Structural, Optical, Magnetic, and Dielectric Properties of Cr3+ Substituted Cobalt Aluminate Nanoparticles

The present study investigates the influence of Cr3+ substitution on structural, optical, dielectric, and magnetic properties of cobalt aluminates in the aluminates series CoAl2-xCrxO4, 0.00 <= x <= 0.20. These series of nanoparticles are effectively synthesized using the co-precipitation method. The cubic spinel aluminate structure is confirmed by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopic measurements. Transmission electron microscopy (TEM) analysis shows the formation of spherical nanoparticles, which are homogeneous in shape and size. The optical properties of the prepared samples have been investigated by means of optical absorbance spectra. The optical band gap E-opt(i) and Urbach energy E-u are estimated. At room temperature, the magnetization of undoped CoAl2O4 reveals a weak ferromagnetic behavior. This ferromagnetism is increased for the sample with x=0.05, and the samples with higher Cr3+ content have a tendency to be superparamagnetic on account of the formation of single-domain particles as a consequence of small particle size. Real part of dielectric constant (epsilon'), dielectric loss (tan delta), ac conductivity (sigma(ac)), real part of impedance (Z'), and imaginary part of impedance (Z) are analyzed as a function of frequency and Cr3+ content. The results show that the dielectric loss (tan delta), dielectric permittivity (epsilon'), and ac conductivity (sigma(ac)) decrease with the increase in Cr3+ content, x=0.05, and then increase with a further increase in Cr3+ content. The increase in these parameters has been explained on the basis of the hopping mechanism. From the Cole-Cole diagram, the value of the relaxation time tau can be calculated and it has been found to increase with increasing the Cr3+ content. Actually, a high dielectric constant makes these materials good for energy storage at high frequencies.