Dielectric relaxations and alternating current conductivity in manganese substituted cobalt ferrite

Manganese (Mn) substituted cobalt ferrites (CoFe2-xMnxO4, referred to CFMO) have been synthesized by the solid state reaction method and their dielectric properties and ac conductivity have been evaluated as a function of applied frequency and temperature. X-ray diffraction measurements indicate that CFMO crystallize in the inverse cubic spinel phase with a lattice constant similar to 8.38 angstrom. Frequency dependent dielectric measurements at room temperature obey the modified Debye model with relaxation time of 10(-4) s and spreading factor of 0.35(+/- 0.05). The frequency (20 Hz-1MHz) and temperature (T 300-900 K) dependent dielectric constant analyses indicate that CFMO exhibit two dielectric relaxations at lower frequencies (1-10 kHz), while completely single dielectric relaxation for higher frequencies (100 kHz-1 MHz). The dielectric constant of CFMO is T-independent up to similar to 400 K, at which point increasing trend prevails. The dielectric constant increase with T>400K is explained through impedance spectroscopy assuming a two-layer model, where low-resistive grains separated from each other by high-resistive grain boundaries. Following this model, the two electrical responses in impedance formalism are attributed to the grain and grain-boundary effects, respectively, which also satisfactorily accounts for the two dielectric relaxations. The capacitance of the bulk of the grain determined from impedance analyses is similar to 10 pF, which remains constant with T, while the grain-boundary capacitance increases up to similar to 3.5 nF with increasing T. The tan delta (loss tangent)-T also reveals the typical behavior of relaxation losses in CFMO. (C) 2014 AIP Publishing LLC.