Thickness-Dependent Thermal Oxidation of Ni into NiO Thin Films

We report thickness-dependent thermal oxidation in Ni (t =10-300 nm) thin films exposed to air annealing and the resulting vibrational, magnetic and electrical properties of Ni films deposited directly on thermally oxidized Si substrate using magnetron sputtering technique at ambient temperature. As-deposited Ni films exhibit face-centred cubic structure with fine crystals and large lattice constant (a(Ni)) at lower t (<50 nm). With increasing t, a(Ni) decreases and approaches to bulk value. With increasing T-A, a(Ni) not only decreases to bulk Ni due to improved crystallization but also reduces below bulk Ni for t >50 due to formation of NiO. The relative fraction of Ni and NiO in annealed films up to 400 degrees C strongly depends on t. Annealing Ni films at 500 degrees C results into complete oxidation of Ni into granular-type NiO. X-ray reflectivity studies reveal that oxidation process occurs from surface of the films converting Ni into NiO possibly through layer by layer process, which is subtle to t. Raman spectra show that intensity ratio between one-phonon longitudinal optical (LO) and two-phonon LO bands decreases and intensity of two-magnon band increases with increasing t for films annealed at particular T-A. This confirms the growth of NiO not only with increasing T-A, but also with t. As-deposited films exhibit ferromagnetism at room temperature. The presence of Ni and NiO in annealed films implies coexistence of ferromagnetic and antiferromagnetic interactions, leading to tunable exchange bias (H-E), whose magnitude strongly depends on the ratio between Ni and NiO. Electrical resistance (R) of the as-deposited Ni films decreases with increasing t and follows the Namba's model. Upon annealing, R increases largely due to oxidation of Ni. The observed results are explained on the basis of thickness dependent thermal oxidation process with increasing T-A.