Manganese (Mn) substituted cobalt ferrites (CoFe 2-x Mn x O 4 , 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 $8.38 Å . Frequency dependent dielectric measurements at room temperature obey the modified Debye model with relaxation time of 10 À4 s and spreading factor of 0.35(60.05). The frequency (20 Hz-1 MHz) and temperature (T ¼ 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 $400 K, at which point increasing trend prevails. The dielectric constant increase with T > 400 K 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 $10 pF, which remains constant with T, while the grain-boundary capacitance increases up to $3.5 nF with increasing T. The tan d (loss tangent)-T also reveals the typical behavior of relaxation losses in CFMO. V C 2014 AIP Publishing LLC.
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