Magnetic Properties of CoFe₂O₄ Thin Films Synthesized by Radical-Enhanced Atomic Layer Deposition

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 (M_s) ranged from 260 to 550 emu/cm³ and magnetic coercivity (H_c) 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.