Growth of Single‐Crystal and Polycrystalline Thin Films of MgAl 2 O 4 and MgFe 2 O 4

Single‐crystal and polycrystalline films of Mg‐Al 2 O 4 and MgFe 2 O 4 were formed by two methods on cleavage surfaces of MgO single crystals. In one procedure, aluminum was deposited on MgO by vacuum evaporation. Subsequent heating in air at about 510°C formed a polycrystalline γ‐Al 2 O 8 film. Above 540°C, the γ‐Al 2 O, and MgO reacted to form a single‐crystal MgAl 2 O 4 film with {001} MgAl 2 O 4 ‖{001} MgO. Above 590°C, an additional layer of MgAl 2 O 4 , which is polycrystalline, formed between the γ‐Al 2 O 3 and the single‐crystal spinel. Polycrystalline Mg‐Al 2 O 4 formed only when diffusion of Mg 2+ ions proceeded into the polycrystalline γ‐Al 2 O 3 region. Corresponding results were obtained for Mg‐Fe 2 O 4 . MgAl 2 O 4 films were also formed on cleaved MgO single‐crystal substrates by direct evaporation, using an Al 2 O 3 crucible as a source. Very slow deposition rates were used with source temperatures of ∼1350°C and substrate temperatures of ∼800°C. Departures from single‐crystal character in the films may arise through temperature gradients in the substrate.