel substrates (indium substituted magnesium gallium oxide spinel). The two-step process entailed the growth of a thin epitaxial film of a spinel ferrite prior to deposition of the desired hexagonal ferrite. The hexagonal ferrite films obtained by this method were more uniform and more strongly bonded to the substrate than films grown without the intermediary spinel ferrite layer. The hexagonal ferrite films exhibited large, rounded hillocks that appeared to be associated with defects in the substrate crystals. Ferromagnetic resonance measurements were made at 60 GHz. External dc magnetic bias fields of about 9 KOe were required for resonance, as expected for barium hexaferrite. Resonance linewidths were about 50 to 60 oersted. Substantial reductions in linewidth should be achievable with further improvements in crystal quality.