A radical-enhanced atomic layer deposition (RE-ALD) process was developed for growing ferrimagnetic CoFe2O4 thin films. By utilizing bis(2,2,6,6-tetramethyl-3,5-heptanedionato) cobalt(II), tris(2,2,6,6-tetramethyl-3,5-heptanedionato) iron(III), and atomic oxygen as the metal and oxidation sources, respectively, amorphous and stoichiometric CoFe2O4 films were deposited onto SrTiO3 (001) substrates at 200 °C. The RE-ALD growth rate obtained for CoFe2O4 is ∼2.4 Å/supercycle, significantly higher than the values reported for thermally activated ALD processes. Microstructural characterization by X-ray diffraction and transmission electron microscopy indicate that the CoFe2O4 films annealed between 450 and 750 °C were textured polycrystalline with an epitaxial interfacial layer, which allows strain-mediated tuning of the magnetic properties given its highly magnetostrictive nature. The magnetic behavior was studied as a function of film thickness and annealing temperature: saturation magnetization (Ms) ranged from 260 to 550 emu/cm3 and magnetic coercivity (Hc) ranged from 0.2 to 2.2 kOe. Enhanced magnetic anisotropy was achieved in the thinner samples, whereas the overall magnetic strength improved after annealing at higher temperatures. The RE-ALD CoFe2O4 thin films exhibit magnetic properties that are comparable to both bulk crystal and films grown by other deposition methods, with thickness as low as ∼7 nm, demonstrating the potential of RE-ALD for the synthesis of high-quality magnetic oxides with large-scale processing compatibility.
Keywords: ALD; CoFe2O4; RE-ALD; atomic layer deposition; cobalt ferrite; epitaxial; magnetic oxides; plasma; thin films.