CoCr$_{2}$O$_{4}$ is a ferrimagnetic material with a cubic Fd$_{3}$m space group belonging to a normal spinel structure attributed to the large octahedral ligand field stabilization energy of Cr$^{3+}$[1]. These spinels belong to a class of mixed oxides in which the Co$^{2+}$ ions occupy the tetrahedral A sites and the Cr$^{3+}$ ions occupy all of the octahedral B sites with the general formula AB$_{2}$O$_{4}$ [2]. Previous studies on spinel compounds have indicated that the Jahn-Teller (JT) effect is responsible for a structural distortion due to the presence of the Ni$^{2+}$ and Cu$^{2}$+ ions at tetrahedral sites. This distortion is caused by the elimination of the orbital degeneracy, resulting in an ordering of the d orbitals and a lowering of the crystal lattice symmetry [3, 4, 5]. In the present work Cu‐substituted cobalt (Co$_{(1−x)}$Cu$_{x}$ Cr$_{2}$O$_{4}$, with x=0.10, 0.50, and 0.90) nanoparticles were synthesized by sol-gel [6] method and calcined at 500 °C. Rietveld refinement of the powder x-ray diffraction (XRD) patterns confirm that the structure is dependent on x, changing from cubic for Co$_{0.90}$Cu$_{0.10}$Cr$_{2}$O$_{4}$, to a mixture of cubic and tetragonal for Co$_{0.50}$Cu$_{0.50}$Cr$_{2}$O$_{4}$, and pure tetragonal for Co$_{0.10}$Cu$_{0.90}$Cr$_{2}$O$_{4}$. This is in agreement with what is expected considering the structures observed in CoCr$_{2}$O$_{4}$ and CuCr$_{2}$O$_{4}$ [7, 8]. The crystallite size (D) was found to be 8±2 nm (Co$_{0.90}$Cu$_{0.10}$Cr$_{2}$O$_{4}$), 9±2nm (Co$_{0.50}$Cu$_{0.50}$Cr$_{2}$O$_{4}$) and 8±2 nm (Co$_{0.10}$Cu$_{0.90}$Cr$_{2}$O$_{4}$), respectively. The size distribution and morphology of the nanoparticles were determined using transmission electron microscopy. The particle sizes of 10±2 nm (Co$_{0.90}$Cu$_{0.10}$Cr$_{2}$O$_{4}$), 8±2 nm (Co$_{0.50}$Cu$_{0.50}$Cr$_{2}$O$_{4}$), and 26±2 nm (Co$_{0.10}$Cu$_{0.90}$Cr$_{2}$O$_{4}$), respectively, was obtained from the TEM. Magnetic properties of the synthesized nanoparticles were studied using a vibrating sample magnetometer. The ZFC and FC curve results show that the two different magnetic phase transitions at T$_{C}$ = 94 K associated with long-range ferrimagnetic order, while at T$_{S}$ = 26 K, a spiral magnetic structure is observed [9]. T$_{S}$ is suppressed for the Co$_{0.50}$Cu$_{0.50}$Cr$_{2}$O$_{4}$ sample because of the cubic to tetragonal structural phase transition. The magnetization as a function of applied field measurements, $M(\mu_0H)$, of Cu-doped CoCr$_{2}$O$_{4}$ nanoparticles indicate that the magnetic properties change from ferrimagnetic to paramagnetic behaviour [10]. The magnetic saturation ($M_{s}$), remanence ($M_{r}$), and coercivity ($H_{c}$) of the samples were obtained from the results of $M(\mu_0H)$ and will be discussed.
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10. Gingasu et al., 2015, Materials Research Bulletin, 62, 52–64.
