**1. Introduction**

Vigorous research has been accomplished on the fundamental, technological and potential applications of nano-ferrites. Nanomaterials of spinel ferrite have several applications in technology that include magnetic diagnostics and drug delivery [1], potential applications that include high density magnetic information storage devices [2], ferrofluid technology [3], magneto caloric refrigeration [4], magnetic recording media, magnetostriction [5], magnetic sensors, microwave devices and electrical generators etc. Ferrites are also used for catalyst and electronic devices. Ferrites are insulators exhibiting various magnetic and electric properties such as low electrical conductivity, dielectric loss, magnetic loss, relative loss factor, moderate dielectric constant, high initial permeability and saturation magnetization. Low eddy current and high resistivity makes ferrites better choice than metals [6]. Doping and thermal changes during synthesis and processing of cobalt-ferrites alter the distribution of metal ions influencing their structure and magnetic properties [7]. Priya et al. [8] doped Al ions, with Cobalt ferrite nano particles. They observed that Al doped cobalt ferrite to be suitable for high frequency applications and magnetic memory devices. Nasir Amin et al. [9] synthesized yttrium substituted cadmium ferrites. They reported yttrium doped Cd nanoferrites can be used in high-frequency microwave absorbing devices. Salma et al. [10] synthesized ferrite series having formulation SrYbyFe2–yO4 (y = 0.00 to 0.10). They observed the dispersion of frequency of ferrites is responsible for the natural magnetic resonance phenomenon and the domain wall pinning. As per the literature net magnetic moment of lanthanide series elements/ions depend on f-orbital electron number in which Er+3 is of small size (89 pm) with large magnetic moment (7 *μ*B) [11]. Magnetic anisotropy of cobalt ferrites if doped get influenced by the existence of Er+3 because of strength in spinorbit coupling. The present work reports the preparation and characterization of erbium doped cobalt ferrites combined by Citrate-gel auto combustion. The studies of CoErxFe2-xO4 with cobalt content x values ranging between 0.000 to 0.030 with step increase of x = 0.005 was reported. The crystallite size decrease with increasing erbium content indicating increase in surface area of the particle making it a good adsorbent. These adsorbents can be used in gas sensors and waste water treatment etc.
