In this study we investigated the structural, optical band-gap, and magnetic properties of CoYFeO (0 ≤ x ≤ 0.04) nanoparticles (NPs) synthesized using a combustion reaction method without the need for subsequent heat treatment or the calcing process. The particle size measured from X-ray diffraction (XRD) patterns and transmission electron microscope (TEM) images confirms the nanostructural character in the range of 16-36 nm. The optical band-gap (E) values increase with the Y ion (x) concentration being 3.30 and 3.58 eV for x = 0 and x = 0.04, respectively. The presence of yttrium in the cobalt ferrite (Y-doped cobalt ferrite) structure affects the magnetic properties. For instance, the saturation magnetization, M and remanent magnetization, M, decrease from 69 emu g to 33 and 28 to 12 emu g for x = 0 and x = 0.04, respectively. On the other hand the coercivity, H, increases from 1100 to 1900 Oe for x = 0 and x = 0.04 at room temperature. Also we found that M, M, and H decreased with increasing temperature up to 773 K. The cubic magnetocrystalline constant, K, determined by using the "law of approach" (LA) to saturation decreases with Y ion concentration and temperature. K values for x = 0 (x = 0.04) were 3.3 × 10 erg cm (2.0 × 10 erg cm) and 0.4 × 10 erg cm (0.3 × 10 erg cm) at 300 K and 773 K, respectively. The results were discussed in terms of inter-particle interactions induced by thermal fluctuations, and Co ion distribution over tetrahedral A-sites and octahedral B-sites of the spinel structure due to Y ion substitution.

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