Grain and grain boundary effects on the frequency and temperature dependent dielectric properties of cobalt ferrite-hafnium composites

We report on the frequency (f=20 Hz-1 MHz) and temperature (T=300-973 K) dependent dielectric properties of hafnium (Hf) incorporated cobalt ferrite (CoF2−xHfxO4 (CFO-Hf); x=0.00-0.20). The dielectric constant (ε') of CFO-Hf is T-independent at T<450 K, at which point increasing trend prevails. A grain bulk-boundary based two-layer model, where semiconducting-grains separated by insulating-grain boundaries, satisfactorily accounts for ε-T (>450 K) variation. Correspondingly, electrical responses in impedance formalism are attributed to the grain and grain-boundary effects which also accounts for the observed two dielectric-relaxations. The results demonstrate that the dielectric phenomena in CFO-Hf can be tailored by tuning Hf-concentration.