Electronic and Structural Properties of Phase-Pure Magnetite Thin Films: Effect of Preferred Orientation

Abstract

Materials in the form of thin films are getting worldwide attention because of their rapid development in the electronics industry. The demand is not only to prepare thin films using low-cost methods but also to induce tunable electronic properties (i.e., ferroelectricity, dielectric/impedance behavior, etc.) at room temperature. Keeping in view today’s demand for electronic materials, iron oxide thin films have been prepared using a low-cost sol–gel method with variation in the sol concentration in the range of 0.2–2.0 mM. Spin-coated films have been annealed at 300°C for 60 and 120 min in the presence of a magnetic field. The magnetite (Fe3O4) phase was observed at 1.4 mM, with preferred orientation along the (220) plane, under as-deposited and annealed conditions. The rest of the concentration range we studied results in the inclusion of small traces of maghemite (γ-Fe2O3) along with magnetite under all the preparation conditions. However, such inclusions result in the shift of preferred orientation from the (220) to the (400) plane of the magnetite (Fe3O4) phase. Formation of Fe3O4 phase has been confirmed using the Verwey transition at ∼ 124.8 K along with the appearance of a Raman A1g band at 667 cm−1. A high dielectric constant (∼ 80.23) and low tangent loss (∼ 0.00239) at log f = 5.0 were obtained at room temperature for 1.4 mM-based thin films. Such behavior may have been observed because of the high grain boundary resistance (5.5 × 104 Ω) and high grain boundary density (0.9939) at a sol concentration of 1.4 mM. An increase in dielectric constant and tangent loss was observed with the increase in temperature from 30 to 210°C. An activation energy of 2.007 eV was observed for the 1.4 mM-based thin films. The conductivity obeys Jonscher’s power law and has been associated with the overlapping large polaron tunneling model. Room-temperature ferroelectricity was observed for iron oxide thin films with maximum polarization (Pmax ∼ 14.74 μC/cm2) at 1.4 mM sol concentration.